Issue No. 18, Article 4/July 28, 2006
Bean Leaf Beetle Densities Expected to Increase: Don't Underestimate Potential Impact of Insect Injury to Soybean Pods
Over the next few weeks, we anticipate bean leaf beetle densities to increase across much of Illinois. During our July 25 teleconference with other extension entomologists in the north-central region, Marlin Rice, Iowa State University, reported that densities of the first generation of bean leaf beetles were greater in many areas of Iowa than they've been during the past two or three years. Kevin Black, insecticide/fungicide technical specialist with Growmark, recently indicated that bean leaf beetles are becoming more noticeable in many soybean fields, albeit below economic levels at this point. Based on the past few winters, which have been very mild, we also anticipate a population surge of bean leaf beetles this summer. So it makes sense to be vigilant and monitor fields for excessive defoliation and pod injury in the coming weeks. Bean leaf beetles, grasshoppers, and stink bugs are all able to cause yield losses in soybean fields during the pod-fill stage of development. Their type of injury is distinctive and will be described in the remainder of this article. So although it has been a long, hot summer already, don't neglect scouting soybeans well through early September for these insect pests.
Typically the second generation of bean leaf beetles begins to emerge in early August and is temperature dependent. If the summer has been very cool, emergence can be delayed until mid-August, particularly in some more northern states of the Midwest. Bean leaf beetles of the second generation will not mate until next spring. Consequently, no development of the ovaries within females occurs during late summer. The second generation of bean leaf beetles will feed on leaves and soybean pods. After the leaves become too old (begin to lose some green coloration), the beetles scrape off the green tissue on the pods and stems but do not chew through the pod wall. The resulting scars on the pods provide an opening for entry of spores of various fungal diseases that are normally blocked by the pericarp. Mild infection results in seed staining; severe infection may result in total seed contamination. As the quality of food diminishes within soybean fields, bean leaf beetles will seek out alfalfa or other leguminous fields. Here they reside until falling temperatures and lack of food drive them into their overwintering quarters, primarily woodlots, within which the insects nestle beneath leaf litter.
Bean leaf beetle defoliation
Bean leaf beetle pod injury
Bean leaf beetle pod injury can contribute to diseased beans
Next spring the adults will seek out alfalfa once again and begin feeding and mating. By the time the adult beetles disperse to soybean fields, they have mated and are ready to begin laying their eggs. To prevent economic losses resulting from pod injury, a rescue treatment should be considered when 5% to 10% of the pods are damaged, the leaves are green, and there are 10 or more beetles per foot of row. Make sure that scouting efforts occur throughout several areas of a field, not just the border rows. When defoliation reaches 20% between the bloom and pod-fill stages, producers should begin to assess the potential need for a rescue treatment based on the cost of an insecticide, the current market value of soybeans, and the overall mix of insects that may be feeding on pods and soybean leaves.
Grasshoppers cause more direct injury to soybean seeds. Because they have impressive chewing mouthparts, grasshoppers often chew directly through the pod wall and take bites out of seeds or devour them entirely. If 5% to 10% of the pods are injured by grasshoppers, an insecticide application may be warranted. So far this season, we have not had many reports of grasshopper infestations; however, as we approach the critical pod development phase, it's worth at least keeping our eyes open for this potential damage.
Grasshopper injury to soybean pod
Many field observers tend to overlook stink bugs and the potential injury they can cause, even though they may be the most important pod feeders in Illinois. Watching for stink bugs, especially in the southern half of the state, should be a high priority for soybean producers. Green stink bugs are believed to migrate northward from overwintering sites (wooded areas beneath leaf litter) as adults. During the early months of summer, the adults feed on berries in trees, especially dogwoods. Stink bugs are first found in soybean fields during August. They undergo incomplete metamorphosis (immature bugs resemble the adults), which requires approximately 45 days from egg hatch to adult emergence. There is usually only one generation of green stink bugs per year in Illinois.
Immature stink bugs (nymphs) have a flashy display of black, green, and yellow or red colors and short, stubby, nonfunctional wing pads. The adults are large (about 5/8 inch long), light green, shield-shaped bugs with fully developed wings. Both adults and nymphs have piercing and sucking mouthparts for removing plant fluids.
Immature stink bug on soybean leaf
Immature stink bug on soybean pod
Stink bugs feed directly on pods and seeds; however, their injury is difficult to assess because their mouthparts leave no obvious feeding scars. Stink bugs use their mouthparts to penetrate pods and puncture the developing seeds. They inject digestive enzymes into seeds, and the feeding wound provides an avenue for diseases to gain entry into the pod. Seed quality also is reduced by stink bug feeding, and beans are more likely to deteriorate in storage.
Other species of stink bugs also occur in soybeans. The brown stink bug has feeding habits and a biology similar to those of the green stink bug. The brown stink bug should not be confused with the beneficial spined soldier bug. Adult brown stink bugs are brown and have a yellow or light green underside, and their "shoulders" are rounded. Spined soldier bugs also are brown and have a white to cream-colored "belly"; however, their shoulders are sharp-pointed. Be sure you are aware of the species present in a soybean field before making a control decision. An insecticide application for control of stink bugs may be warranted when the level of infestation reaches one adult bug or large nymph per foot of row during pod fill.
Adult green stink bug on soybean leaf
The following insecticide products are listed in the 2006 Illinois Agricultural Pest Management Handbook for use in soybeans against bean leaf beetles, grasshoppers, and stink bugs: *Asana XL, *Baythroid 2, *Lorsban 4E, *Mustang Max, Orthene 90S, *Penncap-M, *Proaxis, Sevin XLR Plus, and *Warrior. Products preceded by an asterisk (*) are restricted to certified applicators. Please read the label to determine the amount of product that should be applied per acre and follow all precautions and restrictions.--Mike Gray