If your geraniums and petunias look especially sad this time of year, the tobacco budworm probably is contributing. This caterpillar feeds on the new and developing buds of flowers, causing them to either never open, or to open shriveled and with holes.
Tobacco budworms (also know as petunia, or geranium budworms) become a particularly difficult garden pest in late summer, when the adult moth is actively laying eggs. The moth will lay small eggs on flower buds, and when the caterpillars hatch they burrow into the bud and eat it from the inside. Once the caterpillars are full grown, they will drop down into the soil to pupae, emerging later as an adult moth.
Without seeing the actual caterpillar itself, budworms can be easily detected by checking for flower or bud damage. You’ll find small holes on buds, empty hollowed out buds, or brown shriveled buds and flower tips. There will also usually be caterpillar feces found on the leaves and ground around the plants. The caterpillars will occasionally eat the leaves, but most damage will be found on buds instead.
Biological insecticides such as spinosad and B.T. are usually effective on caterpillars. Both of these products should be sprayed on the buds of the plants, and will kill the caterpillars after being ingested. For geraniums, spinosad (Captain Jack’s is a popular brand) is most effective, and should be used every 7 to 10 days while caterpillars are feeding.