The tomato hornworm. The nemesis of every tomato grower.
There is such a sense of satisfaction that comes when you head out into your garden and pluck that first juicy, red tomato from your tomato vine. But what happens when you turn that tomato over only to find a big hole bored in its flesh?
Or, worse still, what if before you even get any tomatoes at all you find that your tomatoes’ stems and leaves are chewed down to tiny stubs that have no hope of producing fruit?
These are sure signs that the dreaded tomato hornworm has been at work in your garden. Gardeners’ worst nightmare, these destructive caterpillars can do a number on all members of the nightshade family, including tomatoes, potatoes and peppers.
But don’t despair. Every problem has a solution and the same holds true for tomato hornworms. And happily, if you’re growing a pesticide-free garden, there are plenty of organic solutions too.
Read on for surefire ways to rid your garden of these pesky caterpillars and some simple steps you can take to ensure they never find your tomato patch to begin with.
- Tell me more about tomato hornworms
- The lifecycle of tomato hornworms
- Identifying hornworms
- Signs of a hornworm infestation
- Organic control for tomato hornworms
- Barrier fabrics
- Treat with BT Thuricide
- Add some diatomaceous earth
- Try companion planting
- Plant some trap crops
- Release beneficial insects
- Follow good garden maintenance practices
Tell me more about tomato hornworms
As the old adage goes, “know thy enemy” and, when it comes to tomato hornworm control, this is particularly important. By understanding the habits and lifecycle of hornworms, you can more easily find them if they happen to already be munching on your garden and prepare an effective defense to keep them out of your garden in the future.
The hornworms you’ll likely encounter in your backyard belong to one of two species: true tomato hornworms (Manduca quinquemaculata) or the nearly identical tobacco hornworms (Manduca sexta). Both species are caterpillars of the sphinx or hawk moth, which are large, brown moths with a wingspan of 4 to 5”.
Despite their destructive nature as caterpillars, hawk moths are actually one of nature’s best pollinators. This diurnal, or day-active, species has similar flight patterns to hummingbirds and can easily be mistaken for them from a distance.
The lifecycle of tomato hornworms
Adult hawk moths emerge in early spring and begin to lay eggs soon thereafter. Ranging from white to pale green, these future tomato hornworm eggs are laid on -- you guessed it -- tomato leaves and the leaves of other members of the nightshade family. Eggs are usually laid singly on the underside of leaves; however, they can be laid on the upper side of leaves and on plant stems, too.
After about five days, hornworm eggs will begin to hatch tiny caterpillars. These minute hornworms then go through five to six larval stages as they grow larger, reaching their full caterpillar size in approximately three to four weeks.
Once they’ve reached full size, hornworm caterpillars will bury themselves 4 to 6” deep in your garden soil where they metamorphose into large, dark pupa. Inside of these pupae, magic happens as the hornworm quite rapidly morphs into an adult sphinx moth.
Metamorphosis takes about two to four weeks to complete, after which adult moths emerge to begin the lifecycle all over again, mating and laying eggs for another generation of future hornworms.
Under the right conditions, sphinx moths can easily create at least two generations of hornworms per year, which amounts to quite a bunch of hornworms in an unchecked population! Hornworm pupae can also overwinter underground, meaning that last year’s hornworm problem can become this year’s problem too if you don’t take deliberate action to stop hornworms in their caterpillar tracks.
Tomato hornworms have a distinct light green color, which is accentuated by diagonal white striping down the length of their bodies.
Ranging in size from less that 1”, for younger caterpillars, to over 4” long for full-sized caterpillars, hornworms are likely to be the largest caterpillars you’ll find in your garden.
And then there’s the horn. While tomato hornworms’ distinctive black or red horns may look scary, they’re just for show. Intended to deter predators, these fleshy protuberances won’t hurt you at all and, if you manage to touch one, you’ll notice it is quite soft.
Signs of a hornworm infestation
Hornworms will affect any member of the nightshade family. That means tomatoes, but also potatoes, peppers, tomatillos, ground cherries, eggplants and tobacco.
The most obvious sign you have a hornworm problem is the hornworms themselves. Growing up to 4” long, these behemoths of the caterpillar world are pretty hard to miss. That said, their green coloration can help them camouflage well so don’t be surprised if you don’t see any caterpillars too. They could just be hiding.
If you don’t see any caterpillars, other signs of hornworm infestation include:
- Large holes in leaves or leaves that show signs of chewing on the outer leaf margins
- Entire stems or leaves chewed down to stubs
- Holes bored into the flesh of fruit
- Dark pebble-like frass, otherwise known as caterpillar poo, on affected plants
Organic control for tomato hornworms
Even though hornworms are a frustrating garden menace, there is no reason to resort to synthetic pesticides to keep them in line. Today, there are many organic options for pest control, and many of these solutions work exceedingly well for tomato hornworms.
Below are some tried and true methods of hornworm control as well as some protective measures you can take to prevent hornworms from taking up residence in your garden from the start.
If you’re squeamish about bugs this might not be the best solution for you; however, because hornworms are so large, handpicking them off your plants can be one of the simplest, quickest and most effective solutions to a hornworm infestation.
As hornworms don’t bite or sting, you don’t need to worry about handling them with your bare hands. To locate the offenders, check around obviously damaged sections of your plants and be sure to inspect the underside of leaves. Following trails of frass, or caterpillar poo, can help you find hiding hornworms as well. It can be helpful to know that hornworms tend to congregate toward the outer branches of plants during the early morning and evening but move towards the center of plants for shade during the heat of the day.
Once you know you have hornworms in your garden, you’ll need to be vigilant. Try to police your garden once a day for hornworms, carefully picking them off your plants as you find them. While unpleasant, you can squish the offending caterpillars or, for a less messy alternative, plop them in a bucket of soapy water.
If you have chickens, you may be tempted to feed hornworms to your flock; however, chickens often don’t want anything to do with hornworms, possibly because of the high levels of solanine they contain due to the amount of nightshade plants they eat. Reptiles, on the hand, absolutely love hornworms, so if you happen to own a bearded dragon or know someone who does, share the wealth!
Hornworms can be notoriously difficult to spot in the garden, so if you’re having difficulty finding any, try spraying your plants with your garden hose. Sometimes a good blast from the hose is enough to cause hornworms to thrash about a bit and their movement can make them easier to spot.
Hornworms: the other glowworms
Did you know that hornworms glow? Well not all the time, of course, but they do glow under blacklights.
To make hornworms easier to find, try hunting them after the sun goes down with the help of a blacklight flashlight. Under the beam of a blacklight, hornworms’ usually camouflaged green flesh lights up neon, making them much easier to find.
Barrier fabrics, such as floating row covers, are an effective way of preventing hornworms and other pests from ever gaining access to your tomatoes. These lightweight, breathable fabrics are thin enough to allow sunlight and water to penetrate through to your plants below, but thick enough to keep moths and their eggs from gaining access to your plants.
To use, floating row covers should be installed over your garden beds at the beginning of the season, before insects emerge in spring. Covers can be secured to the ground with landscape staples, or with heavy objects such as pieces of scrap wood or rocks. Floating row covers can be purchased online, just search for the length and amount you need.
For a DIY option, many gardeners opt to make their own row covers with inexpensive tulle or a similar, lightweight fabric.
The problem with row covers and hornworms
While floating row covers are very effective against most insect pests, they can be less useful than other methods when it comes to hornworms. This is because tomatoes do need to be pollinated to produce fruit, which means you’ll need to remove your row covers from time to time.
To ensure pollination, after your tomatoes begin to flower, peel back your row covers a bit during the daytime to allow pollinators to do their work. While this may allow diurnal hawk moths an opportunity to lay their eggs, due to limited access, you’ll likely not have much of a problem.
Additionally, because hornworm pupae can overwinter underground and emerge from the soil in spring, if you had a hornworm problem in previous years, floating row covers may not work well as preventative measures.
Treat with BT Thuricide
Approved for organic gardens, BT Thuricide is an all-natural pesticide that is made from a naturally occurring soil-dwelling bacteria known as Bacillus thuringiensis. Available as a liquid that you can mix yourself or a premade spray, BT is exceptionally effective against many pest caterpillars, including hornworms.
As a specialized pesticide, BT only controls caterpillars and worms that ingest treated foliage. That means, when properly applied, BT won’t harm non-target species, such as bees or other pollinators. BT won’t harm humans or household pets either. That said, it is good practice to only spray BT on affected plants and never apply it to plants in flower.
During the growing season, BT should be reapplied every seven to ten days and after heavy rain to ensure your plants are well protected. BT not only helps control hornworms, but it is great at eliminating other pest species like cabbage loopers too.
Add some diatomaceous earth
Diatomaceous earth is another organic garden-approved natural pesticide that works well against hornworms. Made from the fossilized skeletons of tiny marine organisms, known as diatoms, diatomaceous earth comes as a fine powder that can be spread on garden soil or on affected plants. When insects, hornworms or otherwise, come in contact with the sharp points of these fossilized diatoms, their skin or exoskeletons can become pierced or shredded, leading to desiccation and death.
Unpleasant as it sounds for insects, diatomaceous earth is a very potent organic pest control option. Just be aware that, as a generalized pesticide, it can kill any insect, including beneficial insects and pollinators, like bees. For this reason, be sure to take special precautions when applying DE and use it as a spot treatment for existing infestations rather than as a preventative. Additionally, because DE can cause lung irritation, be sure to wear a mask when applying.
When using DE, only dust affected plants or surround the base of plants you’d like to protect and never dust any flowering plants to prevent harming pollinators. Because DE only works when dry, be sure to reapply DE after heavy rain or if your area has been particularly humid.
When properly used, DE can help control young hornworms, generally less than 2” in size; however, larger hornworm caterpillars and eggs are unaffected by it, so try to use DE in combination with other hornworm control measures.
Beyond hornworms, DE has also proven effective against other nuisance insects like ticks, slugs, cucumber beetles, aphids and thrips and is a great addition to any organic gardener’s arsenal.
Try companion planting
Companion planting is a gardening practice in which certain plants are interplanted together to mutually benefit each other. These benefits can range from encouraging pollinator activity to providing natural pest control.
Some of the best companion plants to naturally control hornworms in your tomato or nightshade beds include:
Basil not only pairs well with tomatoes when cooking, but it is also one of the best plants to grow near your tomatoes to repulse hornworms.
Hornworms naturally find basil’s fragrance to be off-putting, while it is said that growing basil near your tomatoes improves the flavor of your fruit. What’s more, basil can benefit from the shade provided by your tomatoes and is less likely to suffer from leaf scorching during the peak of summer’s heat.
Marigolds, particularly French marigolds, are some of the best plants to add to your garden for natural pest control. Marigolds naturally repel hornworms, but other pests as well, such as nematodes, cabbage loopers and mosquitoes.
Lesser-known borage is another great companion plant for repelling hornworms. Considered a medicinal plant, borage is often grown for its pretty blue, edible flowers that can be used as garnishes for salads, ice cubes or desserts. Borage’s fuzzy leaves are also edible in small quantities and have a fresh, cucumber-like flavor.
Companion planting for beneficial insects
While growing plants that naturally repel hornworms throughout your nightshade patch is one way to use companion planting, another option is to use companion plants to attract hornworms’ natural predators.
Insects, like parasitic wasps, lay their eggs on hornworm caterpillars. When those eggs hatch, the wasp larva begin to eat hornworms from the inside out. While parasitic wasps are a caterpillar’s worst nightmare, they can be useful helpers in the organic garden. To attract parasitic wasps, try planting:
- Any member of the umbellifer family. Plants in the umbellifer family are known for their feathery leaves and include plants like dill, carrots, Queen Anne’s Lace, fennel and anise. If you want to use these plants to attract beneficial insects, be sure to let them go to flower as parasitic wasps aren’t particularly attracted to their foliage alone.
Ladybugs and lacewings are also helpful beneficial insects that any gardener should want to lure into their garden. These pest eating machines are particularly useful at devouring hornworm eggs and caterpillars in their smallest larval stages. Plants that can help draw in ladybugs and lacewings include:
Plant some trap crops
Trap crops are plants that are used sacrificially in gardens to draw pests away. A good trap crop is a plant that pests will naturally prefer over the plants you’re trying to protect, which, in the case of hornworms, are your nightshades.
Trap crops work best when located near your garden beds, but far enough from the plants you’re trying to protect that you can lure pests away from your more desirable plants.
An obvious trap crop choice is to simply plant more tomatoes than you need and allow hornworms to munch on your least desirable tomato varieties. You can locate your “sacrificial” tomatoes away from your tomato beds and simply let the tomato hornworms do their worst. Because hornworms can be beneficial pollinators, this is a great option for pollinator-friendly gardens.
If you don’t want to sacrifice any tomatoes, other excellent trap crop choices include:
- Flowering tobacco
Note: If you notice any hornworms in your garden with tiny white eggs on their skin, leave the caterpillars be or locate them into your trap crops. Creepy as it may sound, those eggs are the eggs of beneficial parasitic wasps. The affected caterpillar will not live long enough to be a threat to your garden; however, those eggs will help form the new generation of parasitic wasps that will help protect your garden from hornworms and other pest caterpillars.
Release beneficial insects
While companion planting for beneficial insects is inexpensive and relatively simple to do, if your goal is to bring in lots of beneficial insects for hornworm control and you want to do it quickly, there is another option: ordering beneficial insects online.
Many beneficial insects, such as parasitic wasps, are available for purchase online and will be shipped right to your door. If you want to purchase beneficial insects, be sure to order then when temperatures are warm enough to ensure safe shipping and try to time the release of your insects to maximize their natural pest control abilities.
In the case of hornworms, try releasing ladybugs and lacewings when hornworms eggs are being laid, such as in early summer. Parasitic wasps, on the other hand, should be released when you first spot signs of infestation and when you see adult moths flying around, as this means caterpillars will soon be afoot.
Some of the best beneficial insects you can purchase for hornworm control include:
Note: Despite their small size, parasitic wasps are incredibly effective against pest caterpillars. It’s estimated that 12,000 tiny wasps can clear 500 square feet of garden space!
Follow good garden maintenance practices
The best defense is a good offense, and that holds true with gardens too. Practicing good garden maintenance can help keep garden pests, like hornworms, in check and ensure that any infestations that occurred in previous seasons don’t crop up again and again.
Don’t plant nightshades together
As hornworms are drawn to all nightshade plants, it’s always a good idea not to plant different species of nightshades together to make them a less obvious target and to help prevent an infestation from spreading between plant varieties. To this end, try planting your tomatoes far away from other nightshades, such as potatoes, peppers and tomatillos.
Practice crop rotation
Because hornworm pupa can overwinter underground, crop rotation can help control them naturally, particularly when used in conjunction with floating row covers. By using crop rotation with floating row covers, any sphinx moths that emerge from last year’s tomato beds will encounter barrier fabric instead of tasty plants if they try to migrate to your new tomato patch.
A good rule of thumb is to rotate tomatoes and other nightshades every year. This will help control hornworms, but it will also ensure other pests and pathogens from last year’s garden won’t affect your nightshades this season.
Control hornworm pupae with tilling
While there are many benefits of no-till gardening, such as natural weed control and to protect native earthworms, tilling does come in handy at times and can be particularly helpful if your garden has suffered from hornworm infestations in the past.
Tilling your garden beds in autumn or early spring has been shown to kill over 90% of overwintering hornworm pupae. This makes tilling an incredibly efficient means of hornworm control and certainly should be considered as a useful treatment option for previously infested beds.
Dispose of old plant debris
Whether you’ve had hornworms in your garden or not, a good cleanup of your garden in autumn is an essential aspect of proper garden maintenance. Not only will your beds look tidier and your spring garden prep will be that much easier, but removing old plant matter can help ensure pests and pathogens won’t overwinter into next year’s garden.
If any plants or beds experienced a severe infestation of hornworms or other plant pests and pathogens, it’s good practice not to compost that old plant matter. Diseased plant material can either be burned or bagged and thrown away to prevent pest and disease spread. If you do decide to compost this matter, however, be sure to only use hot composting methods to guarantee those pests and pathogens don’t find their way into your finished compost.
While hornworms can be a nuisance in your tomato beds, they can be effectively controlled and prevented by following basic garden maintenance tips and by utilizing various means of organic pest control.
But even though you may not want hornworms in your garden, keep in mind that, given a little time, they can turn into beneficial pollinators. So, when prepping your garden this season, or policing your garden for pests, try having a little compassion for these hungry little bugs – plant some trap crops, relocate them off your plants or simply protect your tomatoes with a bit of inexpensive tulle.
At the end of the day, your garden may actually benefit when those pesky caterpillars transform into industrious little hawk moths. So, if you don't mind a bit of damaged produce or munched on leaves, try to remember that less is more when it comes to gardens, and it might just be okay to overlook a few hornworms from time to time.