Q. I have a hanging basket of pink petunias that was full of flowers. They were beautiful! Little by little, it seems that something is eating the flowers. It started out with holes in the petals. Now, a lot of the blooms are completely gone. I am puzzled because the plants themselves are fine. Only the flowers seem to be affected. Do you have any idea what could cause this?
A. It sounds like the work of the tobacco budworm larva (Heliothis virescens). The larva, or caterpillar, matures at about 11/4 inches long. The larva's color depends somewhat on its diet and varies from black to pale brown, with reddish and greenish forms as well.
Tobacco budworm larvae can also damage flowering tobacco (Nicotiana spp.), geranium (Pelargonium spp.), ageratum (Ageratum spp.), snapdragon (Antirrhinum majus), roses, strawflower (Bracteantha bracteata) and many other species of flowers. They are major pests of tobacco and cotton, too.
They overwinter as pupae a few inches below the soil surface near susceptible plants. Adult moths emerge in the spring, and after mating, females lay eggs on host plants. The adult moths are pale green with brown overtones and four lighter, wavy bands on the forewing, and they have a wingspan of about 11/2 inches. Because moths are active at night, you probably would not notice their activity.
The caterpillars hatch and feed primarily on buds and flowers; they rarely feed on leaves of host plants. They can cause extensive injury if populations are high. They mature in about three weeks, and then burrow into the soil to pupate. We have two generations in our area every year, with their peak populations occurring in August and early September.
Handpicking is an effective control for small plantings, such as a hanging basket. If the caterpillars make you squeamish, wear gloves to pick them and crush them under foot. Because they are the larvae of moths, Bt (Bacillus thuringiensis) can provide control on some plants, especially petunias where they eat a good deal of the blossoms. It is less helpful on geraniums because they feed inside the buds and consume little of the outer tissue.
Bt is a bacterial disease that affects only the larvae of moths and butterflies that feed on treated crops. The target insect must ingest Bt in order for it to work, so plants have to sustain a little damage. It is very safe for people, pets, wildlife and insects that are not feeding on sprayed plants. Once the larvae get beyond a half-inch long,or if you are treating geraniums, Bayer Advanced Power Force Multi-Insect Killer (cyfluthrin) will provide better control. Cyfluthrin is a synthetic form of pyrethrins, a botanical insecticide made from the flowers of the pyrethrum chrysanthemum. Natural pyrethrins do not provide effective control of this pest.
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