Protecting Sweet Corn from the Corn Earworm with Vegetable Oil and Btk
Sweetcorn is one of the most popular treats from the garden and an easy crop to grow for farmers accustomed to raising field corn. Unfortunately, sweetcorn isn’t easy to grow well because of the worms that often infest the ripe ears. At least three moths lay their eggs on sweetcorn, the European corn borer (Ostrinia nubilalis, Hübner), the fall armyworm (Spodoptera frugiperda, J.E. Smith), and the corn earworm (Helicoverpa zea, Boddie). The caterpillars (larvae) of all three insects can infest the ears of sweetcorn, but in Iowa the most consistent problem is caused by the corn earworm.
The adult female moth of the earworm lays single, white eggs on the emerging silk of developing ears. A female can lay up to 1,000 eggs. The larvae hatch and move down the silk into the ear, feeding as they go. Caterpillars will spend up to two weeks in the ear feeding on the swelling kernels near the tip. They reach 1-1/2 inches in length and can be recognized by their tan head, stripes running the length of the body, and “hairs” or microspines (University of Missouri, 1999.)
Because earworm caterpillars spend most of their time within the ear, pesticide applications to the plant surface are of limited value. However, since the 1940s some growers have successfully controlled earworm by treating the corn silks with oil. In the 1990s a new twist was added. Growers spiked the oil with Btk, a protein crystal toxic to the larvae that is produced by the naturally occurring soil bacterium Bacillus thuringiensis, var. kurstaki. The oil spreads down the silks and smothers the caterpillars, and the Bt toxin is carried by the oil into the ear, where it kills any caterpillars still feeding.