As we move into the month of August, there are a number of insect pest problems that continue to warrant close monitoring on soybeans.

Soybean Aphid

Reports are coming in of aphid populations finally being seen in Ohio soybean fields, especially in northern counties. For the most part, aphids are still being reported at low populations. However, sampling for soybean aphids should still be a primary part of your weekly routine, at least through August. Remember, the threshold, the point when a field should be sprayed so that the economic injury level is not reached, is 250 aphids per plant with a rising population. 250 aphids per plant will NOT cause a yield loss; this is the level when you should spray to keep the aphids from reaching 750-1000 aphids or more per plant. Furthermore, an additionally concern is the growth stage of the soybean crop. The 250 aphid per plant threshold is of use through the R5 soybean growth stage, which is during initial seed growth. As the plants reach R6, or full seed stage, the level when you should spray is higher. Although an accepted threshold at R6 has not yet been determined, we would recommend NOT spraying unless the population is going much higher; probably well above 500 aphids per plant. However, it would be of greatest benefit to growers to make treatment decisions before or during the R5 stage. A final point to make concerns the preharvest interval. Most insecticides have a preharvest interval on soybeans of 21 or more days. Make sure that when selecting an insecticide, you allow enough time between spraying and potential harvesting according to label directions.

Bean Leaf Beetle

We continue to get reports of large numbers of bean leaf beetles feeding on soybeans. We still recommend using established defoliation thresholds of between 15-20% of the entire plant before treatment. A consideration to factor in is the new growth being put on by the plants following recent rains. If a large beetle population is in your field preventing this new growth from emerging, an insecticide treatment might be warranted.

Twospotted Spider Mite

We also are continuing to get calls on twospotted spider mites, especially from areas of the state that remain dry. As we have mentioned in past C.O.R.N. newsletters, these mites could be problems across the entire field, or limited to field edges. If the former, the whole field will need spraying. If only on the edge, you can get by with treating those field edges. Remember, only chlorpyrifos (Lorsban, Nufos, Yuma) and dimethoate are effective against twospotted mites. Before treatment, growers should examine the underside of the leaves to make sure mites are still alive and active. Recent rainfall along with increasing humidity will soon begin to cause twospotted spider mite populations to crash. Whether this occurs soon or sometime in August, growers should make sure mites are still active and feeding on the plant before making an insecticide application. However, with the forecast calling for no rain and hot temperatures, mites will continue to be active.