Relentless spring rains pushed western corn rootworm populations to new lows, according to a statewide pest survey.
“In general, 2020 looks like it’s probably going to be a pretty low pressure year in rotated corn for rootworm beetles,” said Joe Spencer, University of Illinois entomologist and Illinois Natural History Survey principal research scientist.
“It’s a good opportunity to think about how you can manage rootworm beetles, cut some of my costs and maybe reduce the pressure on resistance in rootworm beetles.”
Although overall numbers may be low, populations can vary from field to field and scouting is extremely important to determine infestations.
“There’s a lot of variability. You can have areas with nothing and areas that have rootworms that you don’t even think about. You can’t rely on somebody showing their summary rootworm map for the state. You have to do your own scouting. You have to look in your field because you might have a hot spot. You can’t assume you don’t have a rootworm problem,” Spencer said at U of I’s Agronomy Day.
Like weeds developing herbicide resistance, western corn rootworms are have developing resistance to toxins in Bt corn hybrids. Pest management strategies are important in order to maintain the current technology.
Spencer provided data from trials using hybrids with no rootworm protection, a single trait hybrid with the Cry3Bb1 toxin, a single with the Cry34/35Ab1 toxin and a hybrid with both toxins.
There were over 85 beetles per plant in the hybrids with no rootworm protection.
In plants with the single trait Cry3Bb1, 60 beetles per plant were collected.
“That was the equivalent of not really having any rootworm protection at all. That’s what we know from years of experience, that the Cry3 trait are suffering from a lot of resistance in rootworm beetles,” Spencer said.
With the Cry34/35Ab1 rootworm trait, there were significantly less beetles emerging from plants, indicating that the product expressing that trait provides a significant reduction in beetle populations.
“It’s a little bit strange here, but we looked at SmartStax, which combines the two Bt traits, and we had more beetles coming out than in the single trait Cry34/35Ab1. We would expect fewer, but this speaks to the variability, at a time when the rootworm eggs were already hatching. So, I think this is variability in my system,” Spencer added.
“We found that if you have no Bt trait or the Cry3Bb1 trait, you had very significant damage to the roots. Cry3Bb1 expressed in corn as a single trait hybrid is the same as doing nothing to protect your corn from rootworms. There’s resistance to this trait.
“There was significant reduction in root injury from rootworms with Cry34/35Ab1 and a combination of that trait and the Cry3Bb1. It tells us that the Cry34/35Ab1 trait is providing the value in Bt hybrids. That trait is what’s protecting our corn from all of these rootworm beetles.”
Spencer noted that all of the western corn rootworm populations in Champaign area have resistance at different levels.
“You’re just going to have variability in how many you see. However, if you want to protect your corn you need to make sure you have the Cry34/35Ab1 trait in your hybrids because it is the only Bt that’s providing protection against western corn rootworm in Champaign County plots,” he said.
Given that Cry34/35Ab1 is the lone trait providing protection, it’s important that the technology be protected to extend is viability.
“The low rootworm population in the area over recent years also gives us the opportunity to think that if I want to reduce the possibility that Bt resistance is going to creep up, if I don’t have an economic rootworm population and I can layoff using Bt, I can reduce the pressure that could select for resistance to get worse,” Spencer said.
“So, when you have a low population, if you can avoid using Bt and either if you’re planting continuous corn you rotate to soybeans or go with a non-Bt hybrid and soil insecticide, if that’s possible, that will reduce the rootworm pressure.
“We also have to scout. You can’t just assume you don’t have a rootworm population. Scouting is really important. If you think you’re doing integrated pest management and you’re not scouting, you’re not doing integrated pest management. Scouting is one of the pillars of integrated pest management.”