Why aren’t rains stopping spider mites?

By Ron Hammond and Andy Michel, OSU Extension

The question is being asked why the rains aren’t stopping the mites, which would actually be from a fungal pathogen that can decimate the mite populations under wetter and more humid conditions.

Although rainfall reduces risk of damaging spider mite populations, thunderstorms alone will not eliminate infestations, particularly when rain arrives after large mite populations are established and when rain is followed by dry, hot conditions.  Sources in other states’ newsletters suggest that the mite-killing fungus requires temperatures cooler than 85°F, with 90% relative humidity, to produce infective spores. Periods of at least 12-24 hours of relatively cool, moist, and humid conditions are necessary for the fungal pathogen disperse and infect a spider mite population in a field. In “normal” years, these are conditions we often see in mid-August. So although we are expecting these conditions in the near future, we still urge growers to monitor their fields and spray if the mites are alive and actively feeding. But look closely at the mites, and before pulling the trigger to spray for them, make sure they are still active. Do NOT make miticide applications based on injury symptoms alone; the mites already might be dead!

Another question is how long are the crops susceptible to mite feeding if environmental conditions do not take care of the problem. As mentioned above, we usually would expect the mite populations to decline naturally in later August. However, knowing how strange this summer has already been, we should not assume anything. For soybean, we would usually consider the R6 stage, full seed, when treatment might not offer much protection or benefit. Corn in the Midwest is a different story, mainly because this is a new problem for us. However, in the article that we referred to last week out of Wisconsin, it is suggested that once corn has reached the hard-dough stage, no economic benefit will result from treatment.

The last question or problem growers are having, is that the two main insecticides with mite activity, Lorsban (and generics) and dimethoate, are in short supply and difficult to find. Thus, growers are already needing to use other materials.  We have already discussed bifenthrin products including some that are combinations of bifenthrin and a pyrethroid (Hero), and in combination with clorpyrifos (Tundra Supreme). Presently, reports received suggest that all the bifenthrin products are working. We have also received word that there is a new miticide labeled on corn, Zeal, made by Valent, which contains the miticide etoxazole. Zeal now has a supplemental label for use on corn. It is not labeled on soybean and cannot be used in that crop.

 

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