Figure 1.
Schematic representation of a 5×5 cell landscape for 2 successive years (top) and life cycle of M. aeneus (bottom).
Positions of cells (arable fields in yellow (OSR) or grey (non-OSR crops such as cereals), overwintering sites in green, refuges in purple, empty cells in white) are fixed at the beginning of the simulation. During the rotation cycle, the location of cells with OSR (yellow) changes as OSR is sown only once every 4 years in a field. In spring, after hibernation, adults move into refuges for maturation feeding and later colonise OSR crops for mating and oviposition (orange arrows). The next generation pupates and emerges from the soil in summer, feeds in refuges and finally moves to the overwintering sites at the end of summer (blue arrows).
Figure 2.
Population size and structure in the absence of insecticide treatments.
The size of the population and proportions of insects at different stages of development across the entire landscape were averaged over 300 years of typical daily weather generated for Rothamsted. Green, purple and yellow colours represent overwintering adults, feeding adults and reproducing adults, respectively. Blue, orange and grey colours represent: eggs, larvae and pupae.
Figure 3.
Pest pressure and oilseed rape area during the reproductive period.
The weekly distributions of the average number of insects per infested field (A) and the average area of flowering oilseed rape (B) were derived from a continuous simulation of 300 years of daily weather generated for Rothamsted. In this simulation, 95% of oilseed rape crops were WOSR and fields were not controlled with insecticides. The minimum/maximum, 5th and 95th percentiles, 1st and 3rd quarter envelopes are shown in red, blue and green, respectively. The median of the distribution is shown in black.
Figure 4.
Observed and simulated lambda-cyhalothrin control for 3 levels of resistance.
The measure of control is the reduction in insect number in treated plots relative to untreated plot population. Trials were pooled to reflect 3 different levels of pyrethroid resistance: (A) low resistance (75%–100% control), (B) moderate resistance (35%–75% control), and (C) high resistance (5%–35% control). Mean daily observed values averaged from field trials are shown as blue circles. Average simulated control is shown by the black line.
Figure 5.
Resistance evolution and resulting distribution of time before resistance outbreak in a Monte Carlo experiment.
Increase in resistance allele frequency (A) and distribution of the number of years for this frequency to exceed 50% (B) in a Monte Carlo simulation for 2 inheritance modes: dominant (red) and intermediate (blue). In this scenario, treatment decision followed the recommended high threshold (15 and 5 insects/plant for WOSR and SOSR, respectively) and 95% of oilseed rape was WOSR. The number of repetition was 36 simulations for both scenarios.
Figure 6.
Simulated lambda-cyhalothrin control at resistant allele frequency of 50% for dominant, intermediate and recessive inheritance.
The measure of control is the reduction in insect number in treated plots relative to untreated plot population. The control is simulated for 3 inheritance modes: (A) dominant, (B) intermediate and (C) recessive. For the 3 modes, the frequency of the resistant allele was set to 50%.
Figure 7.
Sensitivity of projected duration before resistance outbreak to sowing practices and treatment decisions.
Box plots of the distributions of the number of years for the resistant allele frequency to exceed 50% with a dominant inheritance (red boxes) and intermediate inheritance (blue boxes) for eight combinations of treatment (C1: single calendar treatment; C2: double calendar treatment; HT: high threshold treatment; LT: low threshold treatment) and sowing practices (W95: 95% WOSR; W75: 75% WOSR). Box boundaries show 25th and 75th percentiles, whiskers show minimum and maximum, thick horizontal line shows the median. The number of repetition was 36 simulations for each combination.
Table 1.
Sensitivity of projected duration before resistance outbreak to weather, surface of non-crop refuge and compound efficacy.