Miniature Monsters
The garden was green, luscious. The tomato beds were full and beautiful. The rain the night before had been long and steady so I had not been out to the garden until late afternoon to pick some chard and tomatoes for dinner. I opened the gate, walked toward the center beds and froze. Instead of a forest of leaves on the South ends of the tomato beds there were only sticks. We were visited. It could only be hornworms - tomato hornworms - the most voracious appetites of very unwanted creatures.
Quickly, a pail of hot water and a little detergent - I had no kerosene. Garden gloves, my glasses, and a call to my husband for help. Despite their size, these worms, the color of the plant, are almost invisible, and it took the two of us a long time to be sure we got them all - 15! The worms hang on tight and wouldn't be so unpleasant to handle if they didn't squirm. But because they are such fast and steady eaters, picking and drowning is a necessity if not a pleasure.
The tomato hornworm, Protoparce quinquemaculata, is the green caterpillar of a Sphinx moth. The caterpillar is 3 to 5 inches long with 8 white stripes on each side and a black horn projecting from the top rear. The horn cannot sting and a red horn belongs to the tobacco hornworm. The tomato horn worm chews the leaves and fruit of tomatoes, eggplants, peppers, potatoes and dill-which is a good trap crop. The larva feed for about one month, molting 4 to 5 times until full development and pupation. Pupae are 2 inches long and have a curved handle on brown spindle-shaped cases. Pupation is in the soil and in the southern states it is 2 to 4 weeks until the adults emerge to lay eggs for a second generation. There is one generation per year in the north. Pupae overwinter 3 to 4 inches underground. The moths emerge in the late spring to early summer and lay their eggs.
The adult, a Sphinx moth, is large, mottled, gray or brown, with 5 orange spots on each side of its body. It has a wingspan of 4 to 5 inches, flies quickly, and is able to hover like a hummingbird. Look for the moths at dusk when they visit flowers.
The moths lay round, greenish yellow eggs singly on the underside of leaves. To try and spot the larvae before they reach full size, look for holes in leaves and fruit and dark colored droppings on foliage. It is said that spraying with water causes the caterpillars to thrash about and be located.
Controls: The best is to hand pick - early. Bacillus thuringiensis (Bt) kills hornworms by invading their digestive systems. It is most effective when the worms are small, of course. Apply it as dust, covering the plant entirely, especially the under sides. Or spray the affected plants every 10 to 14 days until the worms are gone.
Pyrethrum paralyzes on contact and must be applied directly to work. Spray the undersides as well. Usually two applications are needed 3 to 4 days apart.
Parasitic Braconid wasps, Apanteles congregatus small, white, elliptical, papery cocoons (like puffed rice) attach to the hornworms back. The worm is already doomed so do not kill it. Just remove it from your garden so the parasites can reproduce. The adult wasps are just 1/16 to 5/8 inch long and lay their eggs in the bodies of grubs and caterpillars. The larvae hatch and grow inside the host, weakening and sometimes killing it. The larvae pupate on the back of the host.
Assassin bugs and Praying mantis attack tomato hornworms.
Baltimore orioles, barn swallows, blackbirds, downy wood peckers, flycatchers, grackles, phoebes, and sparrows are some of the common song birds that relish hornworms. Moles, skinks and toads dine on these fat green caterpillars.
Preventive Steps: Fall cleanup is critical to reduce overwintering worms. When harvesting is finished, remove all plants and weeds until soil is completely bare. Cultivate thoroughly to 6 to 8 inches, looking for pupae. Two to three weeks later do a shallow cultivation to about 2 inches. Plant a winter cover crop or winter mulch 4 to 6 inches. About two weeks before planting in the spring shallow cultivate about 2 inches. Do another shallow cultivation when planting.
If hornworms were severe in the past year spray Bt on vulnerable plants every two weeks from transplanting them until blossoms form. Remember Bt does not reproduce or overwinter in nature so it must be applied yearly as pests emerge. Bt breaks down in sunlight, the powder being viable for only 7 days after application and liquid spray only about 24 hours. It has little residual effect so it must be applied when feeding larvae are present. Reapply Bt after rain.
Sources: Rodale's Garden Problem Solver, Jeff Ball. Rodale Press, 1988; Insect Pests of Farm, Garden, and Orchard, Peairs and Davidson. John Wiley & Sons, Inc.; Insects, Golden Nature Guide, Zim and Cottam. Simon & Schuster.