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Three ways to control hornworms
In The Grow
By Beverly Shaw
Advanced Master Gardener, Purdue University

Q. Do tomato hornworms turn into a pretty butterfly? I won’t feel so bad squishing them if they don’t. Also, I had some moonflower plants (the poisonous kind that are related to the wild variety, which grow in pig/cow pastures). Anyway, I had some kind of green hornworm eat every leaf off every plant. How can a hornworm eat a poisonous plant and not die? - Linda W., Plymouth, Ind.

A. Tomato and tobacco hornworms are large caterpillars, up to four inches in length. The prominent “horn” on the rear of both gives them their name. They can quickly defoliate tomatoes, potatoes, eggplants and peppers. Occasionally, they may also feed on green fruit. You’re likely to notice the damage before you see the culprit. Hornworms are often difficult to see because of their protective coloring and their tendency to feed on the interior of the plant where they are protected from the sun and predators.

The tobacco hornworm larva is generally green with seven diagonal white lines on the sides and a curved red horn. The tomato hornworms have eight V-shaped marks on each side, and their horn is straighter and blue-black in color. The adult of the tobacco hornworm is the sphinx moth. The five-spotted hawk moth is the adult of the tomato hornworm. Both moths are stout-bodied, grayish-colored insects with a wing spread of 4 to 5 inches. They’re not exactly pretty butterflies but are large and striking. A quick search on the Web proved there are many excited butterfly/moth fanciers that think the hawk moths are fascinating!

If you’re interested in tomatoes and not in moths, you can control them with the following methods:

Handpicking: The large size of hornworms makes it easy to get hold of them. Once removed from the plant, they can be destroyed by squishing them, snipping them in half with shears or dropping them into a bucket of water.

Tilling: Turning up the soil after harvest will destroy any pupae that may be there.

Biological controls: Bacillus thuringiensis, or BT (e.g., Dipel, Thuricide), is also considered effective, especially on smaller larvae. Follow label directions. Natural enemies, such as the parasitic wasp that lays its eggs on the hornworm’s back, are common. If you come across a hornworm with something that looks like fuzzy pieces of rice on it, leave them in the garden so the emerging wasps can parasitize other hornworms. The larvae feed on the hornworm.

The answer to your question about why hornworms can eat poisonous plants is just that those plants aren’t toxic to them. Some birds, animals and insects can eat many plants that would be toxic to humans.

Q. Would you please give me answers to the following questions: are tomato plants self pollinating; and how does a worm get into a peach? - Lovell Caudill, Linton, Ind.

A. You can grow one tomato plant and have fruit. Tomatoes do not require another plant for cross pollination. They do, however, have heavier fruit set with help from external forces.

Tomatoes need help from bees, especially sonicating bees (vibrating wing muscles that cause the entire flower to shake), or from wind or other mechanical shaking to produce a truly heavy crop. For many years, greenhouse growers employed humans with electric vibrators (one brand name Electric Bee) to accomplish pollination. Today, these have been mostly replaced with cultured bumblebees, which do it more efficiently and cheaply.

Several larvae feed on Indiana peaches. Let’s consider two of the most common. There are several generations of the Oriental fruit moth during a growing season. The first tunnels into the end of peach twigs, causing them to wilt and die. Later generations feed on fruit and twigs. Larvae commonly bore right to the center of the fruit and feed around the pit. This can cause the fruit to drop. The wounds often exude a gummy substance, and the area around the wound rots. Damaged fruits that remain on the tree are distorted. Occasionally, the larva may tunnel into the fruit through the stem. In such instances, there may not be any apparent evidence of how the larvae entered the fruit after it is harvested.

Pheromone traps are available for this insect to monitor moth activity and effectively time sprays. Traps are placed in the inside of the tree at eye level or higher, just before bloom. For backyard trees, a single trap can be used, and an insecticide treatment is made at petal fall if the insect is detected. Commercially, sprays for the first generation should be applied six days after peak flight, which coincides with peak egg laying. This often coincides with the time for plum curculio control. Sprays for the second and third generations need to be applied three days after peak flight.

Depending on the anticipated harvest date for the fruit, sprays for the third generation may need to be adjusted or omitted in order to meet the necessary preharvest interval requirements for certain insecticides.

The female plum curculio, the most common larvae found in Indiana peaches, cuts a hole in the fruit with her mouthparts and hollows out a small cavity, then turns and deposits an egg in the cavity. She then cuts a crescent -shaped silt, which extends beneath the egg so as to leave the egg in a flap of flesh. Injury will appear as a one-eighth-inch crescent-shaped cut on the fruit.

This prevents the egg from being crushed by the rapidly developing fruit. After about five days, the larvae will hatch and burrow into the fruit. The larva is a legless, grayish-white grub with a brown head. Its length will be about one-third inch when full grown.

When the larvae are fully developed, they will leave the fruit through clean exit holes. No frass or webbing will be evident. See the answer above for control measures.

Published in the December 14, 2005 issue of Farm World.

12/13/2005