Western Corn Rootworm Beetle Numbers Unimpressive, Likely Good News for Next Year's Corn
There is no question that western corn rootworm beetles are low this year, which correlates with the lack of corn lodging reports received… namely zero.
The last three springs during egg hatch, frequent rains and saturated soils have dramatically increased larval mortality. However, there may be something else at work here.
Ever-increasing adoption of highly effective Bt hybrids for rootworm control is likely contributing to this decline in population. We know that these hybrids typically cause 90%+ mortality in RW larvae. Contrast that with the years of granular insecticides, which effectively protected the roots and crop, but did not do much to lower populations. The European corn borer (ECB) as a key corn pest may offer some guidance here as well. The Bt toxins for this pest are higher in dose, typically leading to mortality in over 99% of insects that contact it. However, ECB have several dozen other potential hosts, ranging from cattails to many common weeds such as ragweed. Yet, throughout most of the country, ECB is now an afterthought in discussions of key corn pests.
In contrast, corn rootworms are essentially limited to corn as a host – meaning that in areas of high-use Bt-RW corn, essentially every host plant is toxic to them. Could we be seeing the early signs of population-level effects of extensive Bt corn adoption? We don’t know the answer, but it is worth thinking about. It is difficult, if not impossible, to tease apart environmental and agronomic factors in a pest with such a huge geographic range. But it’s reasonable to expect that continued adoption and effectiveness of Bt corn will cause a reduction of WCR populations on an area-wide scale. This makes it even more critical that we continue to be good stewards of this highly-effective technology – plant a refuge!
Consider that fewer beetles will lay fewer eggs for next year. Now is the time when beetle populations can be assessed in pollinating corn and soybean for next year’s rootworm risk where corn will be planted. Pollen from a multitude of weeds (e.g., foxtails, volunteer corn, ragweeds, lambsquarters, pigweeds, etc) will draw them in to feed, potentially leading to unexpected lodging. Investigations in these areas during the next few weeks will help make informed control decisions for next year.
