Quantifying rates of random mating in western corn rootworm emerging from Cry3Bb1‐expressing and refuge maize in field cages

Abstract

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is the most significant pest of field maize, Zea mays L. (Poaceae), in the USA. Maize plants expressing Bt toxins targeting the corn rootworm complex have been widely adopted and are the primary insecticidal control measure for this pest in North America. Insect resistance management tactics using various refuge structures have been adopted to ensure Bt products will retain durability. An assumption of the refuge strategy is that males and females emerging from Bt and refuge plantings mate randomly; this has not been tested in the field. We conducted cage studies using field populations of WCR in Indiana, USA, to generate empirical field data on mating rates between beetles emerging from Cry3Bb1‐expressing Bt and refuge maize plants. Two refuge configurations were tested; all refuge plants were labeled using the stable isotope 15N. This mark persists in adult beetles after eclosion, allowing for collection and analysis of isotopic ratios of all beetles. Additional data collected included adult emergence rates, timing and sex ratios for each of the treatments, and head capsule size and dry weights of beetles collected. Treatment had a significant effect on dry weight; mean dry weight decreased in Bt‐only treatments. Fisher's exact test of proportions of mating pairs of refuge and Bt insects indicated that mating was not random in 20% strip refuges and 5% seed blend treatments. We found high percentages of beetles that fed on Bt‐expressing plants as larvae, suggesting that mating between resistant beetles may not be rare even if random mating did occur.

Measuring rootworm refuge function: Diabrotica virgifera virgifera emergence and mating in seed blend and strip refuges for Bacillus thuringiensis (Bt) maize

“…In our 2014 study using natural populations and field cages, we found that non‐random mating occurred in seed blend and strip refuges . Our current results support these findings.…”

“…amount over the average for known unenriched samples) was used to identify beetles that fed as larvae primarily on 15 N‐treated refuge plants. A series of calculations was used to determine the amount of 15 N in excess for each sample . The Purdue Stable Isotope Laboratory recommends that an excess % 15 N > 0.5 be used as a threshold for identifying enriched samples.…”

“…A series of calculations was used to determine the amount of 15 N in excess for each sample. 17 The Purdue Stable Isotope Laboratory recommends that an excess % 15 N > 0.5 be used as a threshold for identifying enriched samples. As larvae in our study had the potential to move between unenriched (toxic, Bt) and enriched (non-toxic, refuge) plants, 23 a conservative threshold of 1.5% (3 × 0.5%) was used.…”

“…We used adult beetle head capsule size and dry weight to determine how physical size predicts mate choice in the field and if WCR fed from Bt maize differed in these parameters. Our previous field test of WCR refuge structures determined that non‐random mating occurred when natural populations of WCR were confined to field cages . The work described here explores refuge function further using unconfined natural populations of WCR, where the identity of mating insects (in terms of Bt or refuge natal host) was determined and refuge function, as measured by the key parameter of mating efficiency, was assessed.…”

“…Our previous field test of WCR refuge structures determined that non-random mating occurred when natural populations of WCR were confined www.soci.org S Taylor, C Krupke to field cages. 17 The work described here explores refuge function further using unconfined natural populations of WCR, where the identity of mating insects (in terms of Bt or refuge natal host) was determined and refuge function, as measured by the key parameter of mating efficiency, was assessed. Based on our past observations and other published work, we hypothesized that non-random mating would occur in this system.…”