Corn Leaf Aphid, an Argument Against Tank Mixing Insecticides

As farmers inspect their corn fields to assess progress toward harvest, many are noticing large populations of corn leaf aphids infesting their plants (Figure 1). A significant number of those farmers have contacted Penn State Extension to discuss the risk that aphids pose to their crop and whether they need to apply an insecticide to control the aphids now. This time of year [September], there is nothing to do about infestations of corn leaf aphid, but it is important to realize the role that growers may have played in causing their own aphid outbreak. Recognizing this connection is key to avoiding the same problem next season.

Corn leaf aphids are common, and they colonize most corn fields in June and July, but they often go unnoticed because their populations remain small. Aphids have an amazing reproductive potential, but the reason that their populations remain small is because they tend to be kept in check by lady beetles, predaceous bugs, and other natural enemies that are common in corn fields, particularly when plants are shedding pollen.

Coincidentally, when corn is tasseling or silking, many growers will elect to treat their fields with fungicides to protect against mid-season pathogens. And many farmers, or their service providers, will insist on adding an insecticide to the application because "we're going over the field anyway and we might as well control some insects at the same time." This argument is made even more convincing by the low cost of adding a pyrethroid to the tank (usually just a few dollars per acre).

Unfortunately, one of the downsides of adding an insecticide to a fungicide application is outbreaks of corn leaf aphids. These outbreaks are typically caused by inadvertently killing natural enemies, not aphids, with the tank-mixed insecticide. With few natural enemies around, the aphid population is free to 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. And the earlier in the growing season that these large populations emerge, the greater their potential to reduce yield.

The best strategy for avoiding this problem, and managing pests in general, continues to be Integrated Pest Management, which dictates that you should 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 the insect populations exceed thresholds, apply an insecticide; if not, don’t. It should be that simple.