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Corn+Soybean Digest

Worm Warning

Whatever you call them — corn earworms or cotton bollworms — they are a mounting menace for southern soybean growers who expect to see higher bean acres in 2009.

Blame it on the increase in corn acres fueled by higher grain prices, a trend that's expected to continue into the next growing season.

Gus Lorenz, University of Arkansas (UA) Extension entomologist, says the burden of bollworms in beans is extremely high in Arkansas. “We've seen a big jump in activity with bollworms,” says Lorenz, pointing out that the worms migrate out of corn and attack the blooms and pods of planted soybeans.

Scott Akin, another UA Extension entomologist, says the damage from bollworms can be more destructive than stink bugs, because they can feed on pods.

“Stink bugs use their piercing mouthparts to suck the juice from seed within the pod, which — make no mistake — can cause yield loss and quality issues,” he says. “But bollworms feed on blooms from R1 all the way up through developing pod stage.

“They can remove large portions of the seed with their chewing mouthparts. As a result, high numbers of this pest can easily decimate a field's yield potential in a very short period of time,” Akin says.

THE ECONOMIC THRESHOLD for this pest in Arkansas soybeans is 15 bugs/25 sweeps, or four per foot of row on standard-row soybeans. It is similar in other states.

“Make sure the sweep net goes deep into the canopy,” says Akin, adding that fields should be scouted closely at least once a week starting at bloom. Also, growers or consultants should examine the plants for visual signs of damage.

“For a drop cloth scouting, the treatment threshold is four per foot of row, or 20 on 5 ft. of row. We sometimes consider a drop cloth a more consistent means of scouting for caterpillar pests since the larvae can be deep in the canopy and hard to dislodge with a sweep net.”

Lorenz says the thresholds “are solidly based, and you should use them to base your decision on whether or not treatment is needed.

“Also, remember that four per row foot on 38-in. rows is equivalent to two per row foot on 19-in. rows and one per row foot on 9.5-in. rows,” he says.

Akin says that while small larvae feed on new, tender leaves and blooms, larger larvae can be found on any part of the plant. They will feed on leaves or blooms, but often prefer pods.

“Small larvae are typically tan in color, but larger larvae can vary from brown, yellow, green to even pink, each having longitudinal light-colored lines along the body,” he says.

“When disturbed, these larvae usually curl up into a C shape, unlike armyworms and most other caterpillar pests found in soybeans. Corn earworm larvae, particularly small ones, are subject to high mortality from natural enemies. For that reason, pesticide treatment recommendations are generally based on numbers of medium and large larvae,” Akin says.

CONTROL OF THIS pest is usually accomplished with any number of labeled pyrethroid insecticides on the market. The entomologists say the rates of insecticide needed for control are typically lower for soybeans than cotton because the larvae are much more exposed to the insecticide.

“Don't just blanket treat,” says Lorenz. “Treat the fields at treatment levels and leave the other ones alone, or you'll have the potential to flare other pests.”

Cost of treatment for bollworms in soybeans can range from $3.50 to $8/acre, depending on the specific pyrethroid and rate used. Akin notes that generic pyrethroids may cost less than name brands, but application rates may vary based on formulation amounts; keep this in mind when choosing an insecticide.

“When soybeans were in the $5-6/bu. range, it was harder to get growers to look at them,” he says. “But with soybean prices much higher, there is more pressure to scout for them (worms) and get them under control.”

Akin also worries that the threecornered alfalfa hopper can sneak up on growers who get a nice stand of soybeans next spring.

“This insect can be serious,” he says. “Soybeans coming out of the ground are easy targets.”

The hoppers have a “piercing-sucking mouthpart, similar to that of stink bugs” he says. “Later-instar nymphs feed in a circular pattern around a young stem. This stem girdling is often not noticeable until much later in the season when a windstorm comes through or you run a sweep-net.

“The plant subsequently falls over and may not be harvestable. Needless to say, an insecticide application at that time would not be helpful since the hoppers that caused the injury are long gone,” Akins says.

The adult has a distinct triangular shape, is bright green in color and is about ¼ in. long. Nymphs have spines along the top of the body.

Early instar nymphs are extremely small and translucent. As the nymphs grow, they become green like the adult, although the later-instar nymphs can also be brown in color.

They overwinter as adults in a reproductive diapause state. They are often found in the winter beneath pine trees where they reside under plant debris. In the early spring they leave pine and move into alternate hosts such as clovers, vetch, dock, wild geraniums and other hosts for the first generation.

“As the first generation of nymphs becomes adults, they move into soybeans in May and June,” says Akin. “Usually there are several overlapping generations in soybeans. Typically, later-planted soybean stands can be more at risk due to when the hopper populations are at the highest.

“We believe the insecticide seed treatments may be an effective option for early season control. We are currently evaluating seed treatments via greenhouse studies,” he says.

Akin says he normally doesn't recommend insecticide seed treatments for later-planted soybeans, “but they may be well justified for insurance against threecornered alfalfa hopper populations.”

DAMAGE FROM THE hopper is often mistaken for soybean cyst nematode damage. Akin says that in severe cases, reports of growers losing three stands of soybean plants in one field in a season have been observed.

“Fortunately, we have several foliar insecticide options for control,” he says. “Many of the pyrethroids labeled for soybean are very effective at controlling threecornered alfalfa hopper.

“Because pyrethroids are a solid choice for this pest as well as bollworm and the green stink bug complex, a well-timed application for above-threshold populations can easily pay for itself,” Akin says.

He recommends treating when 50% of the plants are girdled or if fewer than four to six ungirdled plants per row-foot remain in conventional rows (30-38 in.) and hoppers are still present. “We are, however, currently working on refining the threshold for threecornered alfalfa hopper as production systems have changed considerably since the adoption of this threshold,” he says.

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