Source: Ohio State University Extension
By Anne Dorrance, Andy Michel and Meredith Eyre
Though soil borne pathogens are usually the cause of damping off in Ohio’s poorly drained soils, seedcorn maggot infestation may cause similar symptoms. This maggot is easy to diagnose in the field because it causes very characteristic tunnels as it burrows through any plant material below the soil line (see Figure A). The maggots are small, yellowish-white, and legless (Figure B). If scouting your fields this week, you may find the larvae, or perhaps the pupae, which look like small grains of brown rice (Figure C).
These maggots may also cause mild to severe damage to the cotyledons before the plant emerges (Figure D). If tunneling and feeding is severe the plant will not emerge, resulting in a reduced stand count. If mild, the plant may recover just fine, but will have large scars on the cotyledons. Though the scarring alone does not seem to affect the viability of the plant, it indicates the seedcorn maggot is present in your field.
If the tunneling occurs in the taproot or lower stem, the seedling may attempt to send new roots out above the damaged tissue. This may or may not be successful, allowing the occasional plant to recover. Unfortunately, the more likely outcome is damping off as the plant wilts and dies. The diameter of the tunnel may range in size from the tip of a pen to the width of a piece of rice and vary in length.
The maggots are most likely to be found in fields with high organic matter, especially fields that have had cover crops or weeds recently incorporated into the soil.
No economic threshold exists and no rescue treatment is available. Growers experiencing lack of emergence, reduced stand counts or damping off caused by this maggot may need to replant.
Studies suggest seed treatments may reduce or prevent damage from this maggot and are very effective for seedcorn maggot control. While clothianidin (in Poncho) and thiamethoxam (in Cruiser) work well, our data, along with others, suggests imidacloprid (Gaucho) does not.
For a complete list of labeled insecticides, see http://entomology.osu.edu/ag/.