The hot, dry weather conditions are increasing the need for producers to scout their fields. Reports to Iowa State University (ISU) crop specialists indicate infestations of spider mites in some Iowa soybean fields, as well as isolated fields with more than 250 soybean aphids per plant.
Its unclear right now how serious the aphid threat will be this year. Data collected by Brian Lang, ISU Extension field crop specialist in northeast Iowa, show fewer aphids per plant this year than in 2003, but more aphids per plant than were found in 2004.
Reports of spider mite infestations are primarily from eastern Iowa, which has had drier conditions than the rest of the state this crop season.
“Growers should be aware of the similarities and differences between these two pests, and adjust their management plans accordingly,” said Matt O’Neal, assistant professor, Entomology.
“Two spotted spider mites (Tetranychus urticae) and soybean aphids (Aphis glycines) both feed on soybean leaves, but have very little else in common,” he said.
“Mite damage results in the leaf changing colors to yellow and the leaf surface curling. During heavy spider mite infestations leaves will drop off the plant. Aphid feeding does not result in the leaf turning yellow. Heavy aphid infestations can change the color of leaves, with leaves looking dark gray or even black. This is due to honeydew (a sugary, sticky substance excreted by aphids) that is infested with sooty mold.”
Although spider mites and aphids are small, they are capable of moving great distances. Mites are readily blown by wind, so initial colonization often occurs in the direction of prevailing winds. Landscape features that disrupt airflow, (tree lines, houses, or even telephone poles) can help spider mite populations establish. Cool, humid conditions promote growth of fungal diseases that prevent spider mite outbreaks.
O’Neal says infestations of spider mites and aphids can begin in spotty, isolated areas of a soybean field. However, under certain conditions both pests can spread across large areas. For example in 2003, nearly 3 million acres were treated for soybean aphids in Iowa. During 1988, nearly 3 million acres were treated for spider mites. Spider mite infestations typically occur on field perimeters and can eventually spread throughout a field. Scouting for both pests will require surveying both field edges and interior.
“Growers scouting for aphids need to make the extra effort to survey the entire field, looking for symptoms of spider mite infestations,” said O’Neal. “Yellowing of soybean leaves alone is not sufficient to confirm spider mite infestations. Yellowing can be caused by nutrient deficiencies and some plant diseases. Growers should confirm that these symptoms are due to spider mites by collecting leaf samples and looking for the mites on the underside of leaves.”
Several insecticides are recommended for managing spider mites and soybean aphids, with some products appropriate for both pests. During dry conditions growers are warned to avoid pyrethroids when spraying for other insect pests as these active ingredients tend to flare spider mites.
O’Neal says that under the current weather conditions, if a grower needs to treat a field for soybean aphids that is at risk for a spider mite outbreak, then they may want to consider an organophosphate.
Two organophosphates are recommended for spider mites, Chlorpyrifos (Lorsban) and Dimethoate. Efficacy trials from Michigan State University (http://www.ipm.msu.edu/CAT01_fld/FC07-12-01dimethoate.htm) and the University of Minnesota (http://www.soybeans.umn.edu/crop/insects/aphid/aphid_management.htm) suggest that Dimethoate provides limited control of soybean aphids.
Compared to other organophosphates, Dimethoate has a reduced initial kill rate and residual time, which can lead to resurgence of soybean aphids. Chlorpyrifos (Lorsban) has been shown to be effective against soybean aphids, providing very good, immediate kill. Therefore, if a grower’s field is at threshold for soybean aphids (250 per plant) and the conditions are favorable for spider mite outbreaks, they may want to consider chlorpyrifos for its ability to control both pests. Soybean fields that have received adequate rainfall will have a reduced risk for spider mites and therefore both organophosphates and pyrethroids would be appropriate for soybean aphid management..
For more information about two-spotted spider mites check out the ISU Integrated Crop Management newsletter available on the Web at: http://www.ipm.iastate.edu/ipm/icm/indices/insectsandmites.html. The Iowa Soybean Aphid Task Force Web page www.soybeanaphid.info offers more information on identifying and managing soybean aphids.