Where It’s Dry, Pest Concerns Are High
September 16, 2008 Not everyone’s dealing with wet fields. In areas where dry conditions have plagued soybean crops, some pests have been having a field day even this late in the season, and Extension specialists say there are steps retailers and their growers should take.
The key pests at this stag in such areas as Ohio and Kentucky are two-spotted spider mites, bean leaf beetles, and soybean cyst nematodes.
- Two-spotted spider mitesbecome active when fields are dry, and in Ohio, Ohio State University Extension entomologists have reported some outbreaks, especially along field edges. The two-spotted spider mite feeds on the sap of the soybean plant causing injury. Large spider mite populations, running in the tens of thousands per plant, are enough to kill the crop, causing significant yield losses.
“We continue to think that they will not become whole field infestations, although growers should nevertheless check areas within their fields,” says Ron Hammond, an OSU Extension entomologist. “This is especially important on late-maturing soybeans that were planted late and are still green. Those fields that are starting to yellow are probably past the stage where spraying for mites would be recommended.”
Yellow/bronze discoloration of plants along field edges or within in a field is characteristic of spider mite feeding. “If growers are finding large populations of adults and eggs on the underside of the leaflets, then treatment is probably warranted,” he says. “If mites have built up sufficiently along the field borders, growers should make an edge spray. This should give late-planted soybeans enough time to continue to make good yields.”
- Soybeans grown for seed, especially late-planted or late-maturing beans, should be monitored for bean leaf beetledamage as leaves begin to yellow and pods remain green, say Purdue University entomologists.
Bean leaf beetles scar the surface of pods, but only occasionally feed through the pod to the developing beans, explains John Obermeyer, Purdue Extension integrated pest management supervisor, and Larry Bledsoe, Purdue Extension pest management expert. During pod maturation, this scar often cracks leaving an entry hole for airborne plant pathogens that may cause discolored, moldy, shriveled, or diseased beans that show no outward signs of the pathogen.
Bledsoe says it’s important for pest managers to be able to predict whether economic damage will occur based on the types and numbers of beetles that are present and the stage of pod development (i.e., green, yellow, yellow-brown, or brown pods). Once the pods turn yellow to yellow-brown, they become less attractive and less susceptible to damage. Control is normally not warranted from this point on.
Obermeyer and Bledsoe recommend randomly selecting two plants from five different areas of the field and then counting the number of pods per plant and the number that show damage. Calculate the percentage of damaged pods per plant for the field as a whole. Note if the pods are green, beginning to turn yellow, or are yellow/brown. Also determine the number of beetles per sweep using an insect sweep net. Take five sets of 20 sweeps in the field. Determine the number of bean leaf beetles per sweep. Additionally, note whether beetles are still actively feeding while surveying the field, they advised.
There has been considerable interest in bean leaf beetle and its association with bean pod mottle virus (BPMV). Bean leaf beetle is one of major beetle-vectors of this disease. They spread the virus by feeding on infected plants, ingesting the virus with plant tissue, and then regurgitating gut content after moving to and feeding on an uninfected plant. BPMV symptoms at harvest include green stem and hilum bleeding. Treatment for bean leaf beetle to reduce bean pod mottle virus this time of the year is not recommended, as most disease transmission occurs very early in the season.
- Soybean cyst nematode (SCN)also does damage to soybeans under drought stress. Deemed the “silent robber of yields,” soybean cyst nematodes feed on the roots of young plants, which prevents the roots from taking up vital nutrients. The result is a drop in yields and subsequent economic losses.
Anne Dorrance, a plant pathologist with the Ohio Agricultural Research and Development Center, urges growers to scout their fields for potential SCN populations.
“Where the soybeans are shorter and have fewer pods is a great place to check for SCN on the roots,” said Dorrance, who also holds an OSU Extension appointment. “Dig up the soybeans and shake off the soil, then look for the females. They appear as tiny white pearls, which glisten on the roots.” Notes should be taken and those fields should be sampled in the fall for egg counts through laboratory tests.
“A good accurate egg count in the fall is the first step in soybean cyst nematode management,” said Dorrance. “The best management practice is crop rotation, crop rotation, crop rotation. Once the insect is present, you can’t get rid of it.”
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