Marianne Ophardt: controlling tobacco budworms is difficult

For the Tri-City HeraldJune 27, 2013 

I am not immune to the unpleasing and bothersome damage caused by garden pests. Here’s what’s bugging me in my garden:

The tobacco budworm is an insidious creature that eats holes in my petunias even before they open, leaving them holey and tattered. The tobacco budworm (Heliothis virescens) is the caterpillar of a medium sized (1.5 inches across) greenish brown moth. At night, adult female moths lay eggs on the buds of garden flowers. These eggs hatch into larvae or caterpillars that nibble their way into flower buds and flower centers. When feeding is severe, the flowers may not open at all.

The caterpillar can be elusive, varying in color from light green to red or brown, partly because of the color of the flowers that it is eating. It is hard to find budworms during the day because they are hiding at the base of the plants. There is a better chance of encountering them at dusk, when they come out to eat.

It was once thought that the tobacco budworm would not overwinter here, but mild winters and a lack of deep frost have allowed it to become established in some area gardens, like mine. Control is difficult because it is resistant to most garden insecticides.

The newer synthetic pyrethrin insecticide sprays are the materials most likely to be effective.

If you would rather use an organic insecticide, Bacillus thuringiensis, also known as “Bt,” is an alternative. This a bacteria that disrupts the guts of moths and caterpillars, but to be effective, it must be applied where the caterpillars will ingest it.

The budworm isn’t the only culprit attacking my flowers. Another caterpillar is infesting the daisies and its relatives: the sunflower moth (Homoeosoma electellum). The larvae, or caterpillars, of this moth attack sunflowers and other members of the aster family.

An examination of infested flowers reveals mats of webbing in the center. When you tear apart the center, there is more webbing and caterpillar frass. The adult female sunflower moth is nocturnal and lays its eggs at the base of flowers just starting to bloom, laying about 30 eggs a day.

The eggs hatch in two to three days, and the larvae begin eating pollen and flower parts, and then eat their way into the center and base of the flower. Young larvae are yellow but later change to brown or purple with whitish longitudinal stripes. After about two weeks, they change into a moth.

Control of the sunflower moth is the same as with the tobacco budworm: sprays of Bt or synthetic pyrethrins at dusk. These should applied two more times at five days intervals.

w Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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