URBANA, Ill. – Illinois corn growers in the northern and central parts of the state have come to expect some rootworm damage, but University of Illinois entomologists say putting management plans in place now could help growers avoid major losses.
“Over the last few years, western corn rootworm populations with resistance to toxins present in common Bt corn hybrids have been documented in Illinois,” says Joseph Spencer, insect behaviorist at the Illinois Natural History Survey (INHS) at U of I.
“We’re specifically seeing resistance to Cry3Bb1 and mCry3A toxins, but we know that resistance to these toxins also confers resistance to the structurally similar eCry3.1Ab toxin,” he says. “Cross-resistance among these ‘Cry3’ Bt toxins should be expected for Illinois western corn rootworm populations.”
Resistance to pest-control practices in western corn rootworm is nothing new; the insect is notorious for developing resistance to control tactics such as insecticides and crop rotation. Part of the concern with these recent developments is that there are relatively few Bt toxins available to combat corn rootworm.
“All available hybrids with pyramided traits for corn rootworm use either Cry3Bb1 or mCry3A in combination with a second toxin, either Cry34/35Ab1 or eCry3.1Ab,” Spencer says. “This means where resistance is present in the population, there might be at best only one effective toxin at work.”
There are steps producers can take to manage corn rootworm and possibly slow further development of resistance. Nick Seiter, entomologist in the Department of Crop Sciences at U of I, says the best way to delay resistance to any control tactic is to reduce exposure of the target insect to that tactic in the environment. This can be accomplished using the following strategies.
Apply rootworm control, whether in the form of a Bt hybrid or a soil insecticide, only where it is economically justified. This determination should be based on sampling rootworm adults the previous year. According to surveys conducted by Kelly Estes, agricultural pest survey coordinator for Illinois Extension and INHS, densities of rootworm adults have been relatively low in recent years, although they did trend slightly upward in 2017.
“If you monitor using a yellow sticky trap, the economic threshold is two rootworm beetles per trap per day in corn following corn,” Spencer says. “For rotated corn, the economic threshold is 1.5 western corn rootworm beetles per trap per day in soybean.”
Rotating corn with soybean or another non-host crop remains an effective management strategy in the southern portion of the state. While crop rotation is no longer a reliable method to protect first-year corn from western corn rootworm damage in central and northern Illinois, Seiter notes, all larvae that hatch into soybean still die, and every acre planted to soybean is an acre where larvae are not being exposed to Bt toxins or soil insecticides.
Where monitoring indicates that control is justified in corn, rotate the control measures used from year to year. This means rotating Bt hybrids with different trait combinations and non-Bt hybrids treated with a soil insecticide.
“Follow all refuge requirements for any Bt corn hybrids you plant. In many cases, the ‘refuge in a bag’ or ‘RIB’ approach is now used, but check with your seed distributor on specific requirements for your hybrids,” Seiter says.
Finally, an important step is to monitor the performance of control methods. While lodging is often the cue growers look out for to identify rootworm damage, it’s important to remember that corn can take a lot of damage without lodging, and plenty of factors other than rootworm damage can lead plants to lodge.
“The best approach to evaluating rootworm damage is to dig a representative sample of roots in late July and evaluate them for feeding damage: unpleasant work, but necessary if we want to understand the true extent of the damage,” Seiter says.
Consider planting a small area or a portion of a row with a non-Bt/untreated hybrid as a check strip. Having an untreated patch in the field will allow growers to compare the efficacy of the management tactic vs. the background level of damage where no rootworm protection was used.
Finally, if you experience greater damage than expected in Bt corn hybrids in 2018, let Seiter know by emailing firstname.lastname@example.org. “Your reports will help us document the status of resistance in Illinois and provide updated information to producers,” he says.
For more information, read the full report on The Bulletin.