Horn worm, also spelled “hornworm,” is a type of worm that consumes large amounts of plants and vegetables. Two of the most popular types of horn worms are the tomato horn worm and the tobacco horn worm. Their scientific names are Manduca quinquemaculata (tomato horn worm) and Manduca Sexta (tobacco horn worm). While tomato and tobacco horn worms are not considered harmful to humans, they are extremely harmful to plants, vegetables, and landscaping. Tomato and tobacco horn worms appetites are extremely healthy, so they will nosh on your tomatoes, leaves, and fruits for hours and hours on end if you allow them to. In addition to tomatoes, tobacco and tomato horn worms are also attracted to eggplant, potato, and pepper.
Tomato and tobacco horn worms are typically 3 to 5 inches long and they have a large horn on their rear ends. This horn may look like it can do plenty of damage, but its actually pretty harmless. The tobacco horn worm has a red horn on its rear end and the tomato horn worm sports a black horn.
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If you have ever suspected that you have been bitten by a tomato or tobacco horn worm, chances are it wasn’t a horn worm that bit you. A tomato or tobacco horn worm (or any horn worm or insect for that matter) will do whatever it can to protect itself, especially if you handle it for too long. However, it doesn’t defend itself by biting. A horn worm will spit out the contents of its stomach, it will wiggle and thrash about, and it may even wrap itself around your finger, but it does not have the capability to sting or pierce the skin. It is important to keep in mind, however, that the horn worms wrap can be quite uncomfortable, so it’s best not to handle them for too long.
Another issue with handling tomato and tobacco horn worms for too long has to do with parasite infestations. While the horn worm may be infected by a number of parasites, the most common is the braconid wasp. The larva hatch on the horn worm and it feeds on the horn worms insides until the wasp is ready to hatch. The cocoons are quite visible to the naked eye and they look like raised white bumps on the horn worms body. It’s probably not a good idea to handle a horn worm that shows signs of a parasite infestation, but leaving it in your garden can be a good thing. Once the wasps emerge from their cocoons, they will kill the horn worm host then seek out other horn worms to infest.
If you have a small garden and if you don’t notice white protrusions on any tomato or tobacco horn worms that you may see, it is ok to quickly handpick the horn worms from your garden. You can drop them in a bucket of water or snip them in half. This is considered an effective method for horn worm control in small gardens. Other effective methods of controlling horn worms in your garden include: rototilling and biological treatment. Rototilling means to turn up the soil after harvest. This will destroy any pupae that may be there.
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Biological treatment with Bacillus thuringensis, or BT (e.g., Dipel, Thuricide), will kill horn worms and it is especially effective on smaller larvae. BT must be used with extreme caution because it can be harmful to humans.