The problem with peanut insects is that if you’re not paying attention, you might not see them, which isn’t necessarily a bad thing. Or, you might not see them in time to a stop a yield-threatening outbreak. Or, you might apply an insecticide when it really isn’t needed. It’s tricky.
“There are a lot of insects in a typical peanut field,” says Mark Abney, University of Georgia Extension entomologist. “But most of those insects are not economically important – most of them are just hanging out because a peanut field is a great place to live, and that’s good because we don’t have to worry about those insects.”
Some of the insects are beneficial, and they’re eating the bad insects, he explained during the recent Georgia Peanut Farm Show held in Tifton, Ga. “Some of them will be pests, and some of them we just don’t know either way. We see them, but we really don’t know if they’re causing us economic damage,” he says.
There’s a long list of insects that feed on peanuts, says Abney. “In any given field in any given year, we’re not going to see most of those. In some years, we’ll have outbreaks and see lots of them. Insect pest are also sporadic – we might see them in one field but not in another one nearby. It’s good for growers that we don’t have pests in every field every year, but it’s bad because it makes it more difficult to plan.”
There aren’t valid economic thresholds for most insect pests in peanuts, he adds. “For most crop pests in most major commodities, we have economic thresholds. Also, for most of our peanut insects, there are only one or two insecticide options. And on top of that, the products don’t work very well in many cases,” he said.
Producers list top seven insect pests
One of the first things Abney did when he began working in Georgia in June was to ask the top-yielding peanut growers in the state to rank the most economically important pest on their farms.
These top growers listed corn earworm and tobacco budworm at No. 1. “The No. 2 most troublesome pest was three-cornered alfalfa hopper, something for which we don’t have economic thresholds. And the No. 3 top insect pest was the burrower bug. The growers who had it listed it as No. 1, but a lot of growers don’t have it, so it averaged out to be No. 3 in our survey.”
Lesser cornstalk borer came in at No. 4 and thrips was No. 5, says Abney. Corn rootworm was No. 6 and spider mites and wireworms were tied for No. 7.
“I also asked these same growers how many acres on their farms are treated for corn earworms or caterpillar pests annually. More than 70 percent of the acres are treated for caterpillar pests each year. I believe there are a lot fewer acres that actually are at threshold to be treated for these pests, but growers are spending money to control them, and that’s why they’re No. 1 on their list. And I think it’s the same thing with three-cornered alfalfa hopper. We don’t know if they’re causing us yield loss, but we’re spraying for them anyway,” he said.
Growers have a very good threshold for treating corn earworms and tobacco budworms. It has been validated – four to eight larvae per row foot – but it depends on the condition of the plant. Broad spectrum materials are available for use on this pest, but growers are cautioned not to use pyrethroids if they have tobacco budworms.
“It wasn’t that many years ago that you could go into a peanut field, and if you found a caterpillar that looked like a corn earworm, you could rest assured that it was a corn earworm. Now, you could find something that looks like a corn earworm but it might be a tobacco budworm. And if you spray a tobacco budworm with a pyrethroid, it won’t kill it. Make sure you accurately identify the pest,” he said.
The problem with broad spectrum materials is that they can result in secondary pest outbreaks, so be careful, advises Abney.
“The corn earworm is a real pest that can defoliate peanuts, but follow the thresholds. Peanuts can compensate for a lot of foliage feeding. If you’re treating at sub-threshold levels, it isn’t saving you money or making you money. I’d much rather see us scouting our peanuts more thoroughly and not just throwing insecticides in the tank,” he said.
Producers should watch for granular cutworms and soybean loopers. “I think we’re spraying a lot of soybean loopers in Georgia that we don’t need to spray. Others include beet armyworms, fall armyworms and velvetbean caterpillars. You’ll rarely see only one species of caterpillar in the peanut field. When you decide which insecticide you want to use, you have to think about what will kill most of them. If you spray a pyrethroid, you won’t kill soybean loopers,” he said.
There is no economic threshold for three-cornered alfalfa hopper, says Abney. If you find a threshold, it has not been validated scientifically.
“We tend to spray this pest when we see them flying around in a field. That’s not a scientific threshold, and we don’t know what kind of yield loss we might be having,” he said. “We do know there are more three-cornered alfalfa hoppers now than there were 10 years ago, and we don’t know why. Research has shown that they have preferences for different varieties, but we no longer grow any of those varieties.”
The immature pests may be more damaging than the adults, he says. “And that leads us to think that if we treat those low populations of adults moving into the fields in mid-summer, we can prevent having immatures in the field. We don’t know if that’s accurate.”
The big questions growers have is do I need to treat and when, says Abney. Based on past research, if a grower is within 30 days of digging peanuts, he doesn’t need to treat for the three-cornered alfalfa hopper. “We don’t have any soft chemistries for this insect. If we decide to spray, we’re setting ourselves up for secondary pests. So don’t spray if you don’t have to.”
The stink bug that moves in the ground
If you grow peanuts, then you’ve heard a lot about burrower bugs in the past five to six years, says Abney. You can think of it as a stink bug that moves in the ground.
“It’s not exactly like our stink bugs that feed on cotton, but it’s pretty close. It has a piercing, sucking mouthpart, and it will suck the juice out of a peanut kernel. But it doesn’t take very much of that kind of damage to cost us a lot of money. When you get up to 2 ½ percent damaged kernels, you go from Seg. 1 to Seg. 2. In other words, you go from making money to not making money with three kernels out of 100, and that’s why this insect is so scary. It also increases the risk of aflatoxin,” he said.
Burrower bugs prefer dry conditions, so growers didn’t see many in 2013. “If you had irrigation, burrower bugs did not visit you, though I have talked with people who turned their land and had damage from this pest, so you can’t rule it out completely. This is a very sporadic insect. It’ll be in one field and not in another, and one county can have more than another. But it’s Public Enemy No. 1 for growers who have it," he said.
There currently are no monitoring procedures for burrower bugs and no thresholds. “Cultural controls are the best thing we have for now, including deep tillage and irrigation if you’ve got it. As for chemical controls, we’ve got Lorsban 15G, but the data shows it isn’t that effective on burrower bugs, and it won’t kill them all. We need more chemistry and we need to understand the insect pest better,” he said.
If you don’t scout, then you don’t know what’s in the field, he says. “If you have caterpillars but you’re unsure about the species, you could make a wrong decision when it’s time to make an insecticide application. We shouldn’t be spraying just because we can. If it’s drier in 2014, you might see insect pests that we didn’t see this past year.”