July 25, 2002
Keep an Eye Out for Spider Mites in Corn, Soybeans
LINCOLN, Neb. — Nebraska farmers need to scout their corn and soybean fields now for spider mite infestations, University of Nebraska entomologists say.
Spider mites are appearing earlier than usual in some central and eastern Nebraska cornfields and also have spread to soybeans, which is rare, said Bob Wright, entomologist, at NU's South Central Research and Extension Center near Clay Center.
"The last time spider mites were a problem in soybeans was in the late '80s," he said. Hot, dry weather and drought-stressed crops are perfect conditions for spider mite reproduction and survival.
"We need to pay close attention," he said. "If we have continued hot and dry weather, populations can increase rapidly."
People often miss spotting spider mite activity in the early stages, he said. Spider mites cause yellow spotting on corn's lower leaves, which can be mistaken for normal discoloration at the base of the plant, Wright said. Damage is similar on soybeans and includes leaf spotting and leaf dropping.
Two species, the Banks grass mite and twospotted mites, commonly feed on Nebraska corn, but the twospotted spider mite also feeds on soybeans and dry edible beans. Twospotted mites tend to appear during mid- to late season, increase rapidly and aren't as easily controlled, Wright said.
Deciding when and whether to treat for spider mites is another problem, said Gary Hein, entomologist at NU's Panhandle Research and Extension Center at Scottsbluff.
Treatment is determined by the percent of leaves that have live spider mites on them and the percentage of the leaf damaged by the mites, Wright said. Cost of pesticides and potential crop value also are factors.
Spider mites have natural enemies, such as the lady beetle and other predatory insects, and there are predatory spider mites that feed on the plant-eating spider mites, Hein said. However, predatory spider mites only do well during cooler weather. This summer's hot, dry weather will allow the plant-feeding mites to reproduce faster.
A naturally occurring fungus also can control spider mites, but it favors temperatures below 85 degrees and relative humidity above 90 percent.
The weather plays a large part, Hein said. If cooler and damp conditions were forecast, producers might want to re-evaluate their decision to spray insecticides.
Insecticides that kill spider mites also kill their natural enemies, Hein said.
Hein has seen more natural predators this year as well as more fields that need treatment.
Spider mites have been active in western Nebraska for about a month, but only Banks grass mites have been reported, Hein said. Banks grass mites usually appear earlier in the season and are easier to control. However, twospotted spider mites might still be on their way.
Timing, low yields, drought and reduced and no irrigation water supplies make it difficult to decide whether to treat, he said.
"Marginal or even sub-par yields can be dramatically impacted by the early drying down from severe mite infestation," Hein said.
Also, treating for spider mites this early in the growing season may mean a reoccurrence and more treatment.
"Anytime we treat in July that is a serious consideration," Hein said. "One of the problems we run into is we may need another treatment. Good control keeps the mites at bay for a good two to three weeks."
After the dent stage, corn is unlikely to benefit from spider mite treatment, but that is still several weeks away, Hein said.
Producers who wait too long to treat and have a severe infestation will have difficulty controlling spider mites, he said.
As mite infestations develop, leaves continue to dry up and eventually die. If left untreated, infestations can wipe out yields.
"You have to make a call when the mites are getting at the (economic) threshold," Hein said. "If you let them go too far it's difficult to catch up on them. If you treat too late, it's a waste of money and you'll need to re-treat after a short amount of time."
For more information on controlling spider mites, consult NU Cooperative Extension NebGuide G93-1167-A, Spider Mite Management in Corn and Soybeans, available at local extension offices and on the Web.
07/25/02-SA
Gary L. Hein - Ph.D.
Entomology
Professor
(308) 632-1369
Robert J. Wright - Ph.D.
Entomology
Professor
(402) 472-2128
Sandi Alswager Karstens IANR News and Photography (402) 472-3030
Department: Entomology
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