Understanding warming impact on agricultural system is important because it is related to food security. The response of crop pests to warming is a critical mechanism for the change in crop production, yet some knowledge gaps remain. For example, it is less clear if warming will affect not only the population size but also the population composition of pests (e.g. alate vs. apterous), consequently changing pest colonization. Moreover, warming impact on a tri-trophic system (crop, pest, predator) is less explored. To help fill up the knowledge gaps, we studied soybeans, soybean aphids and ladybugs (an importance crop, pest and biocontrol agent, respectively). We first investigated experimental warming effect (2-4 oC; predicted future warming) on aphid population size and composition (alate proportion). The result showed that warming suppressed alate production (from about 5% to 1 %) without affecting the overall aphid population size. Based on the result, a colonization experiment was then conducted in the field with a 2 (alate proportion under ambient vs. warming scenario) × 2 (ladybug presence vs. absence) factorial design to investigate how warming-induced change in aphid population composition (reduced alate proportion) may consequently affect aphid colonization on soybean plants with/without ladybugs (biocontrol agent).
Results/Conclusions
We found that alate proportion treatment interacted with ladybug treatment. When the initial alate proportion was higher (5%, ambient-temperature scenario), aphids colonized soybean plants faster under the presence than absence of ladybugs. However, when the initial alate proportion was lower (1%, warming scenario), aphids colonized soybean plants slower under the presence than absence of ladybugs. The results were cross-checked by an aphid population growth model, which could help identify potential mechanisms in the colonization experiment. Our findings suggest that warming around the optimal temperature of soybean aphids may not increase their initial population size but limit their colonization potential by changing population composition (alate proportion), consequently making aphids more susceptible to predation. In other words, biocontrol by predators may become more effective in limiting pest outbreaks and protecting crops under future climate warming.