Drought, insects and field crops

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Twospotted spider mites on soybeans are now a concern in the state. Numerous areas in the drier areas of Ohio are already seeing them, and some fields are being sprayed. Mites are showing up not only on field edges but also within the field. Click here for information on treatment decisions and materials to use. 

Currently, materials of choice are chlorpyrifos (Lorsban and generics), dimethoate, and various bifenthrin formulations (including those that are in combination with other materials). Although we do not have much experience with the bifenthrin materials, they do have many mite species on their labels. If you use materials containing bifenthrin, we would like to hear how efficacious they are. Other pyrethroids, while having mites on their label, are listed only for “suppression”; we do not recommend those materials. 

A question we often get is if it is worth spraying during such a dry year. Past experience shows us that when soybeans are protected from mites, later rains in August will allow the soybeans to recover somewhat and give acceptable yields compared to fields where mites are not controlled. Also, remember if the hot and dry weather continues, none of these materials will give long term control; all might need a second treatment later on, even within a few weeks.  Thus, continued scouting is a requirement. And if a second treatment is required, we strongly recommend switching to a different material to help reduce the chances of miticide resistance from developing in the mite. Insecticides/miticides should be rotated just like crops!

Silking on corn is occurring, and growers should be aware that the insects of concern are in the fields already. The two most important pests concerning silk feeding are the adult Japanese beetle and adult western corn rootworm (WCR). For the rootworm, the question is how heavy they will be this year knowing that, in general, populations have been low the past few years.  However, we have heard of fields with higher than expected numbers feeding on the silks. What growers should examine is whether these higher insect populations are in fields where the corn is silking evenly across the area, or if the corn is showing uneven growth, with silking occurring only in pockets. Corn rootworm adults will tend to occur only in the areas that are silking. As other parts of the field begin to silk, the beetles will move to them. Treatment is warranted when an average of 5 or more WCR adults or 2 or more Japanese beetles are found on silks, silks have not been pollinated, or silk clipping to 0.25 inches or less is observed. If silks are wilting and turning brown, pollination is complete.

We are now seeing soybeans entering their flowering stages. Because of the early arrival of many insects this year, we are seeing more defoliation during these early flowering stages than usual. Adult Japanese beetles and the first generation of adult bean leaf beetles are occurring at least 2 weeks earlier than normal. And because of the dry weather in much of the state, soybeans are not growing well and are much smaller in size with low levels of leaf area. This lack of growth results in low LAI levels (leaf area index, a measure of the amount of leaf area present in a field) and thus lower light interception by the plant canopy. When scouting for insect feeding, the 15-20% defoliation threshold on small soybeans with low LAIs will be reached more quickly and have a greater yield-reducing impact than if those soybeans were larger in size with large amounts of leaf area present. Although we still believe it best to keep the defoliation thresholds at the current levels, growers might want to be conservative with these thresholds and make a decision to treat sooner than they would in a “normal” year with adequate moisture and crop growth.

Finally, potato leafhoppers remain active in alfalfa, and with the crop not regrowing as well as expected because of the dry weather, perhaps the leafhopper demands greater attention. If you read our fact sheet on this pest, you will see that when alfalfa is under stress, we suggest that leafhopper thresholds be lowered from the normal ones (http://ohioline.osu.edu/ent-fact/pdf/0033.pdf ); the threshold when under stress is about half of the normal threshold.  This is one time period that leafhoppers should be treated sooner than later if populations are in the field. 

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