Nutrition affects insect susceptibility to Bt toxins

Carrie A Deans,Spencer T Behmer,Marianne Pusztai-Carey,Ashley E Tessnow,Gregory A Sword,Patricia Tamez-Guerra

doi:10.1038/srep39705

Abstract

Pesticide resistance represents a major challenge to global food production. The spread of resistance alleles is the primary explanation for observations of reduced pesticide efficacy over time, but the potential for gene-by-environment interactions (plasticity) to mediate susceptibility has largely been overlooked. Here we show that nutrition is an environmental factor that affects susceptibility to Bt toxins. Protein and carbohydrates are two key macronutrients for insect herbivores, and the polyphagous pest Helicoverpa zea self-selects and performs best on diets that are protein-biased relative to carbohydrates. Despite this, most Bt bioassays employ carbohydrate-biased rearing diets. This study explored the effect of diet protein-carbohydrate content on H. zea susceptibility to Cry1Ac, a common Bt endotoxin. We detected a 100-fold increase in LC50 for larvae on optimal versus carbohydrate-biased diets, and significant diet-mediated variation in survival and performance when challenged with Cry1Ac. Our results suggest that Bt resistance bioassays that use ecologically- and physiologically-mismatched diets over-estimate susceptibility and under-estimate resistance.

Highlights

Variable[36,37,38,39,40,41] and that plant nutrient content can interact with plant defensive compounds to affect insect herbivore performance[33,42]
It was lowest on the commercial diet (CD; Southland Products, Lake Village, AR), which had a P:C ratio that was very carbohydrate-biased relative to the self-selected P:C observed for H. zea[46]; the total macronutrient content (P +C) of the CD was high (62%) relative to what is typical for most plant vegetative tissues[47]
The LC50 values for the Modified Commercial Diet #1 (MCD1) were between 15.7–106.5 times greater than the LC50 values of the CD values, while the LC50 values for the Modified Commercial Diet #2 (MCD2) were between 20.2–408.8 times greater compared to the CD

Summary

Introduction

Variable[36,37,38,39,40,41] and that plant nutrient content can interact with plant defensive compounds to affect insect herbivore performance[33,42]. Our focus for the current manuscript is on plant nutrients Once this baseline has been established, it is possible to study the effects of plant secondary compounds, and interactions between plant nutrients and plant secondary compounds, as environmental factors impacting susceptibility to Bt toxins. We conducted two experiments using H. zea as a model to test whether variation in diet protein and carbohydrates impacts susceptibility to Cry1Ac endotoxins, one of the major plant-incorporated insecticides widely expressed in Bt cotton and corn. We hypothesized that diet protein-carbohydrate content would have strong effects on survival and performance across sub-lethal and lethal concentrations of Cry1Ac, and that the diet most closely matching the self-selected P:C ratio of 1.6:1 would confer the greatest survival and performance for larva when challenged with Cry proteins

Methods

Results

Conclusion