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Close watch on soybean pests could reduce insecticide use

Close watch on soybean pests could reduce insecticide use

Careful monitoring of insects in soybean fields and assessing how much threat they pose to crop yield could result in fewer insecticide applications and greater cost savings, says Angus Catchot, associate Extension professor of entomology and plant pathology at Mississippi State University.

Careful monitoring of insects in soybean fields and assessing how much threat they pose to crop yield could result in fewer insecticide applications and greater cost savings, says Angus Catchot, associate Extension professor of entomology and plant pathology at Mississippi State University.

And, he notes, preliminary studies in Mississippi indicate that defoliation in early growth stages may not be as damaging to soybean yields as once thought, which could result in revision of thresholds for treatments.

“It’s easy to overestimate defoliation — it always looks worse than it actually is,” he said at the annual conference of the Mississippi Agricultural Consultants Association. “The thing that always jumps out is that, if you look at a plant with 35 percent defoliation pre-bloom, it looks like a lot more — 50 percent or more.

“We have a guide to help calibrate defoliation levels, and you can look at photos and train yourself on what to look for before you go in the fields. But, because defoliation can have a big impact on yield at critical growth stages, it’s important that you be able to estimate the defoliation level.”

A 2010 study by graduate student Lucas Owen looked at soybean insect management, with an emphasis on the multi-pest defoliation complex that includes green cloverworms, velvetbean caterpillars, grasshoppers, and others.

“Last year was perhaps the worst fall armyworm situation we’ve ever had,” Catchot says. “There were pastures we sprayed four or five times, starting in June. We saw complete defoliation of soybeans in some areas.

“In the study, looking at early season defoliation of Group IV and Group V beans, we defoliated at V3 and V6 across both maturity groups at levels of zero, 33 percent, 66 percent, and 100 percent.

“Interestingly, at every defoliation level with the Group IVs, we actually saw a numeric yield increase, but no statistical differences at either growth stage at any defoliation level. These results were somewhat eye-opening.”

There were also no differences in yield for the Group IV beans at either growth stage regardless of defoliation.

Defoliation may not warrant spraying

“I was shocked. The first thing that popped into my mind when I saw the results was, if there’s no yield loss at 100 percent early season defoliation, we potentially wasted applications on 150,000 acres of soybeans that we treated for gray loopers last year early in the season.

“Admittedly, had they been my soybeans, I probably would’ve been spraying, too — but if you’ve got just a 5 acre spot that’s defoliated in the middle of a 100 acre field, it’s probably not a catastrophic event and you might hold off on spraying the field. We’re going to follow up on this with more study.”

The soybean insect situation in 2010 was quite variable, Catchot says.“When soybeans were just coming out of the ground last year — anywhere from cotyledon to seedling stage — we started getting calls from across the state about soybean loopers.“Cabbage loopers, I said. There’s no way we’d be seeing soybean loopers in June. Then I started getting calls telling me ‘They’ve got black front legs.’ We started making collections and found we had gray loopers. They look a lot like soybean loopers, but are a different species.

“We’ll have an outbreak of gray loopers maybe every 20 years or so. In high numbers, they can cause extensive defoliation, and numbers were high enough that we treated about 150,000 acres.

Once we figured out what they were, we could make treatment recommendations. Unlike soybean loopers, which require some rather costly applications, we could control gray loopers with very low rates of pyrethroids.”

Controlling soybean bean leaf beetles

Bean leaf beetles are still an ongoing challenge, Catchot says.

“We’ve been working with this pest a long time; in the Delta, since 2005, we’ve had a problem getting control with pyrethroids. When bifenthrin came on the market, it did a good job of control at the high rates when the other pyrethroids alone were not providing adequate control. Last year, I think we started seeing the edge we had with straight bifenthrin start disappearing — probably because we’ve applied it to such tremendous acreages that we’ve selected to the point it no longer has a clear edge.

“To control bean leaf beetles in the Delta region of the state, I think we’re probably going to have to go to a pyrethroid mixture with acephate. We’re seeing better control of bean leaf beetles with a half-pound of acephate mixed with pyrethroids, or with straight acephate alone at the high rates. Also, there are older materials that still work; Sevin XLR, for example, is very effective, or Larvin.

“In the hills pyrethroids still work well — it’s in the Delta where we’re having problems. For Delta growers, I wouldn’t even consider a straight pyrethroid, but would go with a mix right out of the gate, or a premixed product with multiple modes of action.

“If you have a bad infestation, try to go back two or three days later and assess how the application is working. A lot of times, if you’re only checking fields weekly, it may look like you got zero control, when in fact at two or three days you may have had 80 percent control. But after a week, they’re moving back in, there isn’t enough residual to control the new populations moving in, and it may look like a complete failure, particularly in late July and August.”

Other soybean pest problems

Another soybean pest, green cloverworms, is present in most soybean fields every year, Catchot notes. “They show up early, and they’re around the entire season. A lot of people still confuse them with loopers. Green cloverworms have three sets of prolegs, loopers have two. Also, when you disturb them, cloverworms will flip and jump around like crazy, and loopers won’t.

“The confusion occurs when you catch small green cloverworms in a sweep net — they will ‘loop’ like a soybean looper and people confuse this instead of counting prolegs.”

Outbreaks of velvetbean caterpillars occur every 10 years or so, particularly in the hill areas, he says. “We had them in a few spots last year, and they can cause extensive defoliation, but they’re easy to control. You just need to be able to identify them and treat as necessary.”

Soybean loopers can cause massive defoliation when numbers are high, Catchot says. “But, if you know you’ve got soybean loopers, please don’t treat them with pyrethroids or Orthene, even at the highest rates. It’s not worth the risk of flaring them. Instead, use materials that work, such as Steward, Belt, or Intrepid.

“The worst application you’ll ever make is one that doesn’t work.

From R-3 until R-6 is a very critical time for defoliation. If you get 100 percent defoliation at R-3, it can be catastrophic. At R-5, the yield loss is not as steep, but will still result in major yield loss. At R-6, you can still get yield loss, but your chief concern at this point is to keep pest numbers from building to massive levels. Essentially, at early R6 you are guarding against the 80 percent to 100 percent defoliation events.”

Mississippi soybean growers sprayed 650,000 acres last year for bollworms, some as many as four times, Catchot says. “We had higher numbers than we’ve seen in many years, and there were no peaks and valleys — they were sustained for 4-6 weeks, with no really distinct peaks.

“We’re pushing our soybean plantings earlier every year. But now that we’re planting more grain and trying to spread out harvest, we’re planting more Group Vs and more wheat beans, which exposes us more to stink bugs, loopers, and bollworms.

Instances of tolerance to pyrethroids

“Entomologists across the Mid-South are working on methodology for testing on bollworms in soybeans to try and get some meaningful data for growers. We generally get pretty good control of bollworms with pyrethroids, but we’re having some issues of tolerance.

“Due to issues with control in 2010, we are currently recommending mixing an Orthene with pyrethroids to improve control of bollworms. Many tests across the Mid-South have shown increased efficacy when tank mixing these products, rather than relying on a pyrethroid alone.

“Other products are available such as Belt and Steward, but when we have to start switching away from pyrethroids it’s going to get expensive. Another thing to consider is that tobacco budworms were present in soybeans last year in places in the Delta.  Tobacco budworms will infest soybeans, but it’s not something we see on a regular basis. This is important because they are highly resistant to pyrethroid chemistry.”

Red-banded stink bugs were almost nonexistent last year, Catchot says, with only scattered reports of findings across the state.

“I think it was a function of the weather. I’m not expecting that we’ll see many this year — but in case, don’t confuse them with the red-shouldered stink bug. If it has a spine on the abdomen between the hind legs, it’s red-banded; if it doesn’t have a spine, but it looks similar, it’s red-shouldered. It’s important to know which you’re dealing with.”

And he cautions, when applying a fungicide, “If you don’t have bugs in the field at R-3, there is no need to include a pyrethroid with your fungicide application.  When insects are not present at threshold numbers, we don’t see any yield response from just ‘adding in’ and insecticide.

“If you have a situation like this,” he told the consultants, “and you advise your grower to eliminate the pyrethroid, you’ve just paid for your services for the entire year. If there are bugs that need to be treated, by all means include a pyrethroid — but if they aren’t there, leave it out and save the money.”

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