Tomato hornworms and their control

Our beloved tomato plants face many challenges these days. One of these is the tomato hornworm. Tomato hornworms are hugely destructive pests that can decimate tomato plants with their voracious appetites. The caterpillars are large and are capable of completely defoliating plants if not controlled. I speak from experience, as I have already picked at least ten of these devils off my otherwise healthy Celebrity tomato plant this season.  

Tomato hornworms (Manduca quinquemaculata) are the larvae of the five-spotted hawk moth. These worms can measure 4-5 inches long and have pale green bodies that easily blend in with a plant’s foliage. Tomato hornworms have ‘v’ markings along the side of their bodies, black spots, and distinctive black or red horns on their rear ends. 

They are easily camouflaged and you may only be aware of their presence once you start seeing the signs of damage. These signs include munched leaves and fruit. 

The life cycle of a tomato hornworm begins as an adult moth in the spring. This moth has a 4-5 inch wingspan, is grey-brown with yellow spots, and will lay huge numbers of oval, light-green eggs on the undersides of tomato leaves. The eggs hatch into larvae that feed ferociously as they quickly develop into mature caterpillars. When mature, the caterpillars burrow into the soil and enter the pupae stage, emerging as moths and repeating the cycle. Two generations per year are possible.

While the caterpillars themselves may be difficult to spot, the signs of their presence are much easier to identify. First, you may notice fruits with large bites out of them. Next, the leaves will have extensive chewing damage, sometimes to the point of being completely defoliated. The hornworms blend in with their surroundings so look below the damage for dark frass (the droppings produced by the larvae). Look for signs of chewed leaves starting at the top of the plant – where tomato hornworms often begin feeding from – and signs of frass further down the plant. Inspecting underneath leaves can help you find sneaky hornworms or collections of eggs.

There are a few tools to help fight against tomato hornworms. Handpicking is an effective but time-consuming method and the size of tomato hornworms makes them easier to spot, despite their camouflage. 

There are products you can spray that can help control the pest. Bacillus thuringiensis (Bt) is effective against caterpillars as it causes the pests to stop feeding and die. It works by paralyzing their digestive systems, and is most effective when the caterpillars are small. Bt is a naturally occurring bacterium and is considered safe for beneficial insects and humans. 

Another option is a liquid containing the active ingredient spinosad. Spinosad is derived from a soil bacterium and works by disrupting the nervous system of the insects that ingest it. 

A natural insecticide and repellent, neem oil, can also be effective in controlling tomato hornworms. It disrupts the insect’s life cycle and feeding habits. 

Insecticidal soap is a liquid which works by disrupting the outer layer of the hornworm, causing dehydration and death. 

The broad-spectrum insecticide, Sevin, can also be effective against tomato hornworms, but it may also harm beneficial insects. Follow label instructions carefully and apply it when hornworms are small. 

There is one cultural method which can be effective against the tomato hornworm. Since the adult moths emerge in spring after spending winter in the soil, tilling the soil at the start of the year before planting tomatoes, and at the end of the season, can eradicate pupae overwintering in the ground and stop them from emerging as moths to start the life cycle of a tomato hornworm.

I trust these strategies will help you in your fight against the dreaded tomato hornworm!

Tim Lewis is a Georgia Green Industry Association Certified Plant Professional, gardening writer, and former Perry High School horticulture instructor. He can be reached at (478)954-1507 or timlewis1@windstream.net.