With flowers as bountiful and bright as the sunny days they love, petunias (Petunias spp.) symbolize summer. Whether grown as perennials in U.S. Department of Agriculture plant hardiness zones 10 and 11 or annuals elsewhere, they bloom for months at a stretch. When a petunia's flowers unfurl with hole-riddled petals, however, the plant is the victim of a tobacco budworm attack. Budworms are one of three petunia-infesting insect larvae. Each damages the plants differently, but organic measures are available for eliminating all three.
Tiny tobacco budworms hatch from eggs deposited on petunia leaves and buds by night-flying Helicoverpa virescens moths. On petunias, their camouflaging coloration is light green.
Budworms typically feed on the bases of the buds. Symptoms of an infestation include:
- Ragged petals
- Round holes at the bases of the flowers
- Infested buds failing to open
- Black specks of waste on the buds and leaves
Remove tobacco budworms from lightly infested petunias at dusk, when they emerge from their daytime hiding places around the base of the petunias to feed. Handpick them from the plants and drown them in soapy water.
If handpicking is too tedious, spray with ready-to-use, microbial Bacillus thuringiensis var. kurstaki or spinosad pesticide. Worms that eat the pesticides stop feeding immediately, but may stay on the plants for several hours before dying.
To protect honeybees, spray spinosad when the bees aren't likely to forage for at least three hours; late-evening, nighttime or early-morning spraying is best. Bt doesn't harm bees. Treat the petunias during calm, dry weather when no sun is hitting them.
Shake the bottle vigorously and coat the petunias until all their surfaces drip. Reapply after rain or at the pesticide brand's recommended interval, until the infestation subsides.
Wear protective clothing, waterproof gloves, safety goggles and a respiratory mask and follow the label's instructions when working with pesticides.
Named for the light-yellow spots running down the tops of their bodies, variegated cutworms (Peridroma saucia) chew large, irregular holes in petunia leaves. They feed only at night or in very cloudy weather.
During daylight, the cutworms hide in the mulch or soil around the base of the petunias, where their brownish-black color is difficult to spot.
After dark, search the petunias by flashlight and pluck the cutworms from the plants. During the day, use a trowel to scrape a trench in the soil 2 or 3 inches from the stems. The worms curl into a C-position when uncovered. Drown them in soapy water.
For hands-off cutworm control, spray the petunias with Bacillus as you would to eliminate budworms.
Yellow serpentine leafminer worms (Lyromiza trifolii) are the larvae of tiny, yellow-and-black flies. The chances of ever seeing one, however, are miniscule. The female flies insert their eggs into petunia leaf tissue, where the worms create winding, whitish tunnels as they eat their way toward pupation.
Leafminers rarely do more than cosmetic damage, but a heavily infested petunia may experience slowed growth or loss of its affected leaves.
Chemical pesticides have little effect on leaf-protected leafminers. To control them, pick off and dispose of infested leaves at the first sign of the worm's trails.
Plant annual dill (Anethum graveolens) or perennial yarrow (Achilleum millefolium) -- hardy in USDA zones 4 through 9 -- near your petunias. They attract predatory wasps that feed on leafminers.
Control the leafminers with spinosad, applied as you would to manage budworms.