With the persistence of hot and dry weather in parts of Ohio, we are beginning to get more reports of twospotted spider mites beginning to develop on soybeans from Ohio and other states in the Midwest. Spider mites feed on the underside of the foliage with sucking mouth parts and may be very destructive when abundant. Under hot and dry field conditions, 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 dehydration and death of plant. Although mentioned last week in this C.O.R.N. newsletter, it is time to discuss treatment decisions. The following is a rating system to use to determine the need for spraying. This scheme was developed for evaluating infested fields based on observable symptoms and conditions as follows:
1. 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.
2. 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.
3. 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.
4. 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.
5. Extremely high TSSM densities, with much of the field discolored, stunted, with many plants drying down or already dead. Economic population; rescue treatment will only be beneficial if new growth occurs following late summer rain.
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. See the following pictures for signs of early mite injury: http://entomology.osu.edu/ag/images/tssm_injury_far.pdf, http://entomology.osu.edu/ag/images/tssm_injury.pdf, http://entomology.osu.edu/ag/images/tssm_underside.pdf. In addition, it is essential that one use 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.
Another factor that concerns us is the possibility of mite populations exploding when other materials are applied to soybeans. Although no firm data is available, we have seen mites increase when certain herbicides are applied post emergence. Additionally, because mites are often kept in check by various fungal pathogens, we are worried that fungicide applications might, and we would add a big might (again, little data available), interfere with the mite fungal pathogens. Thus, unnecessary fungicide applications could also lead to enhanced mite problems.
If a decision is made to treat, there are only a few options available.
· The two most common materials used in the past are chlorpyrifos (Lorsban and generics) and dimethoate.
· Another material more recently on the market is bifenthrin which is the active ingredient in a number of different formulations. Although we have no experience with this material in Ohio, colleagues in other states indicate that is it a good miticide.
We would also point out that these materials do not offer much control of the eggs, and thus, any treatment this early should be followed up with continued scouting in case newly hatched mites are not controlled by residual activity and begin to build a second time. This might require a second treatment later, especially if hot and dry weather continues. If a second treatment is required, we would also recommend that a different miticide be used, and growers do not make a second application of the same material. Although some pyrethroids are labeled as offering suppression of mites, we do not recommend their use and suggest staying with one of the aforementioned materials. See the following fact sheet for more information on twospotted spider mite on soybean: http://ohioline.osu.edu/ent-fact/pdf/0024.pdf.