You get up one bright and shinning morning and decide to go out and pick some fresh tomatos... only to find this horrific sight: Your formerly beautiful tomato plants have been ravished by giant, four inch long worms. This bad dream can be a reality in many parts of North America, and unfortunately our garden was recently introduced to this menacing creature -- the Tomato Hornworm and Tobacco Hornworm (Manduca quinquemaculata L. and Manduca sexta L., respectively).

Background

At four inches long, Tomato Hornworms and Tobacco Hornworms are large, fat, and green worms with five pairs of prolegs and have a distinctive 'horn' on their rear. Hornworms are known to cause extensive damage to tomato plants, although it won't shy away from taking big bites out of your pepper, eggplant, potato plants (Solanaceae family, i.e. nightshade family) and occasionally green fruit as well. They eat the leaves of the plant and may nibble on green unripe fruit but they typically won't burrow and seldom bite ripening fruit. Hornworms are one of the largest caterpillars in America and are impressive in size and bulk. The only thing more impressive than their size is their appetite--hornworm can make quick work of your plants overnight.

Closely related to the Tabacco Hornworm, the Tomato Hornworm is known by a number of other variant names (tomato worm, tomato horned worm, green tomato worm, tomato fruit worm, etc). Tomato Hornworms and Tobacco Hornworms are often confused as they look similar and it is not uncommon to find a Tobacco Hornworm on a tomato plant and a Tomato Hornworm on a tobacco plant. You can distinguish the two worms fairly quickly: the Tobacco Hornworm has seven diagonal white stripes and a red horn while the Tomato Hornworm tend to have V-shaped marks on each side and their horn is straighter and typically blue or black in color. Tobacco Hornworms tend to prefer the southern United States while the Tomato Hornworm tends to prefer the northern US. While extremely large and slow pests, they are frustratingly difficult to spot due to their shape and color and tend to hide on the underside of the plant. Hornworms also have distinctive droppings which are large and black and tend to accumulate on the ground underneath the plants they are infesting.

So where do these giant green tomato devouring worms come from? They are the larvae either the Five-Spotted Hawk Moth (Tomato Hornworm) or the Sphinx Moth (Tobacco Hornworm). Hawk moths lay their eggs on the underside of the plants leaves where they hatch and eat at the leaves and the green fruit of the plant. Disliking direct sunlight hornworms spend their days eating on the interior of the plant, emerging to feed on the outer leaves at night (thus they are most easily spotted at dusk or dawn). Hornworms have five larval stages, after which the caterpillars typically enter the soil to pupate, only to emerge as a large moth. Hawk moths, with a large wingspan (3.5cm to 15cm; Five-Spotted Hawk Moths are typically 10-13cm / 4-5 inches), are some of the fastest known insects, capable of flying over 30 miles per hour. Some hawk moths are even capable of hovering while they consume nectar from flowers, much like a hummingbird would. Spotting the moth of hornworm caterpillars is difficult because they mainly travel at night.

Frequently hornworms are found with a number of white cocoons attached to its. DO NOT KILL THIS TOMATO WORM! These white sacks are not the caterpillar's eggs--quite the opposite!--the white cocoons are the pupae of the Braconid Wasp. These wasps are parasitic insects that prey on hornworms. These wasps hunt down our garden pests, inject their eggs into their prey where the eggs hatch into larvae and begin eating the internal organs of the hornworm. After these "maggots" have matured they bore through the skin of the hornworm and proceed to spin a cocoon and attach themselves to the worm. From the cocoons emerge adult Braconid Wasp which will begin hunting for other Tomato Hornworms and Tobacco Hornworms to feast upon.

Hints on Spotting Hornworms

- Search for the tomato worms at dusk & dawn. Tomato worms dislike the heat of direct sunlight and eat from the underside of the leaves during the day; they emerge as the sunsets. Due to their color and shape they are very difficult to spot when in plain view, so hunting for them while they are hiding makes them extremely difficult to find.

- Droppings. Hornworms are voracious eaters and produce a lot of large, black droppings underneath the plant they have infested. The droppings look almost like little black grenades. Follow the trail and you are likely to find a hornworm.

- White pupas. Look for these on the ground or on apparent "branches". These are likely parasitic wasp cocoons and can help you find tomato worms.

- Try a blacklight. Some have reported great success hunting for the worms at night by using a black light to help spot the worms.

- Follow the destruction. When all else fails, follow the path of destruction in your garden.

Eliminating this Pest from your Garden

- White Cocoons = Don't Kill! Leave the hornworm where it is. It is most likely fairly developed and already done most of the damage it can. The white sacks attached to the worm are Wasp which will soon emerge to hunt down and kill other hornworms. This is Yahweh's natural biological "insecticide".

- Till your soil. Before you plant your tomatoes rototill your garden soil as this will kill the eggs that were laid in the soil in the winter months. This will prevent moths from emerging in your soil and immediately assaulting your garden.

- Pick them off. Using the tips above to spot them, remove them with your hands or a stick. If they lack white cocoons you can cut them in half with garden shears, drowned in a bucket of water, squished, or fed to the birds.

- Chickens. Some gardeners have reported that bantam chickens will hunt for the worms and remove them from your plants.

- Natural bug repellants. Some have reported that red pepper (dust or liquid) can deter hornworms; others have reported that a mixture of water, vegetable oil, and liquid ivory soap sprayed onto your plants may help make your plants unsatisfying to tomato worms.

- Insecticides. Your garden can be made Hornworms-resistant by the use of chemicals like bacillus thuringiensis, carbarl, permethrin, spinosad, sevin dust/liquid, dipel dust, etc (read the label carefully and do some research before using). Bacullus thuringiensis (BT) is an insecticide that attacks the digestive system of some insects and is not typically considered harmful to humans.

- Insect Collection. Collect the worms (either pull them off or snip the branch they are on) and put them in a jar with a lid with small holes. It will form a cocoon and emerge as a large moth. Your children may enjoy this science experiment! It is also rumored that they are very aggressive and when placed in a closed container with other worms they will fight.

References and Helpful Links

Tamu.edu

Associated Content

Colorado State

KSU

Can You Hear Me Now

Wiki: Tomato Worms

Wiki: Tobacco Worms

Wiki: Parasitic Wasp