Gene-by-environment interactions in agricultural pest management: population effects on diet-Bt interactions in a caterpillar

Given that plant nutrient content is both spatially and temporally dynamic (Lenhart et al., 2015; Deans et al., 2016, 2018), insect herbivores are exposed to an incredible amount of nutritional variability. This variability can constrain insects to feeding on sub-optimal resources, but it can also provide an opportunity for insects to regulate their intake of specific nutrients to obtain an optimal balance. Nutrient regulation has implications for pest control strategies in agricultural systems, as the nutritional state of pest species may impact their susceptibility to insecticides. Deans et al. (2017) showed that diet macronutrient balance has significant effects on the susceptibility of Helicoverpa zea larvae to Cry1Ac, an endotoxin expressed in transgenic Bt crops. This was demonstrated using a highly inbred laboratory strain of H. zea, limiting the applicability of these results to field populations that encompass greater genetic diversity. In this study, we assessed the impact of field-relevant macronutrient variability on the efficacy of two Bt endotoxins, Cry1Ab and Cry1Ac, using three field populations collected from different geographic regions. This was done to further understand the impact of nutritional variability on Bt susceptibility and also to determine the relevance of these effects in the field. While we saw limited differences in Cry susceptibility across populations, dietary effects were highly variable. Across populations there were distinct population-level differences in the interactions between Cry concentration and diet, the type of Cry toxin impacted by diet, and the treatment diet that produced optimal survival and performance. These results show that nutrition can have strong impacts on Bt susceptibility but also that these impacts are strongly affected by genetic background in H. zea. To accurately assess Bt susceptibility in the field, including resistance monitoring, bioassay methods should incorporate the appropriate nutritional parameters and be as localized as possible.