Over-informed on IPM - Episode 023: Corn Earworm
Weekly Scouting Reports: https://extension.unh.edu/resource/sweet-corn-ipm-weekly-scouting-reports
My dad used to have a garden in the back yard and he used to grow sweet corn. He always tells this story, I guess some might call it a joke? He got the water boiling in the kitchen, all ready to cook some fresh sweet corn. Then he went out and shucked the corn right on the stalk, then really quick he picked all the ears, and ran to the kitchen…only to trip and drop all the ears on the ground. Meaning that they were ruined because they weren’t fresh anymore.
Is that a funny joke? Maybe some of you dads out there are enjoying that one.
Well, at some point dad stopped growing sweet corn. He considered it the one crop he couldn’t grow without using pesticide, which he considered an insurmountable hassle. There’s always a farm stand down the road so why not leave that to the professionals. What was the pest that dad was treating for? Corn earworm.
Corn earworm, aka Helicoverpa zea and some of you old school corn earworm fans might know this guy as Heliothis zea, is a noctuid moth native to North America first described in 1850. Corn earworm actually has a pretty broad host range and has earned itself several names, including tomato fruitworm, sorghum headworm, cotton bollworm, because – guess why – people keep finding these larvae everywhere. In the fruit of tomato, in the boll (kind of like the flower) of cotton. You’ll find these larvae in other stuff too… artichoke, asparagus, cabbage, cantaloupe, collard, cowpea, cucumber, eggplant, lettuce, lima bean, melon, okra, pea, pepper, potato, pumpkin, snap bean, spinach, squash, sweet potato, and watermelon.
But how do they get there? Well their mothers leave them on these host plants as eggs. These moths really like to leave their children on corn, just as those ears are developing silks. Silks are essentially pollen tubes and they indicate to moth mothers – probably by their smell – that this is a great time to leave her children on those ears. Those eggs hatch and the larvae burrow into the tip of the corn. The corn kernels are filling out and those larvae at eating away, developing into larger larvae. By the time you harvest the ears and open them up, you could find a surprise inside. Other species of caterpillars attacking corn can be found in different parts of the ear but corn earworm pretty much sticks to the top couple of inches. You might have noticed that sometimes you can find sweet corn at the grocery store that’s kind of like half pre-shucked, with a kind of husk jacket around half of the ears and the tip is cut off. This is a pretty clever workaround for potential corn earworm infestation.
We’re kind of lucky up here because corn earworm does not overwinter well north of, say, Virginia. But it will migrate long distances and is often blown up on big storms. Some of the moths we get here in New Hampshire probably migrated from as far as Texas.
How do we know when this happens? We have very powerful tools for monitoring using lures of synthetic sex pheromone! By monitoring, you can avoid unnecessary applications of pesticide when these moths aren’t here and you can avoid crop injury by making sure to protect your crop when the moths are present. You can try this at home, but many of our growers in New Hampshire have a little help from Cooperative Extension
George Hamilton Intro
Anna: George caught me up on the sweet corn IPM program history, including the state of affairs when he started here and the impacts of this program
George: So growers were saying, geez, what do we do about that we don’t have that data anymore. There was just one farm – Wilson’s Farm – that was doing traps themselves. There were a couple other farms where the traps were lost or run over by a tractor. I got a grant through EPA New England Vegetable & Berry Growers, just to set up a few traps so I could learn when the trap falls were. We had a major storm come through and I couldn’t believe, literally overnight, we went from two or three to two hundred and three. The growers expressed how important that information was to them.
Looking at my growers – I call them my growers – their yields are going to be higher than average. The price they receive is usually higher than average, so I feel very comfortable with the impacts. Normally we’re saving 1 or 2 sprays per year and we’re usually seeing a 10-12% improvement to yield because we’re decreasing the cull rate because of insect damage. So that means there’s roughly $120,00-200,000 worth of product from our roughly 500 acres in production.
Anna: So George has dramatically expanded the sweet corn monitoring network since he started here in New Hampshire. You can find those data by town on our website. If you are growing corn nearby some of these sites, this could be very helpful in determining if your corn is a risk. However, there can be quite a bit of variability from site to site. These moths are often literally falling out the sky after all. So on-farm monitoring will provide you with much better data. I caught up with my old PhD advisor at Virginia Tech, Tom Kuhar, to get up to speed on the state of affairs in sweet corn IPM and some stuff that I’d been scratching my head over:
Tom Kuhar Intro
Based on the experience we’ve had in Virginia over the past three years – we’ve been implementing a sweet corn IPM program using pheromone trapping. One of the concerns we had was the reliability of trap catch, that growers can feel really confident that that trap catch can drive their spray schedule. I’ll point out that there have been times where we’ve put out two trap on the same field and one caught none while the other caught a bunch, which implies a different decision to be made. Obviously the answer there is to get more traps out. Take the average. But do you go with the conservative approach, the one that caught more moths? There’s a lot of uncertainty there so most growers are on a calendar spray schedule, which is unfortunate. We don’t see a lot of corn earworm here in Virginia until the middle of July. It’s possible to get corn going early, get transplants started in the greenhouse, put them on plastic. There’s all sorts of things, depending on how the grower wants to market their corn. Rest assured, if you can get corn up before the 4th of July, you’ve got two things: a heck of a market and not quite the corn earworm pressure.
Another thing that concerns me is bees coming to pollinating sweet corn. Corn is a grass – its not insect pollinated – but the bees love that pollen when its coming out in large numbers. That has concerned me. It hasn’t been a big factor in sweet corn pest management but it scares me when the field is buzzing and you’re coming in there with a spray. I’ve suggested starting with a product less toxic to bees. A diamide, like Coragen, is one that kind of fits that role. It also preserves natural enemies. Things like Orius, the minute pirate bug, that thing will run around eating corn earworm eggs. It’s eating pollen. There’s papers out there showing that it can play a big role if you don’t kill it. That’s all part of the IPM program, using softer chemistries, holding off on spraying, getting corn in early, there’s advantages to that.
Then you get into late season, its just a tough road. I don’t think there are enough good IPM-friendly rotational insecticides out there. I mean, you can try these things, but ultimately it ends up with more damage to your corn. That’s been my experience, as well as other researchers.
Anna: What kind of softer chemistries are you talking about?
Tom: Well, like spinosyns, there’s products like nuclear polyhedrosis viruses, things that are specifically targeted to corn earworm. They’ve got a terrific non-target profile. They’re not going to hurt anything other than the corn earworm. But when you’re trying to fit these product into rotation – and I’m talking late season when pressure is high – its really a tough battle to get equal efficacy with these products. That’s the challenge right now. Brian Nault at Cornell, David Owens at Delaware, and myself, we’re going to be trying some of these programs. Some for organic growers as well, using products they can use. Spinosyns, like Entrust, are probably the best. Pyrethrins used to be ok but we have pyrethroid-resistance problems in our corn earworm, to the point that pyrethrins in products like Pyganic aren’t working very well at all. Pyrethroids are not working very well right now. Sweet corn pest management is a challenge. Researchers are working on it.
Anna: Do you have any thoughts on how to order these products in your rotation, when you’re bringing in new chemistries but keeping old ones too?
Tom: The one thing I keep going with is to bring in the softer chemistries early. I know some are thinking that you’ve got to have you best guns going in blazing early. But those are the ones that are the most disruptive to natural enemies. I just don’t think that’s a wise approach. If you can use Coragen, I like it early. I think there’s a little extra residual. I like that it’s not killing things I don’t want it to kill. I’m not saying run out and use this – I’m not selling anything – but it happens to be more IPM-friendly than other tools.
Anna: All right, so I’ve you’ve been relying on a steady rotation of organophosphates and pyrethroids for your sweet corn spray schedule, it’s time to invest in some new tools. Chemical resistance is something that flies its way up from its overwintering habitat, so there’s not much we can do other than mix things up a bit. If you live in southern NH, there’s a pretty robust monitoring network you can access online. We’d be happy to train you how to monitor these moths for yourself. To be honest, I’d be very happy to supply you with the materials you need if you’re in NH and want to do your own monitoring. Think about ways to move up that planting date. And when corn earworm does comes to your farm, start out with something softer, more selective. Save those bees but, more importantly, save those adorable Orius bugs – those minute pirate bugs - out hunting in the corn!
That’s it for this week. Thanks to my newest mentor, George Hamilton. Thanks to my academic dad, Tom Kuhar of Virginia Tech and, while I’m at it, thanks to my actual dad who volunteers his time in my lab (what a guy!) and for all those backyard veggies growing up, too. As always a special thanks to Jason Lightbown, who wrote and performed our theme music.
"corn earworm" by Whitney Cranshaw Colorado State University, bugwood.org
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