With continued dry weather, the pest we’ve been getting the most calls about is the spider mite. This is just a reminder that vigilant scouting for this pest is a good idea right now.
It is also important to re-scout five days after treatment because many products will not kill the eggs, and populations can resurge. Any necessary follow-up treatment should be made with a product with a different mode of action to reduce resistance development. So, for example, if you used something with bifenthrin the first time, you might switch to Lorsban the second time, or vice versa.
There are two miticides that are newly labeled for spider mite management in soybean and/or some types of corn: Agri-Mek from Sygenta and Zeal from Valent. We at OSU have not tested these products yet, but are currently running trials with both of them in northwestern Ohio.
Economic thresholds based on the number of mites per plant have not been established for spider mites on soybeans. However, a scheme for evaluating an infested field based on observations of the presence of mite and feeding injury has been developed. In making an assessment of a spider mite infested field, it is important that one recognize the early signs of mite feeding, which is the stippling or speckled effect that initially appears on the foliage when foliage is still green. In addition, it is essential that one use a good hand lens to view relative abundance of mites in egg, nymph, and adult stages. The following is a system that can be used to assess a field and determine the need for taking action.
- Mites are barely detected on underside of leaves in dry locations or on edges of fields. Injury is barely detected. Non-economic population; do nothing.
- Easily detected on underside of leaves along edges of fields or perhaps on leaves in dry areas throughout field. Most foliage is still green, but yellow stippling caused by mite feeding is becoming detectable on upper side of leaves with the underside showing mite feeding. Still non-economic; warrants close monitoring.
- Many plants are infested when examined closely, with plants showing varying degrees of stippling. Possibly some speckling and discoloration of some of the leaves. These plants may be limited to field edges, but also might be found throughout field. Field edges might be showing signs of injury. Economic population developing; rescue treatment warranted. Consider entire field spray if mites are common throughout field.
- All plants in area, whether along field edge or within field, are heavily infested. Plants are discolored with wilted leaves, usually obvious from a distance. Severe injury occurring. Economic population; rescue treatment will save field.