Foliage-feeding insects are present in practically all soybean fields during the growing season. Most of these pests have chewing mouthparts and cause a characteristic type of defoliation. Bean leaf beetles feed on the tender terminal leaves and eat small, rounded holes, usually between the veins. Mexican bean beetle adults and larvae crush and strip away the leaf tissue between the veins, giving the leaves a distinctive lacelike pattern. Japanese beetles concentrate on the upper foliage and eat larger holes with only the main veins intact, giving the leaves a skeletonized look. Young green cloverworms scrape the leaf tissues, leaving irregular, shiny windows on the leaf surface; older worms eat irregular holes between the veins, giving the leaves a tattered appearance. Grasshoppers, skipper larvae, and woollybear caterpillars eat large holes in the leaves; often the holes have ragged margins and/or only the main veins are left intact.
Regardless of the pest involved, the effects of defoliation by chewing insects on soybean yield are the same. Foliage feeders reduce the total leaf area available for photosynthesis; consequently, plants may lack the ability to produce enough food for normal growth and seed production. However, soybeans can tolerate considerable defoliation, depending on the stage of growth, plant vigor, and growing conditions. During the early seedling stage, damage to the cotyledons and leaves may result in some stand reduction, but the capacity of plants to recover from early season injury is remarkable. A field with a good stand can compensate for reductions of up to 30 percent of the plant density before yield is affected, if other growing conditions are adequate.
During the vegetative stages when the plants are growing and producing new leaves (prior to bloom), and again after seed enlargement is complete, soybeans can tolerate 30 to 40 percent defoliation without significant loss of yield. Soybeans seem to compensate by adding new leaves and by increasing food production in the remaining functional leaves. Vegetative growth usually slows down or actually stops at the time pods begin to fill; thus, plants are more sensitive during pod development and seed enlargement when more than 20 percent defoliation can cause economic yield loss.
Foliage-feeding insects sometimes can benefit the soybean crop. In full-season beans with rank growth and closed canopies, the outer leaves shade the lower parts of the plant that receive inadequate light. Insect feeding on the outer canopy often allows more light penetration and air circulation, thus promoting blossom set and overall increased photosynthesis.
Check for early-season pests as soon as the plants emerge. Scout for mid-season to late-season defoliating insects in early July and continue weekly until 50 percent of the leaves are yellow. More frequent field visits may be necessary if a pest population is increasing. Estimate the level of defoliation to the nearest 10% on 20 to 30 plants selected at random throughout the field. Walk through the entire field because pest populations often are not evenly distributed. Pull up each plant to examine the total leaf area, not just the upper canopy. Try to identify the predominant pest species associated with the feeding injury. Also note the average stage of plant growth.
If the overall defoliation level of a plant cannot be estimated easily, then sub-sample as follows: (a) select one leaflet at random from the upper one-third of the foliage; (b) select four leaflets from the lower two-thirds of the plant (two leaflets from each half of the branches); and (c) estimate the amount of defoliation on each leaflet to the nearest 10 percent and average (see Figure 4). If the field appears more than 5 to 10 percent defoliated, either examine plants directly or use a drop cloth or sweep net to determine the number and size of stages of each pest species at 5 to 10 locations in the field. Recording the relative numbers of each stage helps determine the timing of insecticide treatments. Also, note the presence of parasitized and/or diseased pest species because treatment may be unnecessary even though defoliation exceeds economic levels. Cloverworms are killed by a fungus disease that causes larvae to become hard, mummified, and covered with powdery-white to light-green spores. The presence of diseased worms usually signals the decline of the pest population.
Spray only when defoliating insects are actively feeding and the following defoliation thresholds are exceeded: at seedling - 40%; at prebloom - 30%; at bloom and podset - 15%; after full green bean stage - 35%. Populations of bean leaf beetle, Mexican bean beetle, and Japanese beetle occasionally reach damaging levels, particularly at seedling stages and when a complex of defoliating insects is present. In most years, green cloverworm populations can be considered a beneficial resource because they host parasites, predators, and fungal diseases that later transfer over to late-season populations of podworms (earworms). For leaf-feeding caterpillars, spray only if defoliation exceeds the thresholds listed above for each maturity stage and provided that green cloverworms and other caterpillars are still active and will cause significantly more damage. Consider the relative size and age composition of the population. Also consider the presence of natural controls, such as cloverworms and other caterpillars infected with the fungal and bacterial diseases. For grasshoppers, consider treatment if nymphs are present and defoliation exceeds the thresholds listed above. Although grasshoppers often are concentrated along field edges or waterways, they sometimes occur in large areas in the center of the field, especially if weeds were present last year. Determine the exact location of grasshoppers in the field and spray only those areas.
