Several worms can damage tobacco late in the season. Control is relatively easy, but it is important to catch infestations early – before damage increases and the small size of the caterpillars makes control easier.
The tobacco hornworm is at the top of the list in terms of damage potential. The late summer brood is usually the largest brood and can extend from mid-August and through mid-September. Hornworm eggs (Figure 1) are laid on the upper third of the plant, so early feeding holes should be easy to see (Figure 2).
Check the underside of the leaf for the hanging caterpillar (Figures 3 and 4).
Scouting for Tobacco Hornworm
Weekly field checks will allow detection of infestations that justify treatment. Look carefully at the lower surface of leaves in the upper third of groups of 5 plants at 10 randomly chosen locations in each field. Check for hornworm eggs and small larvae; record the numbers and approximate size of the hornworms that are present. In some cases, there can be damage but no worms; grackles and other predators can eat them. Hornworms with white egg-like cocoons on their back are parasitized by a small wasp. These worms will not cause any more yield loss. By late August up to 90% of the hornworm population may be parasitized.
Managing Tobacco Hornworm
An insecticide application is usually profitable if there is an average of 5 or more hornworms per 50 plants. Higher rates provide longer residual protection and usually are more effective against larger hornworms. Bt (Dipel, etc.), spinosad (Tracer), Chloranthranilprole (Coragen) insecticides are best used when most larvae are small; control may not be as good against large ones.
Hornworm moths will be flying during the next 4 to 6 weeks. A single insecticide application may not provide control from topping until harvest. It is best to check for hornworms and apply a cleanup spray if necessary to prevent carrying these insects into the barn. Check the restricted entry interval (REI) and harvest interval on the insecticide label before treating.
Yellowstriped Armyworm & Variegated Cutworm
The yellowstriped armyworm (Figure 5) and variegated (or climbing) cutworm (Figure 6) can be found feeding on tobacco, also. Usually, they are scattered on a few plants rather than being widespread. They feed for about 3 weeks. The yellowstriped armyworm is about an inch long and the variegated cutworm about 1.5 inches long when full grown.
Stink bug populations have been up this season, so feeding damage may be seen on scattered plants in a field. Saliva injected as the bugs feed cause the affected leaf to collapse and often scald from bright sun. The insects are usually gone by the time symptom appears, so control is rarely needed.
By Lee Townsend, Extension Entomologist