Part 2, Section 4: Soybean Pest Management
Soybean Pest Management
Insects
Soybeans growing in the northeastern United States are relatively free of insect pests, so the need for control is rare. Some fields in southeastern Pennsylvania, however, may require insect control from time to time.
One reason for the lack of economically important insect pests of soybeans is that soybeans have a great capacity to compensate for damage inflicted by hail and by foliage-feeding insects. Soybean plants can tolerate up to 35 percent defoliation until bloom, about 15 percent while pods are small and soft, and about 35 percent when pods start hardening. Below these levels, defoliation does not adversely affect crop yield, so control is not suggested.
Currently, ten insects occasionally are abundant in numbers high enough to cause economic losses in soybean production. Seven are found on the foliage: soybean aphid, green cloverworm, Japanese beetle, potato leafhopper, Mexican bean beetle, bean leaf beetle, and grasshopper. Usually a field reaches economic thresholds when more than one of these foilage pests are present at the same time. A relatively new pest, the soybean aphid, has spread throughout the Midwest into eastern states and is now a threat to growers in Pennsylvania. The seed corn maggot is a seed-feeding insect that can reduce stands especially when a living, green cover crop is incorporated into the soil prior to planting and when conditions are cool and moist for long periods after the seed is planted. Under extended hot, dry periods, the two-spotted spider mite can cause severe damage. Slugs, which are mollusks and not insects, can cause serious stress to plants before V2. Figure 2.4-1 shows the key periods of soybean insect pest occurrence in Pennsylvania. For more information on pests, see the fact sheet Insect Pests of Soybeans in Pennsylvania, available from the Penn State Department of Entomology at 814-865-1895.
See Table 2.4-17 for a quick reference to insecticides recommended for soybean insect control, and Table 2.4-18 for specific rates, restrictions, and other information.
Green cloverworm. The green cloverworm can be found at subeconomic levels in most soybean fields. Occasionally, however, conditions produce a population explosion of this pest, which in turn results in heavy defoliation of soybean plants. It does not pay to control green cloverworm populations until there are eight or more worms per linear foot of row.
Damage to soybeans is caused by the larval stage. The larvae are pale green with two narrow white strips along each side of the body. They are bare, slender, about 1.25 inches long when fully grown, and easily distinguishable from other insect larvae by the number of prolegs on the abdomen (short, fleshy legs along the middle of the body). Cutworms and armyworms have four pairs, loopers have two pairs, and green cloverworm larvae have three pairs of prolegs. Typically, green cloverworm populations are held in check by fungal and viral diseases.
Adult moths overwinter in buildings, under trash, and in other protected sites. There are two generations per year in Pennsylvania. The first generation develops on alfalfa, clover, and other legumes during May and June. The larvae of the second generation are active from late June to early August and occasionally are serious pests of soybeans.
Japanese beetle. Japanese beetle damage is similar to green cloverworm damage. However, this insect seldom is a major problem in soybean production. Large numbers of Japanese beetles frequently gather in soybean fields during late July and August. The brown, skeletonized leaves resulting from their feeding are fairly obvious. Actual yield reductions caused by Japanese beetle feeding are very low; therefore, control usually is not recommended.
Mexican bean beetle. Both the adult and larval Mexican bean beetle feed on soybean foliage, resulting in skeletonized leaves. Although the Mexican bean beetle is not currently a major pest of soybeans in Pennsylvania, leaf damage has been seen in the southeastern sections of the state. As soybean acreage increases and production intensifies, the Mexican bean beetle may become an important pest of soybeans.
Adult beetles are terrapin shaped, slightly larger than 0.25 inch long, yellow to coppery, and have 16 small black spots. They deposit clusters of 40 to 50 yellow eggs on the under-sides of soybean leaves. Larvae are yellow and covered with numerous branched spines.
The insect overwinters as an adult beetle under trash and in protected sites. There probably are two generations per year. Very little damage is done to soybean fields before August or until the second generation is active.
Seed corn maggot. On rare occasions, the seed corn maggot reduces stands in soybean fields. See the corn insect section for more information on this pest.
Two-spotted spider mite. Spider mites, Tetranychus urticae, can cause economic losses in soybeans under extended hot, dry conditions, as have been observed in recent years. In most years, however, this pest is of little concern in Pennsylvania. However, with the hotter temperatures and drier conditions we have seen over the past decade, we are experiencing more infestations from this pest.
Populations build up under hot, dry conditions because generation time is reduced substantially and fungal diseases cannot infect the pest. At 66°F, a generation is completed in about 20 days. In contrast, at 90°F, the pest completes a generation in about 7 days, and each adult female produces several hundred offspring. At this rate, if one female contributed 50 new females per generation, she alone would be responsible for 6.25 million female mites feeding in the field by the end of one month of 90°F temperatures. From this example, it is easy to see why spider mite outbreaks can occur rapidly during extended hot, dry periods.
Spider mites are minute and are closely related to spiders and ticks. It takes a small hand lens to see them. They are greenish white to green, and in some cases reddish. This mite can be identified from other mites by two dark spots on its abdomen. Mites typically live in colonies and produce a thin web on lower leaf surfaces.
Mites go through four stages of development: egg, larva, nymph, and adult. Eggs are small and oval (microscopic) and generally are laid on the underside of leaves. The larvae have six legs, while the nymphal and adult stages have eight legs. Both the larval and nymphal stages have little impact on yield. Adult females are responsible for most injury to soybeans.
Spider mites have piercing-sucking mouth parts and feed by sucking sap from the plant. Mite injury somewhat resembles herbicide injury or some foliar diseases. Typical damage symptoms are small yellow speckles on the leaves. As injury becomes more severe, the leaves become yellow and develop brown lesions, and the plants die. Rain is the surest way to stop the spread of mite damage. Several miticides can be used, however, if dry, hot conditions are expected to persist (Table 2.4-17).
Potato leafhopper. The potato leafhopper is a pest primarily of alfalfa in Pennsylvania, but can occasionally infest soybeans. The insect migrates into the state around mid-May and begins feeding on plants in the legume and rose family. When infestation levels are high in alfalfa, leafhoppers may move into soybeans. Also, if a soybean field is next to a recently cut alfalfa field, leafhoppers may move to the soybeans and cause injury. If plants are small, economic damage is possible.
Slugs. Slugs are becoming a more common problem in Pennsylvania soybean fields, particularly in the central and western counties. Late-planting fields are more likely to be damaged because of the small plant size and the timing of the slug hatch in the spring. When plants are in the succulent cotyledon stage, slugs can consume the cotyledons and growing point of the plant. When this happens, the field will experience significant stand reduction.
Soybean aphid. This new invasive pest of soybeans was first detected in Pennsylvania during the summer of 2001. In Asia, where it is a native species, the pest has been responsible for significant yield reductions. It is also known to transmit several virus species that can have additional impacts on yield. Studies have shown large reductions in soybean yield when the pest built to high densities. Currently, the threshold for the soybean aphid is an average of 250 aphids per plant with an increasing population. its alternate host is buckthorn, a common shrub species in Pennsylvania. Because of the damage potential from this pest, growers in Pennsylvania should keep a close watch for it in the coming years.