IPM and UW Extension
A lot of corn has, or will soon start to emerge. Over the next few weeks crop advisors will be evaluating stands for emergence. While there are many causes for poor emergence, seed corn maggots and wireworms may be possibilities. Below are some troubleshooting observations for each species.
Seedcorn maggot: There are several generations of seedcorn maggots/year. The first generation peak adult flight occurs at approximately 360 degree days (Base 39 o F) and usually causes most of the damage to corn planted during the “normal” Wisconsin planting season. That peak adult flight, for southern WI, occurred in late April. The second generation peak (1080 dd) is likely to occur within the next 1 ½ – 2 weeks . Late planted corn, sweet corn and especially soybean will likely be affected by the second generation.
The seedcorn maggot adult is a fly and about ½ the size of a common housefly. However, it is the maggot which causes crop damage and they are a cream-colored, legless maggot. Adults will be attracted to recently tilled fields and green/livestock manure to lay eggs. Maggots will feed on the seed and the unemerged shoot. They will not feed on emerged foliage. The cooler weather we had after the first generation peak will likely increase the amount of damage because of longer exposure during the susceptible (below ground) stage.
Seedcorn maggot injury is usually random within the field. Symptoms will include poor emergence and holes in the cotyledon (first leaf) and perhaps the second true leaf. Once the shoot is emerged, that plant is unlikely to have economic yield loss. Dig up the seed if you have poor emergence. You may, or may not, find the maggot depending on your response time when compared to their short generation time. Finding maggots is sound seed is a good sign of seedcorn maggot feeding because saprophytic maggots (non-pest) will not infest sound seed. Conversely, if the corn seed is rotten and maggots are found there is a greater likelihood that something else killed the seed and the saprophytic maggots are only feeding on a rotten seed. When in doubt, navigate to the UW Extension Ag Weather Site, http://agwx.soils.wisc.edu/uwex_agwx/thermal_models/index and click on the Generic Degree Day Calculator. You can print daily and accumulated degree day information for seed corn maggot to see if the planting date may have coincided with the adult flight for your area.
Wireworms: Like seedcorn maggots, wireworms will feed on the ungerminated seed. However, unlike seedcorn maggot, their damage is usually clumped within a field and will have different above ground symptoms. Above ground symptoms can be either holes in the newest emerging leaves and/or wilted whorl leaves. Wireworms have an extended life cycle depending on the species and may last for several years. Timely scouting will usually result in finding wireworm larvae near some of the damaged plants. Wireworms are hard-shelled, copper colored and have three sets of jointed legs. Don’t confuse wireworms with millipedes which are a non-pest. Milipedes are dark-gray and have a fringe hair-like legs the length of their body. Wireworm will move deeper within the soil profile during warmer weather. Millipedes will not. Millipedes typically feed on organic matter and are more common during wet growing seasons.
There are no rescue treatments available for seedcorn maggot and wireworms.
Seedcorn maggot injury symptoms
Wireworm larvae. Photo Credit: Frank Peairs, Colorado State University, Bugwood.org