Corn Leaf Aphid, an Argument Against Tank Mixing Insecticides

This time of year, more farmers are considering applying fungicides to their maturing corn crops. At the same time, many will convince themselves (or their custom applicators will convince them) that there must be value in including an insecticide in the tank of the sprayer because "we're going over the field anyway and we might as well control some insects at the same time." This argument is made more convincing because the cost of adding a pyrethroid to the tank is just a few dollars per acre. I have heard this argument many times. 

If you are willing to listen to an argument not to tank mix, I will make one based on aphids (Figure 1). The corn leaf aphid population in Figure 1 resulted from adding an insecticide to a fungicide application and illustrates the flawed logic of adding insecticide to the tank "because we are going over the field anyway." Most corn fields have some 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 some aphids, all the aphids are unlikely to 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 will grow unchecked into really big, ugly populations (Figure 2). I have only seen large populations of corn leaf aphid like this when they were flared by unnecessary insecticide use. 

The best strategy for avoiding this problem, and managing pests in general, continues to be Integrated Pest Management, which dictates that one should only apply an insecticide (or any pesticide) only when you know it will provide an economic advantage. This advantage is why economic thresholds exist; when pest populations exceed economic thresholds, there will be an economic advantage to controlling the population.

Bottom line: do not just throw an insecticide in the tank unless you need it. To determine whether pest populations are large enough to warrant treatment, scout your fields! If insect populations exceed thresholds, apply an insecticide; if not, don’t. It should be that simple.