Fig 1.
Diagram of the solitary bee lifecycle and potential for pesticide exposure.
(1) The solitary bee lifecycle begins when adult male bees emerge from diapause in spring/summer. (2) Females emerge second, mate, and, without help from their conspecifics, (3) construct their nests either underground or in cavities using mud, soil and leaves. Nests are composed of multiple brood cell chambers that are each provisioned with a ball of pollen mixed with nectar upon which the female lays an egg before sealing the brood cell. (4) After hatching, the larvae feed on their mass provisions and continue to develop. (5) Depending on the species, solitary bees overwinter as either prepupae (i.e., non-feeding larva in the 5th instar inside a cocoon) or pre-emergent adults. The cycle repeats when adults eclose from their cells ready to mate the following spring or summer in synchrony with their host plant(s). However, some species (e.g., Nomia. melanderi, Megachile. rotundata) may produce a first generation of summer-emerging bees and additional full and partial generations during the same growing season. Depicted in the central circle are potential routes of pesticide exposure for adults (gray section) and egg/larvae (open section). *Relevance of route of exposure dependent on life history of individual solitary bee species.
Table 1.
Populations, Exposures, Comparators, Outcomes (PECO) criteria.
Fig 2.
Literature search and selection flow.
The study flow diagram describes the number of publications processed at each step of the evaluation. Literature searches for Web of Science and ProQuest were conducted from January 1, 1970 to February 2021. Targeted internet searches were performed using Google Scholar during March and May 31, 2020 and February 1 and February 15, 2021. In total, 65 publications containing solitary bee pesticide exposure-effects investigations were identified. Since the objective of this systematic review was to scope the literature and identify data gaps, all PECO-relevant studies were included in the sections that follow.
Fig 3.
The number of insecticide, fungicide, and herbicide investigations identified for this review.
(A) The number of investigations identified for various effect types. Open bars signify investigations performed in immature bees or resulting adults. Filled bars signify investigations performed with adult bees. (B) The number of investigations identified for solitary bee life stages. The number of investigations for each life stage conducted under lab, semi-field and field conditions are stacked to produce the total number of investigations conducted. (C) The number of investigations identified for lethal and sublethal endpoints. The number of lethal investigations performed in adults and immatures and the number of sublethal investigations evaluating field realistic and non-field-realistic concentrations are shown in parenthesis, respectively. *Twenty-four investigations did not report whether or not they used field-realistic concentrations or non-field-realistic concentrations.
Fig 4.
Number of solitary bee publications by year.
Web of Science, ProQuest Agricultural and Environmental Science Database, and Google Scholar were searched to February 11, 2021. All results were compiled and organized by year of publication. The number of PECO relevant publications are shown in the black portion of the bar. Publications containing supporting supplemental information are shown in the open portion of the bar. *Since our search was completed prior to the release of all 2021 publications, not all publications for the year 2021 were available for this systematic evidence map.
Fig 5.
Worldwide distribution of solitary bee studies.
The number of publications completed in each country is shown numerically and increases along a color gradient from green to red. This figure was constructed using software developed by Tableau Incorporated (www.tableau.com). The world map was used under a CC BY-SA copyright from OpenStreetMap contributors (www.openstreetmap.org/copyright).
Table 2.
The number of publications investigating the effects of pesticides on commercially available solitary bee species.
Table 3.
Insecticides studied in solitary bees.
The number of investigations that used a formulation or technical grade active ingredient alone are shown in parenthesis.
Table 4.
Fungicides studied in solitary bees.
The number of investigations that used a formulation or technical grade active ingredient alone are shown in parenthesis.
Table 5.
Herbicides studied available solitary bees.
The number of investigations that used a formulation or technical grade active ingredient alone are shown in parenthesis.
Table 6.
Mixtures studied in solitary bees.
The number of investigations that used a formulation or technical grade active ingredient alone are shown in parenthesis.
Fig 6.
Evidence map of pesticide types and experimental conditions.
Distribution of investigations across pesticide types and experimental conditions for adult and immature solitary bees. Colors correspond with investigations of lethal and sublethal outcomes (blue = lethal; orange = sublethal). Bubble size reflects the total number of investigations, ranging from 1 (smallest bubbles) to 87 (66 investigations of lethal and 21 investigation of sublethal effects of insecticides in adult M. rotundata under laboratory conditions); individual publications often investigate both lethal and sublethal outcomes.
Fig 7.
Evidence maps for each pesticide type broken down by pesticide class.
Distribution of investigations across solitary bee species, life stage, and class of insecticide, fungicide, and herbicide. Colors correspond with investigations of lethal and sublethal outcomes (blue = lethal; orange = sublethal). Stacked bar size reflects the total number of investigations, ranging from 1 (smallest bars) to 71 (19 investigations of lethal and 52 investigations of sublethal effects of neonicotinoid insecticides in adult M. rotundata); individual publications often investigate both lethal and sublethal outcomes.