Spider mites might hit soybeans during hot, dry weather

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COLUMBUS – With the persistence of hot and dry weather in parts of Ohio, we have continued to see the beginnings of spider mites in numerous fields, and some fields have been sprayed.
For example, a visit was made last week to a field in west-central Ohio that was in a dry area of the state. The field had no visible spider mite symptoms, except for looking dry.
It was very easy to find mites, not only along the field edge, but also in spots throughout the field.
The rainfall and high humidity lately should slow spider mite development and allow the soybeans to outgrow some of the mite injury.
This is according to Ron Hammond, associate professor of entomology at the Ohio State University’s Ohio Agricultural Research and Development Center
in Wooster, Ohio.
However, a return to hot, dry weather over the next few weeks would likely cause the mite problem to worsen. Thus, vigilance is still recommended.
Refresher course. A refresher article was felt necessary to help growers understand the problem, and how best to deal with it.
The following is information that was presented a number of years ago, including a rating system to determine the need for spraying.
Spider mites feed on the underside of the foliage with sucking mouth parts and may be very destructive when abundant, said Hammond.
Under hot and dry field conditions, he said spider mites thrive on plants that are under stress.
Soybean foliage infested with spider mites initially exhibits a yellowish speckled or stippled appearance.
As plants become heavily infested, foliage turns yellow, then bronze, and finally the leaves drop off the plants as the effect of heavy feeding leads to the plant’s dehydration and death.
In making an assessment of a spider mite infested field, it is important to recognize the early signs of mite feeding, which is the stippling or speckled effect that initially appears on the foliage when it is still green, Hammond said.
In addition, he said it is essential that one uses a good hand lens to view relative abundance of mites in egg, nymph and adult stages.
It is important to note that one field may exhibit a severe spider mite infestation while fields nearby may exhibit minimal or no spider mite activity, Hammond said.
Diagnosis and treatment. During the 1988 spider mite outbreak on soybeans, a scheme was developed for evaluating infested fields based on observable symptoms and conditions, said Bruce Eisley, research associate at the university’s Integrated Pest Management Program.
The series of possible scenarios and assessment suggestions follows:
1. Mites are barely detected at field perimeter or other dry locations within the field.
Multiple plants need to be inspected before mites are found.
Assessment: It is clearly noneconomic and no action is warranted.
2. Presence of mites is easy to detect at field perimeter; however, it is difficult to detect within the field.
Foliage is still green, but there is stippled feeding injury with few mites under lower sides of leaves detectable, but not on every plant.
Assessment: It is clearly noneconomic, but warrants monitoring.
3. Many plants in the field exhibit some sign of infestation with stippling and some discoloration of lower leaves.
Foliage exhibits various levels of stippled feeding injury on relatively healthy foliage.
Field perimeter and dry spots exhibit severely damaged plants.
Assessment: Rescue treatment is warranted at this point, especially if immature stages are in abundance or heavy egg laying is present.
4. Infestation is widespread with discolored and wilting foliage easily detected throughout the field.
Most plants are heavily infested when examined closely.
Severe damage is evident.
Assessment: Effective rescue treatment will save the field.
5. There is total field discoloration and drying down of foliage.
Significant foliage and stand loss is evident.
Assessment: Field may be beyond point of recovery if rescue treatment is applied. However, new growth may resume if treated.

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