I've been traveling around the state with our corn entomologist, Aaron Gassmann, the last two weeks. We've been collecting corn rootworm adults and assessing root injury in commercial corn fields. We are visiting fields that seem to have extreme rootworm pressure, even though the corn is expressing Bt.
Severe corn rootworm feeding can lead to root pruning which makes plant unstable
and interferes with nutrient uptake. Photo by Erin Hodgson.
Aaron has been looking at problem fields for three years and evaluating the susceptibility of corn rootworm to different Bt proteins. He is finding a few fields with increased beetle survivorship compared to the general population. Does this mean corn rootworm are becoming resistant to Bt? Well, according to his recently published article - yes. He is finding fields that have more larvae surviving, or tolerating, the Bt proteins. Although his research showed incomplete resistance in just a few fields in Iowa. This is not a widespread problem at this time and has not been reported from other states.
There are a couple of common scenarios to these problem fields. Most are planted with continuous corn and use the same Bt protein for 2-3 years. The vast majority of beetles are Western corn rootworm. Aaron strongly encourages people to use crop rotation with soybean to knock down the beetle population. He also suggests mixing up the Bt proteins used to delay genetic resistance of the beetles. Also consider using an in-furrow insecticide application for those fields with persistent corn rootworm damage. Of course incorporating the recommended refuge corn will help delay resistance as well.
Western corn rootworm beetle. Photo by Natasha Wright.
During our travels, we saw a few fields with extremely heavy adult emergence and feeding. I would estimate the density of two fields we sampled to be around 10-14 adults per plant. The adults had destroyed the silks and were infesting the ears. Leaf feeding was also very apparent. Some of the fields were sprayed with a foliar insecticide to kill the adults. Corn rootworm had delayed development this year, and adults are emerging over a gradual period. The residual from the insecticide does not appear to kill the adults that emerge after application. The University of Illinois does a statewide sample of corn rootworm adults, but found low numbers this year even though root damage by larval was increased form 2010.
Growers should be evaluating corn roots for damage, even if lodging isn't obvious. You should be able to see adults feeding and mating at this time as well. Knowing more about the current population will help you make seed selection decisions for next year.