https://orcid.org/0000-0003-3630-8667
Figures
Abstract
Although different seasonal cues are important for fall armyworm (FAW, Spodoptera frugiperda J.E. Smith) survival, it is known that the life cycle of this insect is strongly dependent on air temperature, means that its development rate proceeds faster when the weather is warm. To develops the insect needs to accumulate an amount of thermal units, as known as Growing Degree-Days (GDD). However, with the climate change driven by global warming, the GDD pattern must be changed and therefore, the life cycle of this new bug in Mozambique may be different from that observed in its native region. In the present study it is estimated the possible changes of FAW life cycle by applying the GDD method over Mozambique, under two representative scenarios of climate changes, RCP4.5 and RCP8.5 for 2070–2099 relative to present climate (1971–2000). For this purpose, dynamical downscaling process through the regional model RegCM4, nested to global model HadGEM2 were used. The outputs of air temperature dataset from the simulations were used to compute the accumulated GDD and hence the FAW number of generations (NG) during the summer-season over the study domain. The findings indicate that there is a bipolar pattern of GDD accumulation, being negative over most of central and restricted areas in southern region, and positive in northern region, altitude-modified climate areas over central region, and over southernmost areas for both representative climate scenarios, relative to present climate. Meanwhile, there is an increase (decrease) in NG in the areas of higher (lower) increase in air temperature for both future scenarios relative to present climate.
Citation: Sumila TCA, Ferraz SET, Durigon A (2024) Climate change impact on Spodoptera frugiperda (Lepidoptera: Noctuidae) life cycle in Mozambique. PLOS Clim 3(1): e0000325. https://doi.org/10.1371/journal.pclm.0000325
Editor: Djanaguiraman Maduraimuthu, Tamil Nadu Agricultural University, INDIA
Received: August 17, 2023; Accepted: December 11, 2023; Published: January 18, 2024
Copyright: © 2024 Sumila et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: The CRU interpolated dataset can be found at “Dataset Record: CRU TS4.05: Climatic Research Unit (CRU) Time-Series (TS) version 4.05 of high-resolution gridded data of month-by-month variation in climate (January 1901–December 2020) (ceda.ac.uk)”; the weather stations dataset can be found at “INAM—National Institute of Meteorology”. The future climate scenarios can be found at IPCC home page.
Funding: This research was funded by National Council for Scientific and Technological Development (TCAS) who, at the same time, received a salary from an institution of the government of Mozambique and, material resources for this work were provided by the Federal University of Santa Maria (AD and SETF). Angelica Durigon and Simone E.T. Ferraz are scientific productivity Fellows and professors at the Federal University of Santa Maria, mentors of this research. The funders had no role in the study design, data collection and analysis, decision to publish or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
The life cycle and dynamical population of insects are strongly affected by air temperature, which influence on all ectothermic organisms, the so-called cold-blooded animals. Diapause is an expected physiological mechanism of the invertebrate small animals, but some of them, such as the cold-blooded insects have lack of this temperature adjustment process. The development rate of this insect is controlled by its biological clock directly correlated with air temperature. Thus, climate factors particularly air temperature, can impact the physiological metabolism of these species [1–5]. Fall Armyworm, Spodoptera frugiperda, (J.E. Smith, 1797) (Lepidoptera: Noctuidae) hereafter FAW, an endemic and important agricultural pest native from tropical regions of North America, is a cold-blooded migratory insect. Means that, for itself the pest cannot survive under adverse climate conditions, particularly extreme cold or hot weather [3, 6, 7].
Since early 2016, the first record of FAW in West Africa (Nigeria, Ghana, Benin, Sao Tomé and Principe and Togo) was confirmed and rapidly spread throughout the tropical and subtropical regions of sub-Saharan Africa [8–12]. Because of its ability to travel several hundred kilometers in a single night, in the following year, 2017, 28 sub-Saharan African countries including Mozambique, had confirmed the outbreaks of FAW [7, 11, 13]. According to some studies [e.g. 9, 14], the two strains of the species have been found in some African countries and, although the few records of the pest in Mozambique [e.g. 15, 16], it has been reported that the pest keeps spreading throughout the country.
FAW may cause significant damage to agricultural crops with emphasis on maize, one of the two main FAW host crops. The agriculture activities in Mozambique are predominantly driven by smallholder farmers, with around 80% of Mozambican households practice agriculture as their main livelihood, and maize is one of the major staple crops of smallholder farmers in the country. The climate impact-related of FAW on maize (Zea mays L.) is difficult to project due to the complex interactions among insects and this host crop. Meanwhile, the presence of FAW associated with ongoing climate change increase the risk of agricultural activity and thus jeopardizing food security in the country, which since 2017 has recorded some estimated losses on this important host crop [17]. It is important to stress that, unlike the original moths, the African FAW infestation may represent a novel interstrain hybrid population and [18] mentioned uncertain behavioral characteristics of this population.
Climate change is already affecting every inhabited region around the world, and the impacts are more severe in developing countries, such as those of sub-Saharan Africa. Throughout the Intergovernmental Panel for Climate Change-Assessment Reports (IPCC-AR) there is strengthened evidence that the projected climate change simulated by the global models have shown the widespread increase in air temperature, particularly over sub-Saharan African region [19–21]. The southern Africa region, where Mozambique is located, is projected to experience an increase in spatio-temporal variability of air temperature, concurrent with multiple changes in climate impact-drivers [19, 20]. Such as stated by [22] within Southern Africa, Mozambique is one of the hotspots, as it is particularly vulnerable to climate change compounded by high levels of poverty and strong reliance on the rainfed agricultural sector.
Many terrestrial species have shifted their geographic ranges, seasonal activities, migration patterns, abundances and species interactions in response to new established climate environment [23–25]. As stated before, FAW is strongly affected by climate factors and climate change may affect its geographical range, survival, mortality and number of generations per year [26–29]. Some studies [13, 28] have shown that FAW can establish itself in almost all sub-Saharan African countries under current climate but, climate barriers, such as higher temperatures, may limit the spread of FAW to tropical regions close to equator. This statement supports the idea according to which, there are climate-related sensitive thresholds which if crossed have deleterious impact on FAW survival. Although a small portion is somewhat far, most of Mozambique territory are close to equator and the growing season is overwhelmingly hottest. Even though, there is a risk for FAW become transient and permanent population establishment in Mozambique under current climate conditions. Meanwhile, it is not yet known what will be the real impact of the predicted future climate on the dynamics and the life cycle of this insect in the country. So far, no study has been conducted to our knowledge to predict the local response of FAW under future climate change scenarios.
This research aims to estimate the impact of climate change on FAW life cycle, as well as highlight the risk of outbreak due to future climate relative to present conditions in Mozambique. In addition, the study focuses on air temperature-dependence life cycle to map climatically month to seasonal thermal units needs for FAW development.
Data and methodology
Study domain and climatology
Geographically, Mozambique is a country located in the coastal region of Southern Africa, bounded between 10° 27΄ S and 26° 57΄ S of latitude, and 30° 12΄ E and 40° 51΄ E of longitude, that favors tropical climate conditions (Fig 1). The country holds a long north-south coastline, covering about 2700 km and predominantly characterized by lowlands. However, below 20° S of latitude it is observed a marked east-west gradient of topographic caused by plateaus over the central and northern regions of the country. There are important floodplains areas throughout the broadening Zambezi River valley in central region of Mozambique (area bounded by black shaded ellipse, Fig 1). This is an important agricultural region where farmers grow maize year-round. Therefore, besides to favorable climatic conditions the region may become a hotspot for FAW survival and overwintering, enabling seasonal migration between the main growing-season and the flood-recession crop. This migratory behavior probably allowed the expansion of the FAW habitat over the study domain, where by 2017 it had already been confirmed its presence in six provinces over the 3 regions of the country, namely: Maputo, Gaza, Manica [15, 16], Tete, Zambézia and Niassa provinces (Fig 1, red shaded) [30].
There are 10 provinces (Maputo, Gaza, Inhambane, Sofala, Manica, Tete, Zambézia, Nampula, Cabo Delgado and Niassa) and each capital city is represented by the black marks closed to respective symbol (MP, XX, IN, BR, CH, TT, QL, NP, PB and LC respectively). In the same time, these points represent the weather stations used here (left panel). The black dashed ellipse represents the section of Zambezi River basin. The three climatological sub-regions used in this study are shown in the 3 panels left and, the red dashed-bounded areas indicate the regions with FAW presence confirmed [30]. Direct link to the base layer of the map is https://www.gadm.org/download_country.html.
The tropical climate is overwhelming predominantly in the country but, as reported in previous paragraph, the east-west gradient of topography, and because the regional and local characteristics it is possible to describe local climate diversity throughout the territory by Koppen-Geiger climate classification (Fig 2).
Source of data: International Institute for Applied System Analysis (IIASA). Direct link to the base layer of the map is https://www.gadm.org/download_country.html.
Globally, there is an observed rainy season from October to March and, extended dry season from April to September (Figs 3 and 4) (INAM-Instituto Nacional de Meteorologia). The trend line of mean annual precipitation and air temperature, ranges from wetter and hotter, with up to 1500 mm and between ± 25 and ± 27°C (during summer season), to drier and colder with around 300 mm and between ± 18 and ± 25°C (throughout extended winter season), respectively. The central region of Mozambique concentrates the greatest amount of precipitation and the one that records in absolute and mean values, the highest air temperatures year-round (Figs 3 and 4).
Source of data: Instituto Nacional de Meteorologia (INAM).
Direct link to the base layer of the map is https://www.gadm.org/download_country.html.
FAW biology and its dynamical distribution
FAW is a polyphagous pest with a wide host range feeding on more than 186 plant species, sometimes considered one of the major pests of cereals and forage grasses [10, 11, 13]. The larval hatching occurs a few days after the female FAW moth lay eggs on the maize plant leaf. Usually there are six larval instars throughout the FAW life cycle. In the first and second instars the larvae feed on leaves, but eventually they enter in the plant whorl and feed on the unfurled leaves causing extensive defoliation. Duration of the larval stage tends to be about 14 days during the summer and 30 days during cool weather. After completing the last larval stage, the larval pupate into the soil at a depth of 2 to 8 cm, which lasts from 8 to 9 days during the summer, but it may range from 20 to 30 days during the winter. After that the moths emerge from the ground. Under suitable climate conditions, the duration of adult life (moth phase) is estimated at about 10 days, with a range of about 7 to 21 days. Adults are nocturnal, most active during warm and humid evenings and able to migrate to other regions. In warm climate FAW is able to complete the life cycle between 3 to 4 weeks, but in cold weather conditions, sometimes lethal to its survival, may takes considerably longer, up to 45 days or more [3, 28, 31, 32].
One of the first studies focusing in air temperature impact-related of FAW throughout his metamorphosis was made by comparing the development period in each larval stage between summer and winter season [3]. Over the years, several other studies have been done similar analysis. For instance, [33] determined the development rate of FAW at different air temperature levels and additionally computed the amount of thermal units required for the caterpillar to complete each of the larval instar. In the same study was stated that, unlike cold seasons, warmer seasons explicitly may induce to faster development rate, which is reflected in shorter time for each larval stage. Along the same line of research, [34] pointed out that, development time was longer at constant 25°C than at a mean of 25°C which fluctuated between 25°C and 30°C. Alongside, [35] reports the temporal and morphological parameters of the immature stages of FAW for larvae fed on artificial diet under controlled conditions, such as, 25°C, 70% of relative humidity and 14 hour photophase.
Under the perspective of the length of time to complete the life cycle it is observed that there is a significant difference in development time from winter to summer, meaning that colder conditions may delay up to 15 days to complete caterpillar stage Fig 5 [33].
Climate impact-related for FAW survival
Under continue greenhouse gases emissions, global mean air temperature will continue to rise throughout 21st century. This trend of rising global mean air temperature, usually projected trough future climate scenarios or Representative Concentrations Pathways (RCPs), is expected to reach and/or exceed 1.5°C (2°C) under RCP4.5 (RCP8.5) for 2081–2100 relative to 1985–2005 [20]. However, the referred increase of global mean air temperature will not be regionally uniform. The simulation of the numerical models indicates that over sub-Saharan African countries for example, it is virtually certain that there will be more records of extremes (warm) air temperature than colder as the global mean increases. Additionally, some regionalized simulation have point out that, Mozambique as a country of tropical climate, is expected widespread increase in mean air temperature [22, 37, 38].
Besides to the likely shortening of aforementioned FAW life cycle, the hot and humid climate over tropical and subtropical regions of Mozambique, may allow the insect overwinter and lives year-round, and becomes endemic in the country. These hypothesis can be supported by the field results of [15, 16] and from some few forecasting studies of FAW year-round, potential distribution and global extent, including over sub-Saharan Africa. For instance, besides to established climatic limits for FAW survival, [13] stated that much of sub-Saharan Africa may host the FAW populations year-round. Another important results found through Species Distribution Model (SDM) simulation, indicate the Indian Ocean coast in Southern Africa region (implicit indication of the present study domain) as the only area in southern Africa which has suitable climate environment for FAW survival [28]. It may also mean that, during the southern hemisphere summer season, FAW moths can migrate over long distances throughout the Mozambique regions, and establish transient populations which can jeopardize agricultural crops in the country. This behavior was observed over the native FAW region, and reported by [6, 7, 13, 35]. According to these two studies, FAW moths can travel several hundred kilometers in a single night, behavior aimed at escaping adverse climate conditions, demand for host plants and ultimately the maintenance of the species. The previously highlighted findings are reinforced by the warm and humid tropical climate, the prevailing climate feature in Mozambique. As known, the strongest adverse climatic factors for FAW survival and distribution are extreme air temperatures, both minimal as well as maximum. But extreme maximum air temperatures are overwhelmingly more prevalent in the present study domain. Although there are regions of climate modified by altitude, where lower air temperatures are usual during winter season, thereby supporting the above-mentioned statements.
The thermal units required for FAW development
The range defined as the sum of daily mean air temperature is called thermal units or Growing Degree-Days (GDD) required for FAW development. However, there is some difference of air temperature required throughout the FAW development stages [33, 39–41]. For instance, [42] and [41] stated that when mean air temperature is between 15 and 25°C then, daily fluctuations above and below these borders increase pupal and larval development rates and decrease adult deformity. Quoted by [42] the developmental time at constant air temperature in the laboratory ranged from 66.5 to 18.4 days at 18.3 to 35.0°C respectively, with incomplete development at or below 15.6°C. During the larval instars for FAW populations in the American regions, the feeding rate increase between 25–30°C [40], while period until pupation decrease between 28.9–33.4°C [43], larval developing rate increase between 21–30°C [42] and, the lowest viability to pre-pupal stage occur at 32°C with duration decreasing when temperature range between 18–32°C [39]. Furthermore, considering the lack of diapause, the above-mentioned studies were conducted under some defined lower and upper air temperature thresholds (cardinal temperatures) for FAW survival and development. For example, in [44] the lower development threshold was found to be equal to 7.4°C, meanwhile [45] considered 10°C, and [31] 13.8°C. The most fairly conservative upper development threshold mentioned among the several studies is around 39.2°C [44].
Recently, important result was published in the study of [33]. The authors pointed out that the development rate of FAW increase linearly with daily increasing temperatures between 18 and 30°C. Otherwise, the larval mortality was highest (lowest) with temperature below 18°C (between 26 and 30°C). Note that from 30°C and beyond, there is a trend to reverse larval mortality, means that the development rate becomes nonlinear (reversal trend) above these thresholds and FAW survival is limited (Fig 6). As can be seen in the same figure, the larval (egg-adult) development period is 34.39 days (71.4 days) at 18°C, and 10.45 days (20 days) at 32°C, supporting the results according to which FAW development rate is reduced during winter and increased throughout summer season [33].
Source of data: Adapted by the author through the results presented by [33].
As a consequence of the correlation described in previous paragraph, the response of FAW to air temperature can be described through a mathematical models and air temperature response functions, such as described briefly by [46–52] although its applications have been commonly used for agricultural crops. However, the physiological response to air temperature is not linear as suggested in most of mentioned papers, but there is a response function similar to parabolic trend line with downward-facing concavity. First of all, the cold-blooded insect develops following an asymmetric quadratic function through which is observed an exponential increase of development rate, ranges from lower threshold (lower cardinal temperature, Tb) up to maximum development rate (optimum air temperature range, Top), around 25°C. Then, decline sharply from Top to upper threshold (upper cardinal temperature, TB). If by any chance the maximum air temperature increases beyond TB, the insect will be exposed to the lethality conditions for its own survival (Fig 7).
FAW development rate and its growth range limits, as a function of lower (Tb), optimal (Top) and higher (TB) cardinal temperatures in Celsius degrees. Source: Adapted by the author through the results presented by [53] by the author.
Climate data
For a comprehensive assessment of the impact and implications of climate change on FAW life cycle, it was necessary to use the output of regional numerical model simulations that span a reasonable range of the likely climate change impact-related. The climate environment for FAW survival was simulated through regional model RegCM4 nested by global model HadGEM2 for baseline (1971–2000) and future climate (2070–2099). The future climate simulations used here were run under two Representative Concentration Pathways, mid-radiative forcing (RCP4.5) and strong-radiative forcing (RCP8.5). These climate scenarios represent the assumption according to which, the increase of global mean surface air temperature by the end of twenty-first century (2081–2100) relative to 1985–2005 is likely to be 1.1°C to 2.6°C under RCP4.5 and 2.6°C to 4.8°C under RCP8.5 [20]. Although brief short description of the downscaling procedure and the regional model RegCM4 nested to global model HadGEM2 setup provided above, full details can be found in [54].
The GDD FAW requirements and number of generations
For each development stage and therefore throughout the life cycle, FAW needs to accumulate amounts of thermal units. However, the insect grows and develop when air temperature is above Tb and below TB. The daily sum of air temperatures between this range and leads the insect to complete its life cycle is referred as thermal units or Growing Degree-Days (GDD,°C day). This index indicates the thermal units above Tb and below TB to be accumulated for the specie during each day.
Since there are no detailed studies for African climate environment and Mozambique in particular, the FAW life history and climate-related data from experimental studies conducted in other parts of the world were used to define the cardinal temperatures for computed the GDD and Number of Generations (NG) throughout the study domain. The conservative findings of air temperature thresholds range from 10 to 40°C and, there is no hatching and no survival at 40°C or more. Some experiments indicate that when FAW is reared on an artificial diet there is no development between 35 and 37.8°C [e.g., 28, 31, 33, 39, 41–43, 45]. Hence, based on the experimental data the lower and upper cardinal temperature for FAW population growth and development used in the present study, were left unchanged at Tb = 10°C and TB = 40°C, respectively. Meanwhile the optimum range (Top) is from 25°C to 28°C. As discussed early, the development rate of FAW increase linearly with increasing of air temperature from Tb to optimum range, then declines when air temperature increases above this boundary up to TB, as can be seen in [54], Fig 6.
Note that, although in Mozambique is not usual to record air temperatures below 10°C, FAW overwinters only in warm and humid areas. Hence, even if temperatures are above Tb but below 20°C for example, the development rate may slow down. So, for the purposes of calculation were used the bilinear method, which considers daily maximum and minimum temperatures, Tb and TB, in addition to two Top ranges, as described in the next section.
Some of the crop models applied in most of the studies to simulate the phenology of crop or small animals use the canonical form of GDD method. This mathematical form described by Mcmaster and Wilhelm (1997), considers only the daily mean air temperature relative to lower cardinal temperature. But there are several studies using different mathematical forms [e.g., 41, 42]. For instance, [33] using the canonical method, found 390 GDD with Tb = 12.57°C as the thermal requirements for FAW complete its life cycle. Similar results was found by [31], with Tb = 13.8°C were estimated 346.2°C day throughout FAW life cycle. To get minimum GDD required to complete a generation, [27] and [28] set Tb = 12.0°C and found 559 and 400°C day, respectively.
Under suitable climate conditions, after the moth lay eggs, it will be hatching from 3 to 4 days and, during this period the estimated GDD accumulation is around 36°C day. With the same climate conditions and available host crops, the larval (pupal) stage may take from 10 to 13 (7–14) days to complete the stage. During this period, it is expected to accumulate around 255°C day (97°C day). Hence, throughout its life cycle it is expected that FAW accumulate around 400°C day (Fig 8). These estimates were made under Tb = 13°C and TB≈39°C [31, 33, 39, 41, 42, 44, 45] (Fig 8).
Adapted by the author through the results presented by [31, 33, 39, 41, 42, 44 and 45] by the author.
For the purpose of GDD calculation and considering the assumptions presented previously, the mathematical model used in this study is the bilinear model, according with:
(1)
(2)
(3)
with n corresponding to the single day of GDD calculation and GDDsum is the sum of GDD for n days.
If Tavg < Tb, then Tavg = Tb, and if, Topt1 < Tavg < Topt2, then Tavg = Topt, then Eq 1 is applied. When Topt2 < Tavg < TB, so Eq 2 is applied. Note that, in the same time where maximum daily air temperature (Tmax) exceeds 40°C is set equal to 40°C, and minimum (Tmin) is set at 10°C when it is less than 10°C. The most common situation in the present climate conditions is observed when minimum Tmin and Tmax are above and below Tb and TB, respectively. Meanwhile, for the computation of the Number of Generations (NG) of FAW, it was applied the methodology established by Parra (1981) and Haddad et al. (1999), quoted in [39] and [55]. According to this method, the mathematical model is written as following:
(4)
then
(4.1)
Replacing Lc from 4 in 4.1 is obtained:
(5)
Where Lc is the life cycle, is the daily mean air temperature, and N is the number of days which NG is calculated.
The results presented here are regarding to summer season, because the main agroclimatic factors related to FAW survival are observed during this period. In other words, the growing season of the FAW host crops override to summer season in Mozambique, where the agricultural activity is predominantly rainfed; on the hand, the summer season over the study domain is in the same time the rainy season, during which is observed warm and humid climate, conditions that may favor or not the FAW survival in tropical region.
Through the combination of GDD and NG results under the three experiments (baseline, RCP4.5 and RCP8.5), the five categories of GDD index linked to the possible NG were created, related to five levels of risk (very-low, low, medium, medium-high and high) and producing the colors scale for vulnerability. This index allows the delimitation of the risk and vulnerability to the FAW in Mozambique. The starting point to define the risk and vulnerability levels is the NG determined in the reference climatic conditions. The criterion for classifying risk and vulnerability levels is: very low for NG less than one, low for NG greater than or equal to one and less than 2, medium for NG greater than or equal to two and less than four, medium-high for NG greater than or equal to four and less than 6, high for NG equal or greater than 6. From there are determined five levels of GDD index, each one associated to the color scale (Table 1). These categorizations were adapted by taking into account two previous studies [26, 56].
Discussion and results
Under current climate scenario and during the growing season, from October to March, the largest GDD accumulation (≥2600°C day) is concentrated throughout the coastline of north and central regions of the study domain (Fig 9A). Alongside to this result, there is a narrow hotspot through Zambezi River basin and around XX and IN weather stations. In the same summer season, the lowest GDD accumulation (<2540°C day) is observed over the plateau areas, inside the central and northern regions. These results are closely matched with highest and lowest monthly and seasonal air temperature records over Mozambique. The lowest air temperature is recorded in the regions where the climate is modified by altitude, meanwhile in the low-lying areas the highest records occur.
Fig 9A, Fig 9B and Fig 9C illustrate GDD for summer-season (OND-JFM), but Fig 9A represent accumulated while Fig 9B and Fig 9C represent the difference between each RCP relative to reference period. Fig 9D, Fig 9E and Fig 9F shown the GDD for OND quarter, and Fig 9G, Fig 9H and Fig 9I for JFM. While Fig 9D and g are the total GDD, Fig 9E, Fig 9F, Fig 9H and Fig 9I corresponds to the difference between each scenario and respective quarters, relative to reference period. Direct link to the base layer of the map is https://www.gadm.org/download_country.html.
There are positive and negative responses for RCP4.5 radiative forcing relative to reference scenario. The differences range from less than -100 to more than 100°C day but, the positive response prevailing (Fig 9B). Additionally, the regions with highest GDD in reference period are those with negative response under RCP4.5 scenario. Meanwhile, for the strongest representative scenario (RCP8.5), there is a widespread trend to reduction the GDD accumulation (Fig 9C). These negative responses ranges from -500 to just under 50°C day relative to reference period, and around XX and IN weather stations the signals are sharper and more significant over the north and central regions. The altitude-modified climate regions remain with a single response from the other regions and in this case, lesser significant GDD reduction. Important to note that the trend of this reduction is more significant along north-central coastline, around XX and IN weather stations, and throughout Zambezi River basin for both scenarios and also, more pronounced feature under RCP8.5 than RCP4.5 radiative forcing’s (Fig 9B and 9C).
When the summer season is broken, the similar GDD accumulation pattern is observed during both OND and JFM, but smoothly higher in the first quarter (Fig 9D and 9G). The difference between RCP4.5 relative to reference period allows to observe the come up of a bipolarization among north and southernmost (positive response), against central-southern (negative response) regions (Fig 9E and 9H). Once again, the pattern of this bipolarization is more comprehensive during OND months (Fig 9E). Otherwise, the bipolarization pattern almost demise during OND months under RCP8.5 scenario, because the negative response is extended northward (Fig 9F). Although these findings do not be the same during JFM months, the pattern remains for both RCPs but with a westward GDD gradient occurring, being negative through coastline and positive inland (Fig 9H and 9I). An important worth to highlight is that, for both spatio-temporal scale there is the more negative predominance along the Zambezi River basin and around XX and IN weather stations, and more positive in altitude-modified climate regions. In addition, throughout the summer season, the OND months contribute more to the variation and establishment of this GDD feature relative to JFM (Fig 9E and 9F, against Fig 9H and 9I).
The GDD spatial pattern showed in Fig 9, is closely related to the climatology of the mean air temperature during the extended summer season in Mozambique (Fig 10). As can be seen for reference period (1971–2000) and from October to March, the highest mean air temperature is concentrated on the coastline of the central region and along the Zambezi River basin. Nevertheless, relatively similar values are observed in the coastline of north region and northwest areas of XX and IN weather stations. The lowest values occur in low-lying areas of the southernmost region, and over central and northern plateau regions (Fig 10A). Under both RCPs scenarios a generalized positive response is observed in the study domain. However, the overwhelming positive response occur over central region and northwest areas of XX and IN weather stations (Fig 10B and 10C).
Climatology of the mean air temperature for reference period (panel a) and the difference between RCP4.5 (panel b) and RCP8.5 (panel c) relative to reference period (1971–2000). The average is relative to extended summer season, from October to March (ONDJFM). Direct link to the base layer of the map is https://www.gadm.org/download_country.html.
This reduction of GDD comes from projected increasing of daily air temperature above Top and TB. For instance, similar results was found by [27] and [28]. According to these studies, under climate change scenarios, areas of climatic suitability for FAW establishment are expected to gradually decrease over time mainly due to heat and dry stress. However, in the same studies is claimed that, if determined climate or weather conditions are met, the established persistent FAW populations could, in turn, serve as a source of seasonal invasions and migrate into less favorable climatic regions. This latter statement may be usual if the combination of available GDD and irrigated crop fields in the suitable hotspot of Zambezi River basin allow seasonal FAW populations during projected warmer climate in Mozambique. FAW may still survive and become established in significant areas in Mozambique, because the suitable thermal units for its life cycle is still favorable. Moreover, the reduction (increase) of GDD in low-lying (plateau) regions may be as manifested shift of suitable area for FAW survival. This scenario may be the trigger to activate the migration behavior of the insect for the most suitable new regions for FAW development in the country. Similar behavior was observed in native regions of central-north America, as well described by [6] and [7].
The NG of FAW throughout the extended summer season (ONDJFM) and in each quarter (OND and JFM) is shown below (Fig 11). The mean NG during the summer season ranges from around 0.6 to 1.2 under reference scenario. As can be seen, the spatial pattern of NG overlaps the accumulation of GDD in summer season, with greater NG (lower NG) occurring in warmer (colder) climate regions (Fig 11A). Relative to reference period, there is a positive response of both RCPs, means that the NG will increase under futures scenarios. Moreover, besides to the possible suitable agroclimatic conditions for FAW survivor along Zambezi River basin, the greatest increase in NG is observed in this region. Note that similar response is observed in the northern areas of XX and IN weather stations. The smallest increase of NG is concentrated in most of the interior of northern region, around southernmost (MP weather station), coastline of XX and IN weather stations, and over plateau areas of the country (Fig 11B and 11C).
Direct link to the base layer of the map is https://www.gadm.org/download_country.html.
As pointed out for GDD from a quarterly point of view, it is noted that the largest (smallest) contribution to summer season NG pattern is observed during OND (JFM). The OND (JFM) months contribute with 1.2 to 1.4 (less than 1, except over northwest of XX and IN weather stations) NG for the summer season as a whole (Fig 11D and 11G). The difference between future and reference scenarios during OND months is predominantly positive, keeping more significant in the central region and around northern areas of XX and IN weather stations. But there is observed negative signal over some areas in northern and southernmost (Fig 11E and 11F). The pattern observed in OND is similar to that observed during the JFM months, but less pronounced for positive response and more pronounced for negative. The latter response occurs mainly on the plateau regions and southernmost of the country (Fig 11H and 11I).
It is noted that during the summer season, there is a tripolar pattern of the response for FAW-NG. After the discussion of accumulated GDD in the same period, the increase in FAW-NG was expected because there is an inverse relationship between NG and GDD manifested in Eq 5. After all, for the insect complete its life cycle, it needs to accumulate a certain amount of GDD. Hence, there is opposite responses between GDD and NG but both in phase with the regions of highest and lowest heat stresses, simulated by [54]. Even more interesting is that the most pronounced answer is concentrated on Zambezi River basin and surrounding areas, the currently most important maize-belt in Mozambique. However, if it is considers valid the assumption stated by [8], according to which Zambezi River basin may acts as a reservoir for this pest, providing suitable climate for overwintering or making perennial infestation, where it can then recolonize in cooler climates in the adjacent highveld from November to April, the increasing of NG will enhance the risk of FAW on agricultural crops in the region, especially on maize.
The present findings converge with those found by [27], according to which although several studies, e.g., Altermatt 2010; Karuppaiah & Sujayanad 2012, quoted in [27] and [57] highlight the expansion of insects geographic ranges in warmer climates, little attention has been given to a possible reduction in or disappearance of pest suitability due to a warmer and drier climate. The findings of [54] shows an increase in air temperature and a decrease in mean precipitation that reduces or nullifies the suitability of many areas for FAW over Mozambique. This may be explained by the fact that when higher air temperatures favor the insect life cycle and developmental rates, the same air temperatures may also affect the FAW survival if this increase exceeds the TB, making the insect facing the heat and dry stress. Another important finding is that the increase in air temperature in cold regions may enhance the insect fitness and survival, and hence, there is a shift in unsuitable to suitable areas in colder places (plateau areas) and a reduction in or disappearance of suitable conditions (most of low-lying areas) in some regions of Mozambique.
Table 2 shows the sum of mean GDD required for FAW to complete its life cycle, computed through Eq 3. The mean time for FAW complete its life cycle is around 40.5, 26.8 and 21.7 days for reference, RCP4.5 and RCP8.5, respectively. Hence, the mean GDD required for each corresponding abovementioned scenario are 431.2, 412.4 and 338.1°C day. There is a clear falling trend of GDD and shortening the life cycle as radiative forcing become stronger. This result is a consequence of the increase in air temperature in the same direction as the climatic scenarios considered in the present study. In laboratory experiment studies, [39] find out 463 GDD, [33] converge more with the present results, with 391.61±1.42 GDD requirements for egg-to-adult development thermal time. However, Ramirez et al. (1987) quoted by [26] found a significantly higher value of GDD, 599°C day.
The gray-shaded lines indicate the mean accumulated GDD in the same period in the study domain.
Under current climate the estimated number of generations is higher in hottest regions and northern coastal of Mozambique, where the mean global number of FAW generations during the summer season is around 1.07. It means that, it is expected to occur around one generation per month from October to March and the largest contribution came from the central region of the country. Hence, throughout the summer season, the largest number of FAW generations occurs in TT and QL with 7.68 and 7.02, meanwhile the lowest number of FAW NG is observed in LC and CH weather stations with 4.68 and 5.55, respectively (Table 3). There is a little difference between OND and JFM seasons.
Unlike to reference period, the mean monthly number of FAW generations under both future scenarios is above one in all weather stations (Tables 4 and 5). However, the largest and smallest records continue to be recorded around the same weather stations as those of the reference period. From reference period to RCP4.5 and RCP8.5 scenarios, the global mean of FAW generations increase from 1.07 to 1.35 and 1.82, respectively. In addition, the monthly values are slightly higher than one, overestimating in several cases one and half, and in a few others reaching two generations per month. It is important to worth highlighted that the greatest contribution to the total generation in the central region, the most affected region, occurs during OND season, a pattern that is not observed over northern and southern regions of the study domain.
When the monthly difference between NG in each future scenarios relative to reference period is made, there is a clear positive response in all weather stations and surrounding areas. But, as noted in previous results, the most significant response occurs for RCP8.5 than for RCP4.5. Moreover, the most positive response still occurring over central region, with 3.81 (9.29) and 2.18 (8.20) NG for QL and TT respectively, under RCP4.5 (RCP8.5). Surprisingly, the order of magnitude of FAW NG response in LC weather station is included in the group of the second highest, contrary to what was observed in seasonal and global time scale. This unexpected result becomes from the higher response under both radiative forcing’s. Besides to these results, the lower positive response occurs in MP weather station.
Considering the results discussed above and supported on FAW-NG, regions of risk and vulnerability were bounded and classified according to Fig 12. It can be easily observed that there are two regions marked out as hotspot with more than 6.5 NG throughout south hemisphere summer season for reference period. At the same time, over the regions of humid tropical climate with cold winter (altitude-modified climate in restricted central and northern region) and subtropical climate in the southernmost of the country, the smallest number (less than 5 NG) of FAW generations is observed (Fig 12A). The difference between RCP4.5 and reference period (Fig 12B) is entirely positive, with the huge extension of maximum risk and vulnerability (more than 2.5 NG relative to reference period) occurs along almost the entire Mozambican coastline and around the “narrow strip” of the Zambezi River basin (Fig 12B). Likewise, but most strong response occurs for RCP8.5 scenarios relative to reference period. It is observed from 4 to more than 6.5 NG over the similar hotspot identified in current climate (Fig 12A and 12C).
Direct link to the base layer of the map is https://www.gadm.org/download_country.html.
The GDD index represent the delimitation of risk and vulnerability areas for FAW outbreak (Fig 13). Thus, it can be defined risk as the possibility of occurrence (highest or lowest) of FAW in any region of the study domain. As can be seen the minimum level of GDD index occur in restricted plateau areas, meanwhile the low level is noticeable over most of north region and some areas around MP, CH and TT weather stations. Furthermore, medium and medium-high GDD index is predominantly observed over most of central and southern region of Mozambique, with emphasis to the strip of greatest increase of NG (Fig 13). This latest finding indicates that where the GDD index is medium-high represent the region of highest risk and extreme vulnerability to outbreak of FAW.
Direct link to the base layer of the map is https://www.gadm.org/download_country.html.
Conclusion
Our findings indicate that for both RCPs scenarios it is expected a significant reduction of GDD for FAW life cycle along north and central coastline of Mozambique. Besides to this negative GDD response there are two important hotspots, one of them in northern areas of Xai-Xai (XX) and Inhambane (IN) weather stations, and other throughout the narrow strip and surroundings areas of the Zambezi River basin. Meanwhile, most of the positive GDD response for future climate is observed over the plateau regions, where the air temperatures records are the lowest under present climate in Mozambique. Meaning that the regions with the highest (lowest) GDD accumulation in the present climate have negative response (positive response) in the GDD under RCP4.5 and RCP8.5. The referred response is stronger during OND quarter than JFM, and under RCP8.5 than RCP4.5. The results linked to decrease and increase of GDD are the consequence of significant increase in number of daily air temperatures above TB, and between Tb and Top, respectively. This statement is one of the findings highlighted by [54].
Concerning to NG it is expected that FAW takes out from 37.7 to 41.3 days to complete its life cycle during the summer season in the present climate environment. However, under future climate scenarios, it is observed a reduction in the time to complete the life cycle, as representative scenarios become stronger relative to reference period. This shortening of time to complete the FAW life cycle is closely related to decreasing in the mean GDD accumulated during summer season, forced by increasing of air temperature under both future scenarios simulated over Mozambique. The immediate consequence of the shortening of the FAW life cycle is the increase in NG, with the largest contribution occurring during OND quarter. Hence, more than 2.5 generations are projected to occur along north and central Mozambique coastline, besides to surroundings of Zambezi River basin, and north areas of XX and IN weather stations. These are the regions of medium-high risk and higher vulnerability to FAW outbreak. From here it is possible to suggest that increasing air temperature allow speed up the insect life cycle leading to a faster increase in FAW populations through the largest NG per year. However, unlike this statement the FAW survival and its development rate may also reduce as air temperature increases up to exceeding its survival thresholds.
The year-round availability of maize combined to warm and moist climate over most of the regions mentioned above, may favor a suitable agroclimatic environment for FAW populations to become pandemic in Mozambique. For this reason, through natural migration it should not rule out the possibility of seasonal migration from the maize belt to other regions of Mozambique, just because throughout the life cycle FAW moths use maize growing areas as “stepping-stones” to self-survivor of the species.
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