Sunflowers and Moth Control

The sunflower moth (Homoesoma electellurn) lays its eggs on the flower heads, and the larvae, commonly called caterpillars, feed on the developing seeds. A larva bores a hole near the top of the sunflower seed and consumes the entire kernel then leaves by the same hole. Small areas of silken webbing on mature seed heads indicate that the larvae are feeding on the seeds.

Larvae Description

Sunflower moth larvae are 1/2 to 3/4 inch long with brown to black stripes on top and yellowish-green on their undersides. Each larva typically eats about nine seeds in three weeks and a severe infestation can remove 30 to 60 percent of a seed head.

Checking for Danger

To figure out if there are so many sunflower moth larvae on your sunflowers that you need to do a chemical treatment, you need to determine the severity of the infestation. There are two ways to do this. You can randomly select sunflower buds and count the moth eggs in the bracts on the outer whorl. These eggs will eventually become larvae. Twelve larvae on one seed head is regarded as a serious infestation. You can also set commercially available pheromone traps to see if there are sunflower moths in the area.

Biological Control

Bacillus thuringiensis, also called Bt, a soil-borne toxin, is an organic solution to kill the larvae of butterflies and moths, including sunflower moths. It will not kill adult sunflower moths or their eggs or pupae. You can buy Bt powders that you mix with water at most garden supply centers. University of California's integrated pest management program recommends spraying Bt on the caterpillars when the plant is in the shade. It typically takes several hours for the caterpillars to stop feeding and two or three days for them to die. Bt only kills smaller caterpillars, so you should make repeat applications one or two days apart to eliminate newly hatched larvae.

Applying Insecticides

Use an insecticide containing the active ingredients carbamate, endosulfan or methidathion, applied according to manufacturer’s directions. Apply insecticides when the sunflowers begin to shed pollen; this is when the eggs of the moths have hatched and the larvae are beginning to grow but before the seeds have formed. The larvae on the developing seed head are then exposed. If you wait too long, the hard exterior of the seed head will block insecticide from reaching the larvae. Apply an insecticide early in the morning or late in the afternoon to minimize the effects on bees and pollinators that are active in the sunlight.