All you have to do is drive through the countryside in dry areas and you can notice that spider mites have moved from the edges of fields deeper into the field. The symptoms aren't always as pronounced away from the field border, but lighter colors and wilting plants can give it away.
Some farmers have sprayed. Others haven't. Maybe they don't believe the problem is severe enough. Maybe they think they've already invested enough in this crop. But at $15 per bushel, harvesting even one extra bushel is important.
Based on what happened in 1988, a similar drought, spider mites caused significant yield damage, far greater than one bushel per acre, in areas that remained the driest. Even fields that were sprayed, perhaps later than other fields, came up short in yield.
Here's what the Purdue University Corn & Soybean Field Guide says about the two-spotted spider mite. The Guide is published by the Purdue Diagnostic Training Clinic.
First, damage can continue all the way through to harvest, or the R7 stage, if left unchecked. Heavily infested leaves will turn brown and die. Once the damage is done it is not reversible on infected plants.
To detect mites you can shake plants over white paper and look for movement. You're dealing with a pest, technically a member of the spider family that is very tiny and hard to see. If you want to look for mites or eggs laid by mites on the underside of leaves, you'll need a hand lens.
Recommendations are that if signs of damage are present, you can find mites, and hot, dry weather is predicted to continue, you may want to consider treatment. Except in areas covered by pop-up thunderstorms, most of Indiana remained dry through most of last week.
There are a couple of pesticides that do a good job of controlling these mites. There are also miticides, but they tend to be much more expensive.
The bottom line is that you have seen classic symptoms along field borders and haven't walked farther in to the field to check for mites, now might be the time to do so.