How Do I Treat Petunia Plants That Have Caterpillars?

Hand pick caterpillars from your petunia plants. Focus not only on the top of the leaves, but the underside of leaves and the stems, including the base of the stem near the soil. Drop them into a bucket of sudsy water to dispose of them, or use an alternate method. Check in the daytime and again in early evening, as some caterpillars feed at night or on cloudy, overcast days. Hand picking is the most effective way to eliminate caterpillars, requiring no expense and no pesticides.

Keep your flowerbeds neat and tidy. Remove dead leaves, wilted foliage, spent blooms, and other plant debris from the soil. Plant debris are common hiding places for caterpillars, as most caterpillars hide during the night, coming out to feed in the morning.

Apply a bacterial control such as Bacillus thuringiensis if other control measures aren't effective. The bacterial, which is a naturally-occurring substance, kills the caterpillars by attacking their cells and digestive system. As a result, the caterpillar starves and dies within days or a few weeks. Check with your local Cooperative Extension agent first, as different bacterial products are effective against different types of caterpillars and may not work on the caterpillars that have attacked your plants.

Encourage caterpillars' natural enemies, such as Hyposoter exiguae, which includes a variety of wasps that are effective predators of several types of caterpillars. Adult Hyposoter wasps lay their eggs in the body of the caterpillars, which hatch inside the caterpillar. Trichogramma is a tiny parasite that lays its eggs in hundreds of caterpillars. Avoidance of pesticides is the best way to encourage these helpful insects, as most caterpillars are pesticide resistant. Trichogramma can be purchased in most garden centers and nurseries.