Carbon Isotope Ratios Document That the Elytra of Western Corn Rootworm (Coleoptera: Chrysomelidae) Reflects Adult Versus Larval Feeding and Later Instar Larvae Prefer Bt Corn to Alternate Hosts

Abstract

In much of the Corn Belt and parts of Europe, the western corn rootworm, Diabrotica virgifera virgifera LeConte, is the most important insect pest of maize. The need for additional basic knowledge of this pest has been highlighted while developing resistance management plans for insecticidal genetically modified crops. This study evaluated the possibility of tracking feeding habits of western corn rootworm larvae using stable carbon isotope signatures. Plants accumulate different ratios of (13)C:(12)C isotopes, usually expressed as δ(13)C, according to whether they use the C3 or C4 photosynthetic pathway. Herbivore biomass is expected to reflect the δ(13)C of the food they eat. For the current experiment, western corn rootworm larvae were grown on different species of plants exhibiting different δ(13)C values. The δ(13)C values were then measured in elytra of emerged beetles. When beetles were unfed, biomass reflected larval feeding. When beetles were fed for 31 d postemergence, δ(13)C values of elytra almost exclusively reflected adult feeding. These results suggest the use of caution in the interpretation of δ(13)C data aiming to document larval diet history when adult feeding history is unknown. The technique was also used to evaluate western corn rootworm larval choice between alternate hosts and maize with and without genetically modified (Bt) traits aimed at their control. Propensity for feeding on alternate hosts versus maize was biased toward feeding on maize regardless whether the maize had Bt or not, suggesting western corn rootworm larvae were not repelled by Bt. These data will be helpful for regulators in interpreting western corn rootworm feeding data on Bt maize.

Effect of adult feeding on the δ15N signatures of different tissues for Popillia japonica

“…Overall, our results suggest that the d 15 N signatures of both soft tissues and elytra of adult P. japonica feeding on 15 N enriched grape leaves change over time, and this change is variable between tissues, with soft tissues showing a higher turnover rate than elytra over time. The d 15 N signatures of both soft tissues and elytra from recently emerged adults that were not fed were not significantly different from each other or from the d 15 N signatures of larvae collected in the same location, which is consistent with previous research showing the isotopic signatures of newly emerged adult beetles can be reliable indicators of larval feeding (Ponsard et al, 2004;Hiltpold et al, 2014;Tsurikov et al, 2015). The elytra of P. japonica had lower d 15 N signatures compared to soft tissues when adults fed on enriched leaves, suggesting that the integument is often isotopically different from other more metabolically active tissues (Tiunov, 2007;Maraun et al, 2011).…”

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

“…A pipette (Eppendorf Research Plus, Hamburg, Germany) was used to deliver the labeled fertilizer directly into each of the holes at the rate of 1 ml of solution per hole. Natural abundances of stable isotopes have provided a field‐tested method to track how generations of a polyphagous pest utilize different host plants (Gould et al., ; Hiltpold et al., ). The 15 N stable isotope of nitrogen provides a reliable, inexpensive, and efficient method of marking WCR (Murphy & Krupke, ).…”

Host search behaviors of specialist and generalist root feeding herbivores (Diabrotica spp.) on host and non-host plants

“…Although something other than corn was producing WCR beetles in the 1800s, it is noteworthy that corn is currently the only known host plant of WCR in nature 43 . Our previous studies demonstrated that WCR beetles can be produced experimentally on a number of non-corn hosts 13 , 47 – 49 , though no natural production of WCR has been documented without corn 43 .…”

“…Corn roots contain not only feeding stimulants 35 , 36 but also likely provide the best-fit nutrients for corn rootworm development. When roots of both species are present, later instar WCR larvae choose to eat corn roots even those expressing insecticidal toxins (Bt corn) rather than eating one of the better alternate hosts 49 . The differences in the search patterns of the specialist root-feeding larvae, that share intimate coevolution adaptation with these host plants 50 , may reveal that neonates of these species acknowledge their best-fit nutrients.…”