A new artificial diet for western corn rootworm larvae is compatible with and detects resistance to all current Bt toxins

Abstract

Insect resistance to transgenic crops is a growing concern for farmers, regulatory agencies, the seed industry, and researchers. Since 2009, instances of field-evolved Bt resistance or cross resistance have been documented for each of the four Bt proteins available for western corn rootworm (WCR), a major insect pest. To characterize resistance, WCR populations causing unexpected damage to Bt maize are evaluated in plant and/or diet toxicity assays. Currently, it is not possible to make direct comparisons of data from different Bt proteins due to differing proprietary artificial diets. Our group has developed a new, publicly available diet (WCRMO-1) with improved nutrition for WCR larvae. For the current manuscript, we tested the compatibility of all Bt proteins currently marketed for WCR on the WCRMO-1 diet and specific proprietary diets corresponding to each toxin using a susceptible colony of WCR. We also tested WCR colonies selected for resistance to each protein to assess the ability of the diet toxicity assay to detect Bt resistance. The WCRMO-1 diet is compatible with each of the proteins and can differentiate resistant colonies from susceptible colonies for each protein. Our diet allows researchers to monitor resistance without the confounding nutritional differences present between diets.

Characterization of DvSSJ1 transcripts targeting the smooth septate junction (SSJ) of western corn rootworm (Diabrotica virgifera virgifera)

“…A proprietary mixture of additional ingredients was added to SHD to stimulate feeding and/or allow for the biological maturation of WCR. In internal testing, this diet typically generates less than 30% mortality in the negative control, similar to other WCR laboratory diets 17 . On Day 0 of each respective bioassay, the dry diet was mixed with RNase free water (negative control) or a liquid test substance of interest to achieve the desired concentration.…”

“…WCR neonates were provided with seven different lengths of DvSSJ1 dsRNA incorporated into a standard artificial diet as described in Method. Acceptable control mortality of WCR associated with this diet was set at 30% given the variability of WCR performance in laboratory bioassays 17 . WCR mortality was significantly greater than the control for each test substance at 60 base pairs (bp) of length and greater (Table 2).…”

Multidimensional approach to formulating a specialized diet for northern corn rootworm larvae

“…Although the ultimate goal of insect artificial diet research might be to serve as a comparable substitute for natural foods 25 , only a few diet formulations can be considered to have achieved that goal over the past 60 years 35 . However, artificial diets are also desired to enable the study of insect species of interest 41–43 , to reduce labor or space 41 , to reduce time for host plant rearing 44 , and to provide a standardized diet for insect bioassays 21,23 . This is the first report of a diet formulation developed specifically for NCR larvae.…”

“…diet had nearly zero contamination after 11 days when clean laboratory practices were applied 21 , but contamination was also eliminated from 5 other WCR diets using the same techniques 22 . This diet was also compatible with all current Bt proteins targeting WCR larvae 23 .…”

Characterization of Corn Root Factors to Improve Artificial Diet for Western Corn Rootworm (Coleoptera: Chrysomelidae) Larvae

“…Diet assays for western corn rootworm larvae are typically limited to 10 d (Pleau et al 2002, Siegfried et al 2005, Ludwick et al 2018). Here we extended the bioassay period to 15 d, which allowed the detection of differences in multiple larval development stages.…”