Soybean Insect Management
Soybean production in North Carolina ranges across the Coastal Plain and into the Piedmont. Within this geographic range the likelihood of economic loss to insects is highly variable. In general, pest insect abundance in soybeans increases in a southerly and easterly direction. For example, soybeans in a mid-coastal county (e.g. Carteret county) may experience economic infestations of several different pests, whereas insect associated soybean loss in the interior of the state (e.g. Franklin county) would likely be infrequent and limited to corn earworm feeding.
Soybeans harbor many species of insects. Several have the potential to be pests under high populations but most are either beneficial predators or parasites, or perform no function of direct interest to the soybean farmer. Those insects that do feed upon the plant have traditionally been classified as either foliage feeders or pod feeders, although a few may feed on both plant parts. Most of these plant feeding insects have chewing mouthparts (e.g. caterpillars and beetles) but some pierce the plant and suck juices (e.g. stink bugs and leafhoppers). A few species feed on the underground stems, roots, and nodules. Approximately 20 different plant feeding insects may be found with relative ease in soybean fields, but their individual or collective abundance or damage usually does not exceed the economic threshold. The corn earworm is by far the most important insect pest of soybean in North Carolina. Stink bugs, soybean looper, velvetbean caterpillar, green cloverworm, and bean leaf beetle infrequently occur at high populations but may sometimes reach damaging levels; soybean looper, stink bugs, and velvetbean caterpillar may occur with some regularity in coastal counties below the Albemarle Sound. Other plant feeding insects are rarely important to the soybean farmer. There are several readily available and excellent references on soybean insects and their management. For added information on the identification, biology, and other subjects, consult the publications listed at the end of this chapter in the “References” section.
The soybean plant has an amazing capacity to tolerate insect attack when grown under favorable conditions. Low levels of defoliation, root/nodule feeding, and flower/pod feeding will have no effect on yield. Whereas yield or quality of cotton, most vegetables, and other crops be can be adversely affected by low numbers of insects, the presence of insect pests in soybeans does not mean that yield loss has begun. Research based thresholds describe the level of pests or defoliation required to produce loss and should be used when deciding if control actions are necessary. Since soybean is a low value per acre crop, close attention must be given to producing the highest possible yield at the least cost. It is easy to unwisely increase the cost of insect control by not recognizing the plant’s potential to tolerate some insect damage. On the other hand, a common problem that some NC soybean growers demonstrate is a lack of attention in detecting and controlling damaging insect problems when they do occur.
Managing crop pests involves a consideration of agronomic, economic, and biological factors. Soybean is a low value crop that can not support high cost pest management techniques. Therefore, low cost practices which avoid the build-up of insect pests and/or enhance the soybean crop’s ability to tolerate insect injury are emphasized. These “avoidance practices” include several cultural techniques that: (1) help ensure a healthy, competitive, and compensating crop; (2) avoid infestations of pests by site selection; (3) seek to make the crop unattractive or unsuitable for some pests, and (4) preserve biological control organisms for pest suppression. Avoidance techniques can greatly reduce insect damage to soybeans but can not always be used in full force, or may not work as well as hoped. In those cases, damaging infestations may occur and “remedial techniques”, such as insecticide use based on scouting and treatment thresholds.