Editor's note: Get Growing, written by the La Plata County Extension Office's Master Gardener Program, appears every other week during the growing season. It features timely tips and suggestions for your garden and landscape.
By Darrin Parmenter
For those of you who are lucky (and brave) enough to have the space, patience and resources to grow sweet corn, you most likely have been visited by its most common pest, the corn earworm (Helicoverpa zea). Every year, the moth migrates north, and eventually, the female moth will lay single, white eggs (as many as 1,000 total eggs) on the corn silks.
Once they hatch, the larvae mosey around until they find a nice place to feed - usually the swelling tip of the fast-growing ear. Initially, the larvae eat in harmony with each other. But as they grow up (kind of like teenagers), they get more aggressive and end up cannibalizing other lavae, until there is usually only one earworm left per ear.
The larvae typically reach 1½ inches and are easily recognized by a couple of different ways: a) tan heads with stripes along the body; b) feeding at the tip of the corn ear; or c) shrieks and groans when husking the ear. Ewwww, gross!
Control is always a challenge because the pest is greatly protected by the same thing that protects the tender kernels: the husk.
However, many researchers, growers and gardeners are seeing success with the application of refined vegetable oil on the corn silks, a technique that has been around for ages.
Apply the oil to the silks when they reach maximum length and start turning brown. A quick and effective way to do this is to use a machinery oiling can. The oil will wick down the silks toward the ear and smother the caterpillar.
If you want to step it up, try adding a water-soluble form of the naturally occurring soil bacterium Bacillus thuringiensis, var. kurstaki (Btk) at the rate of one part Btk to five parts oil. This organically accepted microbial pesticide (trade name Dipel, Thuricide, Biobit) has to be consumed by the caterpillar to work and will paralyze its digestive system. Look for Btk at our local nurseries.
Darrin Parmenter is horticulture agent for La Plata County Extension.