Soybean Insect Control
Part 2, Section 4: Soybean Pest Management
Soybean Pest Management
Historically, soybeans growing in the northeastern United States have been relatively free of insect pests, so the need for control has been rare; however, some recently arrived exotic species can build to damaging numbers, necessitating insecticide usage in some parts of Pennsylvania, particularly in southeastern portions of the Commonwealth.
One reason for the historical 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. However, insect-pest challenges faced by Pennsylvania soybean growers now include soybean aphid and stink bugs (particularly brown marmorated stink bug), which are not defoliators. These insect species are phloem feeders that use needle-like mouthparts to extract sap from plants, so plant tolerance based on tissue removal simply does not apply. Moreover, populations of these sucking insects can grow quickly, making their management even more challenging.
Currently, ten or so insect species occasionally are abundant in numbers high enough to cause economic losses in soybean production. Most are found on the foliage: bean leaf beetle, soybean aphid, green cloverworm, Japanese beetle, potato leafhopper, Mexican bean beetle, bean leaf beetle, and grasshopper. Stink bugs, particularly brown marmorated stink bug, will feed on foliage as well as pods. A field usually reaches economic thresholds when more than one of these foliage pests are present at the same time. Soybean aphid has spread throughout the Midwest into eastern states and has been a threat to growers in Pennsylvania, typically in odd-numbered years (e.g., 2005, 2007, 2009), but this pest species did not materialize in 2011 or 2012 and we are unsure of what to expect for 2013 and 2014. 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.
Bean leaf beetle. Bean leaf beetle is a native insect species that can be problematic on an introduced legume crop—that is, soybeans. Populations of bean leaf beetle tend to be most troublesome in the eastern half of Pennsylvania and appear more common in areas with abundant woods, where the beetles spend the winter. In spring, bean leaf beetle colonizes the earliest emerging soybean fields, where they can appear to cause significant damage to cotyledons and young leaves of seedlings. However, this damage, despite appearances, is rarely economically significant and plants typically grow through the injury. Moreover, insecticidal seed treatments that are quite common on soybean varieties tend to provide adequate protection. Bean leaf beetle has two generations per year and the mid- to late summer generation feeds on foliage and growing pods. If substantial populations feed on pods, economic damage can occur. Bean leaf beetle is the primary vector for bean pod mottle virus, which can significantly sap soybean yield and is sporadic in Pennsylvania.
Brown marmorated stink bug. The brown marmorated stink bug is a relatively new pest species in Pennsylvania soybeans, but its large populations in some parts of the state in 2010 and 2012 caught many folks by surprise and caused significant yield losses. This pest species has been most common in Pennsylvania in the southern tier of counties, but the populations have been relatively spotty, seemingly associated with fields in proximity to woodlots. In soybean fields, populations accumulate on field edges, often adjacent to woods or corn fields. These edge-accumulating populations are often evident after the pest is gone via a stay-green syndrome where the edges of the field that were infested senesces much more slowly (stays green) than the rest of the field. This syndrome appears to be a physiological response of the plant to stink bug feeding. Stink bugs tend to accumulate in soybean fields when pods start to appear; they seem able to feed on new growth, but mostly concentrate on pods, feeding with their needlelike mouthparts through the pod on the developing seeds. Soybeans in the R4 growth stage are most susceptible to stink bugs, and beans become less susceptible to damage at the R7 stage and beyond. A range of insecticides is available against this pest species, but few offer significant residual control, so multiple applications may be necessary.
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 due to natural control provided by some beneficial fungi as well as insect predators. 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. 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). Note that populations of mites vary in their susceptibility to dimethoate, with some appearing to be resistant, so treated areas should be monitored carefully to ensure adequate control is achieved. If resistance is suspected, consider alternative materials (Table 2.4-18).
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 builds to high densities. Currently, the threshold for the soybean aphid is an average of 250 aphids per plant with an increasing population. The soybean aphid population in Pennsylvania is largely migratory with individual winged aphid being blown here from states to our north and west. Aphid populations have been economically damaging in odd-numbered years (e.g., 2005, 2007, 2009) when some fields harboring thousands of aphids per plant. Our most recent outbreak year was 2009 and many counties experienced large aphid populations; however, this outbreak was less uniform than previous outbreaks and some counties never developed damaging populations. In 2011, when we expected large populations of aphids again, they did not materialize. Populations were again low in 2012, so we are not sure what to expect in 2013 or 2014. Natural enemies, particularly ladybird beetle adults and larvae, play an important role in moderating soybean aphid populations and ill-advised applications of insecticides can disrupt natural control. Soybean aphid has a secondary host plant, buckthorn, where it spends the winter. Buckthorn is uncommon in the southern parts of the state, but increases northward until New York state where it is quite common. It seems that soybean aphid populations are worse in areas where buckthorn is most common. Beginning in June, growers should regularly scout their fields for aphids. Timing of the insecticides (at 250 aphids per plant) is the most important detail of gaining control.