Insects Affecting Plant Stands

Bryan Jensen, UW Department of Entomology and Division of Extension

This spring has been difficult.  It never ceases to amaze me how hardy corn and soybean seed and seedlings are.  Because of this prolonged cool/wet weather I am sure a lot of people will soon be in the field evaluating stands.  Please, keep in mind that some insects can also affect plant populations.  The longer it takes for seeds to germinate and seedlings to emerge means the greater the plant’s susceptibility to insect damage.  When evaluating plant stands it will be important to determine if insects are at least part of the cause for poor plant stands.  There are some clues you can use to verify.

Seedcorn maggot injury is usually randomly distributed in a field.  Symptoms in corn and soybeans includes a range of damage that includes poor emergence or holes in the cotyledon(s).  Dig up the seed if you have poor emergence.  Because of their short generation time and your response time you may, or may not, find the maggot present.  Finding maggots is sound seed is a good sign of seedcorn maggot feeding because saprophytic maggots (non-pest) usually don’t infest sound seed.  Conversely, if the seed is rotten and maggots are found there is a greater likelihood that something else killed the seed.  Because of the brief peak adult flight periods, you are likely to find a narrow range of planting dates that are affected by seedcorn maggot.

Similar to seedcorn maggots, wireworms will also feed on the ungerminated corn seed.  However, unlike seedcorn maggot, their damage is usually clumped within a field.  Above ground symptoms can either be holes in the newest emerging leaves and/or wilted whorl leaves depending if the larvae are feeding at or above the growing point.  Timely scouting will usually result in locating 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 (usually) 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 and will not be easily found.

Don’t focus on a single symptom.  Rather look for other clues that will verify your diagnosis.  Although rescue treatments are not available, verifying their presence can explain reduced plant stands.

Seedcorn maggot injury on soybean
Seedcorn maggot injury on corn

Seedcorn maggot injury to soybean and corn.  As long as the growing point is healthy there is minimal, if any, long term damage.

Wireworm larvae