Caterpillars Infesting Mississippi Field Corn

There are several caterpillar pests that commonly infest field corn in Mississippi.  These include corn earworm, fall armyworm, and southwestern corn borer (Figure 1).  In field corn, the southwestern corn borer is the most economically important of the three.  Transgenic Bt corn hybrids are very effective in controlling this insect pest.  In non-Bt corn there are several insecticides that are effective against this pest, however application timing is critical.  Insecticides must be applied before larvae bore into the stalk.  The previous treatment threshold for southwestern corn borer was based on egg and larval counts.  However, this is a very time consuming process and the eggs and larvae are difficult to find.  Based on research conducted at Mississippi State University, a new treatment threshold was developed based on pheromone trap captures.  This threshold is detailed in Figure 2.

Fall armyworm typically infests field corn later in the season.  This insect does not overwinter in Mississippi and must migrate from south Texas, Florida, and areas further south each year.  Bt corn provides good to excellent control of fall armyworm. However, even in non-Bt corn, fall armyworm infestations generally do not warrant insecticide applications.  The treatment threshold for fall armyworm in whorl stage corn is 100% infestation or every stalk infested.  It is not considered to be economical to treat fall armyworm infesting corn ears.

Corn earworm infests corn in Mississippi annually.  Infestations may occur during both the whorl stage and the reproductive stage.  Depending on the technology, Bt corn provides low to good control of this pest.  In non-Bt corn, the treatment threshold for corn earworm in whorl stage corn is the same as fall armyworm, 100 % infestation.  In sweet corn, corn earworm can reduce quality of ears for fresh market and processing and insecticide applications on a frequent basis are required.  However in field corn, treating corn earworms infesting ears is not considered to be economical.  Typically, corn earworm feeds on the tip of corn ears where the smallest kernels occur.  These kernels due to their smaller size and lower weight are more likely to be discharged from the combine with the cobs and leaves.  Several multi-year studies supported by the Mississippi Corn Promotion Board using natural infestations and artificial damage indicate that corn earworm infestations do not impact field corn yield and treatment for corn earworm infesting field corn ears is not recommended.

Figure 1. Southwestern corn borer, fall armyworm, and corn earworm.

Figure 2. 2014 Southwestern corn borer treatment threshold.