Issue No. 20, Article 5/August 10, 2007
Whiteflies and Twospotted Spider Mites in Soybeans
Although the focus of a lot of recent insect scouting in soybeans has been soybean aphids, other critters are being found in soybeans in several areas of the state. We have received numerous reports of whiteflies in soybeans in southern and central Illinois, and twospotted spider mites have shown up in areas of the state that were not blessed with ample rainfall. It is likely that whiteflies are causing little to no economic damage to soybeans, and soybeans have progressed enough in development that spider mites probably won't be much of a concern either. However, the presence of either of these pests should be noted and their populations monitored. It's also critical that people not confuse either of these pests with soybean aphids.
We started hearing about whiteflies in soybeans in mid-July, occurring mostly in drier areas of Illinois. On July 26, I observed fairly large numbers of whiteflies in soybean plots at the Brownstown Agronomy Research Center (Fayette County). I was using a sweep net to sample the plots for Japanese beetles and discovered whiteflies accidentally. Although I counted dozens of whiteflies per leaf, their numbers were not quite large enough to create the "white clouds" that others have observed while walking through soybean fields.
Whitefly adults on soybean leaflet, Champaign County, August 7, 2007. Note the alate and white dward soybean aphids. (Photo courtesy of Joe Spencer, Illinois Natural History Survey).
We know very little about the effects of whiteflies infesting soybeans in Illinois. Whiteflies are related to aphids (in the same order, Homoptera) and have piercing-sucking mouthparts. However, although soybean producers in Illinois have encountered whiteflies many times over the years, no one has yet indicated that they suffered yield losses as a result. To quote from the ESA Handbook of Soybean Insect Pests (page 98; published by the Entomological Society of America): "Most reports of large whitefly populations in the northern United States have not been associated with specific species; consequently, the potential importance of whiteflies in these areas is difficult to assess."
In the southeastern United States, Bemisia tabaci is the species thought to injure soybeans, although there is some controversy about this insect. Before 1986, B. tabaci, called the sweetpotato whitefly, was an occasional pest of crops. In 1986, B. tabaci became an extreme pest of crops in Florida, and entomologists believed that a new biotype, B, of this species was responsible for the increased severity of injury. Some entomologists believe that biotype B of B. tabaci is actually another species, B. argentifolii, although this has not been universally accepted. Regardless, the whitefly now is called the silverleaf whitefly. Bemisia tabaci has caused serious injury to soybeans, and yields have been drastically reduced. Its impact seems to be more pronounced in warm, dry weather. It's important to restate that we do not know what species of whiteflies people are finding in Illinois soybeans. (An excellent Web site with details about this whitefly is the University of Florida's "Featured Creatures")
Whitefly injury to soybeans results in a yellow speckling of leaves, which might resemble injury caused by twospotted spider mites. When injury is heavy, the leaves become dry. However, unless the infestations are extremely heavy, it is not likely that whiteflies will cause economic yield losses.
Lots of whiteflies on a yellow sticky trap (for western corn rootworm adults) from a Champaign County soybean field, August 7, 2007 (photo courtesy of Joe Spencer, Illinois Natural History Survey).
Twospotted spider mites have been observed at the edges of soybean fields in areas of Illinois that have received lesser amounts of rainfall. Robert Bellm, University of Illinois Extension crop systems educator in Edwardsville, observed symptoms of spider mite injury to a soybean field in Pike County on August 7. Spider mites have been found elsewhere by others over the past 3 weeks, so it's important to keep track of their populations' progress. As indicated previously, soybean development may be far enough along that application of a miticide would not pay for itself in yield benefits. However, if the infestation is severe enough along field edges, spot treatments might be justified.
Symptoms of twospotted spider mite injury along the edge of a soybean field in Pike County, Illinois, Auguts 7, 2007 (photo courtesy of Robert Bellm, University of Illinois Extension).
Other insects to watch for in soybeans in August include pod-feeders such as grasshoppers and stink bugs. Although these insects often are ignored, they can cause significant yield loss if infestations are heavy. Bean leaf beetles also pose a threat to soybean pods, although they typically do not cause direct injury to the beans inside the pods.
Our soybean insect season is not over yet. Keep a close eye on these late-season developments.--Kevin Steffey