Do you have flower buds that have been damaged by some kind of caterpillar? At this time of year, that is probably the result of the tobacco budworm, Heliothis virescens, also known as the geranium budworm. This caterpillar has white stripes running lengthwise along the abdomen with numerous erect hairs on its body but is quite variable in color. They are commonly brownish but they can also be reddish, purplish, or greenish depending on what they are eating. This caterpillar grows to be as large as 1 3/4 inches in length.
Tobacco budworms do not survive the harsh winters in Minnesota. Instead, they find their way up to this state during late summer by riding on air currents from the south. Because of this, their appearance in gardens is unpredictable from year to year. Tobacco budworms are commonly found on petunias, geraniums and nicotiana while occasionally damaging roses and other plants. The larvae damage flowers by chewing deep holes into the buds. Flowers can still emerge from these damaged buds but flowers have large holes in the petals. Tobacco budworms may also eat the flower blossoms, giving them a ragged appearance.
There isn't any preventative action you can take to avoid this insect. Your best bet is to examine your flowers routinely and handpick and destroy any caterpillars you find. If you have a large number of susceptible flowers in your garden, you may also choose to use a residual garden insecticide, such as bifenthrin, esfenvalerate, or permethrin. Bacillus thuringiensis, while effective on caterpillars, does not do a good job against tobacco budworms. It works as a stomach poison, but unfortunately the tobacco budworm does not consume enough of this insecticide when it is chewing into the buds.
Published in Yard and Garden News, September 1, 2007