Effects of Pesticide Selection and Exposure to Pesticides and Bt on Survival and Development
Overall, exposure to both pesticide types, OP (OP PTx) and Pyr (Pyr PTx), decreased survival likelihood (OP PTx: z = 14.06, p < 1*10-4; Pyr PTx: z = 15.53, p < 1*10-4), while there was no main effect of Bt treatment (BtTx) on survival (Figs. 1A-B; Suppl. Table 3). Compared to control-regime populations, pesticide selection significantly improved survival after pesticide exposure (OP Reg:PTx: z = -6.15, p < 1*10-4; Pyr Reg:PTx: z = -10.58, p < 1*10-4), reinforcing previous observations of evolved pesticide resistance (Figs. 1A-B, compare OP Reg:PTx and Pyr Reg:PTx; Suppl. Table 3; Birnbaum et al., 2021). Interestingly, there was a positive significant interaction between OP and Bt treatment (OP PTx:BtTx: z = -3.26, p = 1.10*10-3) indicating that larvae dually exposed to OP and Btt had improved survival compared to larvae exposed to OP only (Fig. 1A, OP PTx:BtTx; Suppl. Table 3). There was no significant interaction between Pyr exposure and Bt treatment (Pyr PTx:BtTx) or between either pesticide selection regime and Bt treatment (OP Reg:BtTx, Pyr Reg:BtTx) on survival (Figs. 1A-B; Suppl. Table 3). Thus, while pesticide selection resulted in increased pesticide resistance, pesticide selection regime did not interact withBtt exposure to influence survival compared to control-regime populations. However, dual exposure to OP and Btt improved survival outcomes compared to pesticide treatment alone.
Pesticide selection regime (OP Reg, Pyr Reg), pesticide exposure (OP PTx, Pyr PTx), and Bt treatment (BtTx) individually delayed development (OP Reg: z = -7.28, p < 1*10-4; Pyr Reg: z = -2.72, p = 6.60*10-3; OP PTx: z = -16.89, p < 1*10-4; Pyr PTx: z = -12.68, p < 1*10-4 ; BtTx (OP): z = -11.31, p < 1*10-4; BtTx (Pyr): z = -10.5, p < 1*10-4), indicating a cost from each (Figs. 1C-D; Suppl. Table 3). There was no significant interaction between pesticide regime and Bt treatment on development (OP Reg:BtTx, Pyr Reg:BtTx; Suppl. Table 3). However, resistant individuals developed faster than susceptible individuals in the presence of pesticides, as indicated by a significant interaction between pesticide regime and treatment (OP Reg:PTx: z = 4.61, p < 1*10-4; Pyr Reg:PTx: z = 9.13, p < 1*10-4; Fig. 1C-D; Suppl Table 3). There was also a significant positive interaction effect between both pesticide treatments and Btt exposure (OP PTx:BtTx: z = 7.4, p < 1*10-4; Pyr PTx:BtTx: z = 3.98, p < 1*10-4), indicating that larvae exposed to both Btt and pesticide treatments developed faster than those exposed to only pesticide or only Btt (Figs. 1C-D; Suppl. Table 3). Overall, exposure to pesticides delayed development, representing a cost to fitness, but this effect was mitigated in pesticide-selected individuals. Btt exposure lengthened development time regardless of evolution regime but concurrent pesticide exposure partially mitigated this effect.