Assessing recovery of stream insects from pesticides using a two-patch metapopulation model

Pesticides can exert lethal and sublethal effects on streams organisms. Field studies have shown that non-polluted upstream patches promote population recovery from such effects. Nevertheless, the dynamics and potential carryover effects on the upstream patch are largely unknown. We used a metapopulation model with 2 patches to simulate lethal and sublethal effects on the downstream population of an insect with one generation per year, which was structured into early and late instars aquatic life stages, and an adult terrestrial life stage. We examined the implications for the recovery time of a range of scenarios covering different pesticide effect combinations, migration and exposure types. We found that recovery time responded most strongly to a reduction in reproduction in terms of the reduction of the intrinsic growth rate during the early instar aquatic life stage. For 60 of 96 scenarios with pesticide exposure in consecutive years, no recovery occurred within one year if the intrinsic growth rate was reduced by 50% or more. Without migration between patches (32 scenarios), the polluted downstream population went extinct in 5 of these scenarios. Migration lead overall to slightly faster recovery, albeit this was scenario dependent, but also to a carryover of the pesticide effect from the polluted downstream to the non-polluted upstream patch (up to 25% reduction in the minimal population size). A sensitivity analysis revealed that recovery time was most sensitive to the parameters length of the intrinsic growth phase during early instar aquatic life stage and to migration mortality of the late instar aquatic larvae, and least sensitive to the adult emigration rate and timing of pesticide application. Our study highlights the important role of sublethal effects for population responses to pesticides and that migration buffers against effects, but also carries effects over to non-polluted patches.