Hornworms

Have your ever noticed small holes in the leaves, fruit and stems of tomatoes, peppers and eggplants? Are small brown droppings present on your garden plants? If so, you may be observing the presence of the tomato hornworm.

However, as frustrating as an infestation can be, there are a number of steps you can take to control and eliminate hornworms in your garden. We’ve prepared this guide on hornworms and their life cycle to help you learn how to get rid of them. With a little hard work and the careful use of insect-killing products approved for organic gardening along with the use of beneficial insects, you can repel and prevent the tomato hornworm from eating your precious fruit before you do.

Hornworms are voracious eaters. If left undisturbed on a plant, hornworms can completely defoliate your garden vegetables in just a few days. With the larvae eating the leaves and new stems of plantings, the planting may die, although plants can typically recover if the hornworms are removed early.

However, even if the plant survives the initial leaf feast, if you don’t remove the larvae, they will move on to other, more prized fruits of your garden. Later in the season, the tomato hornworm feasts on the actual tomatoes, peppers and eggplants, making your fruit or vegetables unmarketable and unappetizing.

If you’re farming with the goal of making money, a hornworm infestation can hit your pocket hard. Even if you have a small garden, hornworms can erase hours of work and spoil the joys of a homegrown vegetable on your family table.

Life Cycle

Hornworm eggs are normally found on the underside of leaves but may also be found on the leaf surface. They are very small, translucent green spheres (1 to 2 mm in diameter). Hatching usually occurs 3 to 5 days after eggs are laid. In the wild, newly emerged larvae will immediately begin to feed on the leaf they were laid on.

Once hatched, the larvae will eat and grow over the next 2 to 3 weeks, and as they do, they will progress through 5 instars, or stages. Between each instar the larvae molt, shedding their skin to allow for new growth. In the first instar, larvae are usually about 7 mm long. By the end of the fifth instar, they can measure up to 70 mm in length. During this time of growth, larvae need to remain in constant light at 27° C (81° F). If not, once the larvae pupate, they will enter diapause, extending the pupal stage to months instead of weeks.

Toward the end of the last instar (2 to 3 weeks), the dorsal aorta will become visible as a dark line running down the dorsal surface of the caterpillar. Once this line begins to pulsate, the larva will be ready to pupate. This is your cue to prepare pupation chambers for the larvae. Transfer 1 larva into each clean vial or bottle and add an inert material, such as sawdust, shredded paper, or potting soil, to loosely cover the caterpillar. Cap the container, then wrap it completely in newspaper or foil. In the wild, larvae burrow into the soil to pupate because they require total darkness.

Once the larvae have been secured in their chambers, pupation should be complete in about 7 days. This stage marks the time for metamorphosis of the caterpillar to the adult moth. After pupation, the external features of the adult moth are apparent on the outside of the pupal case.

Emergence of the adult moths will occur 1 to 2 weeks after the pupae have been placed in the habitat. Once out, adults will climb onto a branch or other vertical surface to expand their wings. In nature these moths primarily feed on plant nectar, so provide them with a sugar water food source in the classroom habitat. The lifespan of the adult is usually 2 to 3 weeks.

Organic Tomato Hornworm Control

If you’re looking for a way to effectively go about killing tomato worm without harming other beneficial insects or normal garden worms, consider Monterey BT spray. BT, which is short for Bacillus thuringiensis var. Kurstaki, is a naturally-occurring bacteria that will poison all manner of caterpillars, including tomato hornworm, cabbage worms, cabbage loopers, cutworms, and many other types of larvae.

If you would prefer not to use a spray but still want to use BT Kurstaki to kill tomato hornworms, you might consider Garden Dust. Garden dust is a powdered form of Bacillus thuringiensis var. Kurstaki, and when it is dusted to coat the surface of plants, it works just as effectively as the spray form does.

If you are looking for an alternative choice in how to kill tomato worms, consider pyrethrin sprays. There are many variations on these on the marketplace today, including powerful pyrethrin-only options such as PyGanic. There are also blended formulas that combine pyrethrin with other insecticides, such as Safer Brand Home And Garden Spray.

Spinosad-based sprays such as Monterey Garden Insect Spray can also be an effective way to kill tomato hornworms before they can devour your plants. Monterey also produces a tomato & vegetable spray, but it has an identical spinosad formulation as their catch-all garden insect spray.

Environmental Tomato Hornworm Control

For this pest, environmental controls are absolutely key to ensuring your survival. And one of the best options is the use of beneficial insects.

Beneficial predatory wasps are a major form of tomato hornworm control. These wasps will lay their eggs inside or on top of the hornworm, and when the wasp larva hatch, they will devour the hornworm. Trichogramma wasps are the most commonly available predatory wasps, and while they won’t harm humans or pets in any way, they love to eat garden pests!

Lacewings and ladybugs are also really useful when trying to kill tomato hornworms, because they happily eat the eggs that are on the bottom of leaves. While you still need to remove already-hatched larvae by hand, you can reduce future generations that might hatch with these two friendly beneficial insects.

Companion planting can be beneficial in the fight against tomato hornworms, especially around tomatoes. Planting basil nearby will improve the flavor of your tomatoes while simultaneously repelling tomato hornworms, flies, mosquitos, and some other moths. Similarly, borage will attract bees and other beneficial insects while repelling tomato worms. Finally, marigold is a wonderful draw for all manner of beneficial insects, including parasitic wasps, and its aroma tends to repel or confuse most caterpillar species so they don’t move in.

Preventing Tomato Hornworms

One of the most effective means of tomato hornworm control is simply not allowing the five-spotted hawkmoth to reach your plants in the first place. To do that, use floating row covers over your plants until they need to be removed for pollination.

Once you have removed the floating row covers, you can use diatomaceous earth sprinkled on the leaves and stems of your plants as a repellent measure. Diatomaceous earth is harmless to humans and pets, but to all forms of caterpillars, it’s sharp and will cut up their tender bodies. It can also cause hornworms to dehydrate and die if they are sliced up often enough.

Tilling your garden at the end of the harvest season can help reduce future hornworm populations. Tilling will dig up any overwintering insect populations that might be lingering in your garden beds, leaving them exposed for birds to eat or other insects to devour. Till again in the early spring before planting to make sure you got them all.