Kentucky grown soybeans, particularly those in the southwestern portion of the state, will be at elevated risk of damage from soybean podworm.

This is the same insect know more widely as the corn earworm. This is the case yearly because we plant so many double crop beans. However, this year due to planting delays and possibly wheat harvest delays, we may see a significantly larger portion of our soybean crop planted significantly later than in most years.

If you noticed Dr. Chad Lee’s comments on late planting in our Grain Blog he mentions two important factors that will affect the impact of corn earworm as well as general production.

Certainly the late planting date is a major problem, but the second is lack of closure in the canopy. Dr. Lee’s comments were directed at seeding rates needed to obtain canopy closure and thus some important agronomic benefits (for example, weed suppression and light interception). In addition, an “open canopy” situation is also associated with more damage by corn earworm. Apparently, corn earworms have greater survival in open canopied fields.

Additional, risk factors for corn earworm damage is the association of soybeans fields with maturing corn fields. When corn approaches maturing and begins to dry down, the plants become less attractive to female corn earworm moths looking for a place to lay their eggs.

Image 7. Corn earworm damage to soybean pod.

These risk factors are compounded by the fact that corn earworm is not easily detected in the field. The insect generally does not feed to any great extent on the foliage so it does not produce any easily seen evidence of it’s’ presence. One MUST get out into the field and look down into the canopy at the blooms, petioles and pods.

One can obtain some early warning of the presence of these pests by checking the UK-IPM web pages.

Corn earworm is one of the insects that we trap for at both Princeton and Lexington, KY. (Those of you in the southern tier of counties, especially the purchase area, may want to check the University of Tennessee traps at Jackson and Milan.)

The Kentucky graphics will illustrate to you the presence of the adult moths as indexed by these traps. Remember the moths are not your target; it is the caterpillars that do the damage. But the caterpillars will begin appearing sometime after the moth flight, depending upon temperature. Also, our graphics provide you with a view of what this years’ population size as compared to previous years, and a rolling five year average.

You should be scouting from late bloom through maturity. Late planted fields, especially those in which a closed canopy did not develop, and are located near maturing corn fields are at greatest risk.

Adults are buff to light green moths with a wingspan at rest of about 1/2". Eggs are white to pink, about 1/30" wide and laid singly. Larvae (worms) are very small to 1 1/2" in length when full grown. They are usually tan to pale green with several dark stripes down the back. However, color may be quite variable, with some individuals almost black.

These insects feed almost exclusively on pods. They eat away the pod wall and completely consume the seed.

If you are using a shake cloth in wide rows the threshold is greater than 2 worms per row foot. If you are using a sweep net in narrow rows the threshold is 9 worms per 25 sweeps on average. In both cases one should sample in at least five locations in each field and average the results.
So, just to review; factors for increasing risk from this pests are: late planting, late blooming, open canopy and nearness (especially during bloom and early pod set) to maturing corn. The greatest area of risk will be the Purchase Area and the southern tier of Pennyrile Area counties.