Drought '99 Ohio State University Extension Dry Weather Weaving a Tangled Web of Trouble, July 1, 1999 By Jennifer Kiko WOOSTER, Ohio -- Hot temperatures and very dry weather conditions are wreaking havoc in Ohio's corn and soybean fields, and farmers have more than slow crop development to worry about. Ohio State University's Ronald Hammond, entomologist at the university's Ohio Agricultural Research and Development Center in Wooster, said two-spotted spider mites have been detected in Franklin, Madison, Clark, Champaign, Darke, Miami, Pickaway and Wayne counties, among others. The mites can devastate soybean crops. Hammond said significant amounts of rain in the next one to two weeks would help curtail the mite infestations, but field monitoring would still be needed and problems could continue to persist. The spider mite was a major problem during the 1988 season when widespread drought conditions decimated many crops, Hammond said. "Ohio's current dry weather conditions and early mite problems are similar to conditions farmers experienced in 1988." Spider mites feed on the underside of foliage and can be extremely destructive when infestations are heavy. They thrive on plants that are under stress from hot and dry field conditions. Infested soybean foliage will turn yellow then bronze, and the leaves will fall off due to dehydration. Plants will eventually die. Hal Willson, field crops entomologist with Ohio State University Extension, said during the spider mite outbreak of 1988, a scheme was developed for evaluating infested fields. It's based upon these observable symptoms and conditions: -- A few mites are detected around the perimeter of the field or in dry locations. Multiple plants need to be inspected before mites are found. Assessment: Rescue action is not worth the cost. -- Presence of mites easy to detect at field perimeter and dry spots. Difficult to detect within field. Foliage is still green but feeding injury has occurred with a few mites detected under the lower side of leaves on several plants. Assessment: Rescue action is not economical, but warrants monitoring. -- Entire field exhibits signs of infestation with speckling and some discoloration of lower leaves. Foliage exhibiting various levels of feeding injury on relatively healthy foliage. Field perimeter and dry spots exhibit severely damaged plants. Assessment: Rescue treatment warranted at this point, especially if immature stages are in abundance or heavy egg laying is present. -- Infestation is widespread with discolored and wilting foliage easily detected throughout the field. All plants are heavily infested when closely examined. Severe damage evident. Assessment: Effective rescue treatment will save field. -- Total field discoloration and drying down of foliage. Significant foliage and stand loss evident. Assessment: Field may be beyond point of recovery if rescue treatment is applied. New growth may resume if treated. When treatment is warranted, growers have three insecticides or miticides available, according to Hammond. These include Dimethoate, Lorsban (Chlorpyrifos), and Warrior. Label directions and rates should be followed. Growers should note that a second application of Lorsban is often needed three to five days later if the egg population within the field is high. Warrior is labeled for suppression of mites only. When making an assessment of a spider mite-infested field, Hammond said it's important to recognize the early signs of mite feeding -- a speckled effect that appears on the foliage while it's still green. Growers may want to use a hand lens to view relative abundance of mites in egg, nymph and adult stages. One field may exhibit severe infestations while neighboring fields have minimal or no spider mite activity. However great or small the infestation in fields, both Hammond and Willson agree that all Ohio growers should be on the alert for spider mites and begin monitoring fields immediately. For more information about spider mites, contact OARDC's Department of Entomology, (330) 263-3725 or Extension Entomology, (614) 292-8358; obtain a copy of Extension Bulletin HYG-2012-92, "Spider Mites And Their Control" from your local county office; or search for information on Extension's website, Ohioline (http://ohioline.ag.ohio-state.edu/). -30- All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status. Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension. TDD No. 800-589-8292 (Ohio only) or 614-292-1868   | C.O.R.N. | Newsletter | Ohioline | Publications |