What's eating your petunias? - pest control

Sunset, June, 1994 by Lauren Bonar Swezey

WHY ON EARTH HAVE my petunias stopped flowering, when just a couple of weeks ago they were in glorious full bloom?" Any gardener who grows petunias is likely to ask this question come late spring or early summer.

No, it's not because you forgot to fertilize, although that may slow flowering; so can neglecting to deadhead. But in either case, you'll get at least some new flowers. When very few flowers open--and those that do are chewed--you're undoubtedly seeing the effects of the geranium budworm (Heliothis virescens or Helicoverpa virescens, also called the tobacco budworm).

This voracious 1/4- to 1 1/2-inch-long greenish or brownish light-striped caterpillar eats pinhead-size holes in buds of petunias (as well as of garden geraniums, nicotiana, and border penstemon), which prevents the buds from opening. It also feeds on open flowers and foliage, often taking on the color of the flower it's been feeding on.

Geranium budworm used to be confined mostly to milder climates, but now even gardeners in cold climates such as Denver see its effects. The moth can survive not only through relatively mild winters, but also in protected areas around buildings and on geraniums brought inside for winter.

ONE SPRAY OF BT WIPES 'EM OUT

What can a gardener do about this pesky creature? Some sources say budworm is difficult to control because the insect spends part of its life hidden in buds, but we had great success in Sunset's test garden using Bacillus thuringiensis kurstaki (BT), a bacterial insecticide that is nontoxic to mammals.

For the best control, spray BT at the first sign of damage, when larvae are small (BT isn't as effective against large ones). Look for holes in buds, chewed leaves, and black droppings. You may also see larvae crawling around on leaves or inside buds.

Apply BT according to label directions and thoroughly spray buds and leaves (top and bottom) until the solution drips off. BT doesn't kill instantly, as harsher chemicals do. The larvae stop eating within a day after ingesting the bacteria on treated buds and leaves but continue moving around until they die from bacterial infection. Additional applications may be necessary. Monitor plants at least weekly and look for new damage, which is still green on the chewed edges (the old damage is brown and decayed).

You can also reduce budworm damage by handpicking and destroying insects and infested buds.