Fortunately, through scouting and the use of economic thresholds this problem can be avoided.
Over the past few summers, we have heard from increasing numbers of growers suffering from really large populations of corn leaf aphid in their corn fields (Figure. 1). If you dealt with this problem, you probably won't forget it any time soon as you had thousands of aphids per plant and lots of honeydew (a polite term for liquid excrement from aphids) on leaves. This sugary snack often draws in wasps and other insects looking for a sweet treat, but it also complicates matters because it is often colonized by sooty mold, which turns leaves black, limiting their photosynthetic capacity.
The image shows corn leaf aphid population from 2015 that resulted from a grower including an insecticide unnecessarily in a tank mix when he was applying fungicide to corn. The insecticide was disproportionally hard on the insect predators that can keep aphid populations in check.
At this time of year, farmers are considering their options for mid-season pest control. More and more farmers are choosing to apply fungicides to corn mid-season to limit the potential influence of fungal infestations. Often, these same growers will include an insecticide in the tank with the thinking that "they are going over the field anyway and the insecticide can only help." But the aphid population in Figure 1 results from this flawed rationale. Most corn fields have a few aphids but the populations are non-economical and are kept in check by lady beetles and other predatory insects. When an insecticide is unnecessarily applied to a field with a few aphids, all the aphids may not be killed, but the application will be particularly hard on the beneficial insects, particularly lady beetles and their larvae and predaceous bugs that eat the aphids. With these predators out of the way, the small aphid populations are able to grow unchecked into really big, ugly populations like those seen above.
The best strategy for avoiding this problem, and managing pests in general, continues to be Integrated Pest Management, which dictates that one only applies a pesticide when the pest population exceeds an economic threshold. Do not just throw an insecticide in the tank unless you need it. To determine whether pest populations are large enough to warrant an insecticide spray, scout your fields! If the insect populations exceed thresholds, apply an insecticide; if not, don't. It should be that simple.