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Soybean pests to watch for in 2009

Representing a team of university and Extension entomologists from Arkansas, Louisiana, Mississippi and Tennessee, Roger Leonard spoke at the recent 2009 Tri-State Soybean Forum.

“We get together on an annual basis and try to coordinate recommendations so producers can call any of us and get the same answers,” said the LSU AgCenter entomologist stationed at the Macon Ridge Research Station in Winnsboro. “That helps quite a bit because folks are beginning to farm across (state borders) and, with cell phones, you never know who will call you.”

There are a number of common soybean IPM issues among the states:

• Early-season feeding by the three-cornered alfalfa hopper on soybean seedlings has resulted in considerable lodging of plants that is not detected until the late season, especially in soybeans double-cropped behind wheat.

• Mississippi populations of the bean leaf beetle are demonstrating resistance to pyrethroid insecticides, the primary chemical control strategy.

• Outbreak levels of corn earworm created significant problems in Arkansas during 2006 and in 2008. “Some folks missed those populations and likely suffered yield losses.”

• Stink bugs are the primary yield-limiting insect pest problem in Louisiana. “We have a new species called the red-banded stink bug.” (discussed in accompanying story)

• Defoliation problems with soybean loopers continue to plague many producers in late-planted soybeans.

• The persistence of “green bean syndrome” and plants that don’t mature normally. (see accompanying story)

Seed treatments

Soybean insecticide seed treatments have been in use for the last few years. The primary insecticides being used at this time are formulations of Gaucho and Cruiser.

A regional project “has shown anywhere from a 3-bushel to 8-bushel seed yield increase when these treatments are used on soybeans planted in early production systems. The results are much less consistent for later-planted soybeans.”

Three-cornered alfalfa hoppers

In double-cropped soybeans behind wheat, injury from feeding by the three-cornered alfalfa hopper can cause “girdles” on the plant stem. As long as the plants don’t lodge later in the season, “you might not see much of a yield loss. However, we are seeing numerous instances of lodging around R5/R6 growth stages.”

Locating the early-season feeding is very difficult for consultants. “Typically, by the time it’s found, the insect is long gone and there’s nothing that can be done. The feeding usually occurs before the beans are about 10 inches tall.”

Fortunately, seed treatments will provide high levels of mortality for this insect. “When the insects feed on these systemic products at two weeks after planting, nearly 100 percent mortality of the hoppers has been observed. And, depending on whose (research) we look at, residual (protection) can occur up to 21 to 28 days after planting.”

To try and find out how serious this early-season feeding is — and how it relates to yield losses — a regional and state-wide project simulating early-season injury to the main stem and late-season lodging is ongoing. “What we’re finding is unless you see greater than 40 percent of the plants lodge, you’re unlikely to see a significant yield loss. That’s even though you have a high percentage of the plants that have been fed upon and the girdles are obvious.”

Bean leaf beetle

For several years, the bean leaf beetle has been a persistent pest throughout the Mid-South growing season. Recently, Mississippi began to observe problems in controlling the pest. Entomologists “began to make collections and run a series of assays to see if the issue was related to pyrethroid resistance.”

One problem with this pest is its similar appearance to less-threatening beetles. “A number of different beetles can be found in soybeans. The bean leaf beetle is sometimes gray without spots, gray with spots, or a bit red with spots. It should be distinguished from the 12-spotted cucumber beetle and the banded cucumber beetle — which aren’t as damaging as the bean leaf beetle.”

In Mississippi, the bean leaf beetle problem is particularly acute in the Delta region where it is becoming more difficult to manage. “Fortunately, there are other insecticides available for control. They may cost a little more but they work.”

Corn earworm

This has been a sporadic pest but can be very damaging if it isn’t detected with timely scouting during the flowering and early pod development stages.

“It isn’t very common in Louisiana. In fact, in 20 years, I’ve seen only a handful of fields I’d have treated for it.”

However, according to Gus Lorenz, Arkansas Extension entomologist, there have been significant yield losses to the pest in his state. In severe cases, all pods have been removed from plants by uncontrolled corn earworm.

“We’re also seeing a shift in susceptibility to pyrethroids in corn ear worms. If you’ve used a pyrethroid in cotton on bollworms and it didn’t work, it likely will not work on earworms in an adjacent soybean field. Be careful and pay attention to what’s going on in other crops.”

Since 1997, the frequency of pyrethroid resistance in this pest has been going up.

“Fortunately, most pyrethroids are still working in many areas. Even if we see failures, it’s very low levels of survival that exist.

“Field test results from Virginia — in a similar situation but with much higher populations of earworm than we have — shows a number of products they tried worked well. So, we have plenty of control options as alternatives to pyrethroids. However, you have to find the insect or you’ll be in trouble.”

Soybean looper

The soybean looper is one of three late-season defoliating pests — including the green cloverworm, which is very susceptible to most products, and the velvetbean caterpillar, which is often dealt with through preventative treatments — in the Mid-South.

“The soybean looper is probably the toughest of the three to manage because of the way it migrates and the fact that it’s also resistant to a few insecticides.”

In the past, control of the pest has come from Steward, Larvin and Intrepid. “But in 2008, I visited a number of fields that experienced lower than expected control with Intrepid, especially at lower rates. This particular insecticide had a wide rate range that varies among the three states. In Mississippi, during many years producers can get by with 2 to 4 ounces of Intrepid; Arkansas, with 4 to 6 ounces; and Louisiana recommends nothing less than 6 ounces.”

In 2008, “Louisiana producers had to re-treat fields that had already been treated with 6 ounces of Intrepid. That gives me cause for concern because Intrepid is a long residual product. I’ve never had to re-treat with that insecticide before.”

The good news is that Coragen and Belt are two new products that will offer options for control of soybean looper in the future.

e-mail: [email protected]

TAGS: Soybeans
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