Insect control key to profit in soybeans

Soybean acreage is expected to be up in 2008 across the upper Southeast and for growers to match the optimism of high prices with the reality of profit they must pay attention to at least four insect species that can take profitability out of a bean crop.

North Carolina State University Entomologist John Van Duyn says four of the many insects that cause problems in soybeans in the upper Southeast are:

  • Soybean stem borer.
  • Lesser cornstalk borer.
  • Stinkbugs.
  • Corn earworms.

Most are more of a threat because of their sporadic occurrence in soybeans.

Soybean stem borer

The soybean stem borer was first reported in North Carolina in 1974, according to Van Duyn. “We had a lot of research on this particular pest early on, and it just seemed to go away. These insects didn't go away, they spread westward to the Midwest and throughout our region,” he recalls.

The North Carolina State entomologist says the biology of the soybean stem borer is not well documented today — whether it has changed since the 1970s is not clear. What we do know about the biology is that it seems to be ideally suited to no-till farming.

The adult is about 15 mm long with a dark gray, elongated body. The antennae are longer than the body and lie parallel to the body when at rest. Eggs from soybean stem borer are yellow, elongated, and about 1.5 mm long.

The larval stage consists of four larval instars varying in length form 1.5 to 15 mm long. The pupa is about 15 mm long and maintains the yellowish color of the egg state when it first forms, but turns dark brown late in the life cycle.

Though there are a number of weed hosts, including cocklebur and ragweed, soybean stem borer typically over-winter in the upper South as mature larvae within soybean stubble. Common weeds, if left in a soybean field, may also provide the insect with a source for tunneling.

Some damage occurs when the immature larvae feed within the plant. However, Van Duyn says the biggest threat to soybeans comes when the immature borer cuts the plant stem from the inside. In many cases this may appear to be weather or nutritional-based lodging.

In past years, the pest has been the biggest problem in fields that are planted to soybeans after soybeans, especially if the beans are not harvested soon after maturity. As the value of soybeans continues to grow, scouting for this pest will become more critical as growers strive to produce higher yields.

Tillage is the best way to manage soybean stem borers. Maximum benefit will come from deep tillage, but just disking and disrupting these pests will produce good results, according to Van Duyn.

Growers who plant beans in wide rows and at higher plant populations will likely be affected most by this pest. Rotation, variety selection and tillage is the best management plan in fields with a history of this pest. Narrow tillage and timely harvests are also good tools, Van Duyn adds.

Lesser cornstalk borer (LCB) is a dry weather pest and the drought year of 2007 produced a banner crop of these pests. “We saw large acreages damaged and in many cases re-planted,” Van Duyn says. He adds that the worst cases seemed to come in fields in which wheat stubble had been burned.

“LCB appears to be fire-tolerant, and if these critters survive fire in stubble, then there is an abundant supply of tender seedlings coming up. It has a wide range of hosts, including most of the common weeds and crops we see in the upper Southeast,” Van Duyn says.

Lesser cornstalk borer have alternate bands of purple and green encircling its body. Larvae are typically three-quarters of an inch or so long and can frequently be found in tunnels created by the insect in soybeans, corn, wheat and a number of other crops.

Larvae remain in the soil in a silken tube just below the soil surface and\injure plants by boring into and tunneling up the stem of the plant. Violent wiggling when disturbed is a non-scientific, but highly effective way to identify LCB adults.

LCB damage is frequently confused with wireworm damage. However, wireworms do not produce the silken webbing found at or about the soil line of plants damaged by LCB.

Lesser cornstalk borer over-winters as a larva or pupa. The moth emerges early in the spring and lay its eggs on the leaves or stems of plants. The small larva begins to feed on roots and leaves, later the larva constructs underground silken tubes at the soil surface from which they feed on plants.

It becomes fully grown in 2 to 3 weeks after egg hatch, and pupates in silken cocoons under trash on the surface of the ground. The moth emerges from these cocoons in 2 to 3 weeks. Subsequent generations are not economically important.

In the upper Southeast, stinkbugs over-winter in edges of vegetation and litter. They emerge fairly early. “Last year we started catching nice populations early, then the Easter freeze really knocked populations down, and we never saw high populations in most areas of the state again,” Van Duyn notes.

Stinkbugs feed on fruiting plants. Green stinkbugs tend to feed on broadleaf plants and brown stinkbugs feed on broadleaf and grasses. In soybeans they can cause aborted pods, aborted seed within the pods, shriveled seeds and basically lower quality and yield.

Primary stinkbug damage to soybeans in the upper Southeast comes from green and brown stinkbugs. There is some data that indicates more damage comes from green stinkbugs, though this may in part be due to increased populations of green versus brown.

All adult stinkbugs are shield-shaped. Green and southern green stink bugs are bright green and measure 14-19 mm long. The major body regions of the green stink bug are bordered by a narrow, orange-yellow line. Brown stink bugs are dull brownish-yellow in color and 12-15 mm long.

Green stinkbug eggs are barrel-shaped and yellow to green when first laid, and later turn pink to gray. Eggs of brown stinkbugs are kettle-shaped and slightly smaller than eggs of the green stink bugs.

The critical time for stinkbug damage in soybeans is in reproductive stages R2 through R6. Once beans get past the R6 stage there is little damage from the adult stage. “We may see more brown stink bugs in 2008, because of the increase in wheat acreage planted in the fall of 2007. Stinkbugs build up in wheat. When the wheat is cut in the spring, brown stink bugs typically move to corn. Soybeans can provide an ideal host as the corn plays out. Most of the adults that come out of wheat are brown stinkbugs,” he explains.

Bidrin, Warrior, Capture and Mustang Max in some combination have provided excellent control, according to Van Duyn.

Soybeans have to be at the perfect stage to be vulnerable to corn earworms — that's the good news. The bad news is that infestations that hit soybean fields just before, during or just after bloom can cause rapid and devastating damage.

In soybeans, corn earworm caterpillars start as eggs, they survive in bloom clusters, and when the get large they eat soybeans. Medium to large larvae are the primary concern in soybeans, according to Van Duyn.

Soybeans are highly compensatory plants and can survive fairly high worm counts with minimal damage — if conditions are right for regrowth.

Corn earworm, is the most common and destructive insect pest of soybeans grown in Virginia, according to Virginia Tech Entomologist and IPM Leader Ames Herbert.

Corn earworm caterpillars are light green to brown, with small dark spots and dark brown heads. As they get older, they can stay green or brown or turn yellow — even pinkish, depending on the host crop.

Earworms typically grow to about 1.5 inches in length. Typically these insects curl into a ‘C’ shape when disturbed. They have dark stripes that run the entire length of their body and are distinguished from other soybean-damaging caterpillars by four pairs of prolegs. These insects are typically covered with dark hairs.

Early planting and use of Group IV and early Group V varieties both increases the chances that soybeans will be beyond the susceptible flowering stage when the moths fly and enables natural enemies to increase their populations before earworms arrive.

Full-season soybean fields are therefore much less likely to develop earworm outbreaks.

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