Answer: You're dealing with the geranium bud worm, which also attacks petunias and other plants. The geranium budworm is a difficult insect to control with insecticides because it is inside the bud, safe from most insecticide sprays. You can try to control the budworm by a bi-weekly spraying of the unopened geranium buds, nearby stems and leaves with Orthenex, a systemic insecticide. When spraying petunias cover the entire plant. The spray will protect a plant for about two weeks.
Insecticides containing Bacillus thuringiensis/Bt (Thuricide, Dipel, etc.) are effective biological controls when used on some plants. However, the insect must eat the Bt in order for it to be effective. On plants such as geranium, where the caterpillars drill into the buds and eat little of the outside surface, Bt is not effective. On petunia, where caterpillars eat a great deal of the blossom, Bt can provide a marginal amount of control.
Best wishes with your garden!
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