My last article was about how insects survive the Nebraska winters and how small spring insect populations can grow to high levels given the right environmental conditions. Their population growth depends on a variety of factors, and weather conditions are some of the most important. Since predicting the weather in the long run is difficult, it is also difficult to predict outbreaks of most insect pests.
For the most part, insects are ectotherms. This means that they do not produce their own internal body heat like we do, but use heat from their surroundings. Since their development depends in large part on temperature, we can use temperature for some insects to predict their activity and appearance. One such insect is the seed maggot, a common pest of Nebraska corn and soybeans.
Seed corn maggots overwinter as pupae and then appear as small adult flies in early spring. After mating, the female flies are attracted to rotting organic matter such as rotting animal manure or plants, where they lay eggs in the ground. The eggs hatch in a few days and the larvae (maggots) feed on germinating corn and soybean seeds. Nebraska has several generations of seed corn maggots each year.
Insecticidal seed treatments typically protect corn and soybeans from injury from seed maggots. However, when insect densities are high and conditions cool and humid, they can damage or even kill seed treated seedlings. This has been happening in Nebraska in recent years, especially with farmers incorporating animal manure or green manure into the soil just before planting.
There are several cultural tactics a farmer can use to prevent serious damage from seed corn maggots. Delaying planting until warm soil temperatures encourage rapid seed germination allows the plant to outgrow broken corn kernels. Planting for at least two weeks after adding fresh manure or other fresh organics to the soil can decrease its attractiveness to female flies.
A more accurate way to determine when to plant is to use degree-tag models. Seed corn maggots develop when temperatures are above 39 ° F. Therefore, degree-day models for this insect track the time and temperature after January 1, when temperatures are above 39 ° F. The idea is to avoid planting during the emergence of the top flies. The maximum occurrence of flies for the first three seed corn maggot generations occurs when 354, 1,080 and 1,800 degree days have accumulated.
Fortunately for us in Nebraska Mesonet, degree-day calculations are performed for several insect pests. For information on seed corn maggots, see the drop-down menu at https://mesonet.unl.edu/page/data. The location in Mesonet, Nebraska, contains other interesting weather-related data as well as information about the degree day for long-handled borer and western bean cutworm. You can also find information about seed corn maggots and other insect pests in the University of Nebraska-Lincoln’s CropWatch newsletter at https://cropwatch.unl.edu/.
