Tomato hornworms are a very common caterpillar in North America. They eat a wide variety of popular veggie plants including members of the Hemlock family, such as:
The tobacco hornworm wreaks havoc on tobacco crops.
These enormous and rather dazzling-looking caterpillars are relentlessly destructive. They eat leaves whole and also cause massive damage to stems and fruit.
Even though they are large and flashy looking, they can hide with ease in the vegetable garden because their coloring and markings tend to blend in with the crops they pillage. For this reason, you may find yourself surprised by areas of mass destruction without ever having seen one of these little devils at work.
You are most likely to find this damage beginning in midsummer, and if you don’t put a stop to it, tomato hornworms can ruin your entire growing season and your crops. In this article, we will provide sound advice for identifying and dealing with these pests early and efficiently. Read on to learn more.
What Do Tomato Hornworms Look Like?
Tomato hornworms (aka: Manduca quinquemaculata the Five-spotted Hawk Moth) are quite large. They measure between three and six inches long, and they are quite sporty with a bright green body, seven white stripes running diagonally and a prominent red or black horn at the rear end.
The parent moths are also quite large with a wingspan of four or five inches. The moths are brown or gray with zig-zagging white markings on the wings and brown or orange spots on their heavy bodies.
You may have heard these moths referred to as Hawk Moths or Sphinx Moths. They are rather impressive because they can fly very fast, and they can hover like hummingbirds.
The Life Cycle Of The Tomato Hornworm
The pupae overwinter in the soil. At this stage, they are dark brown.
In the springtime, adult moths emerge from the soil and mate. Then they lay their eggs, which are green and spherical, on the undersides of desired plant leaves.
The eggs hatch within five days and the larvae begin their above-ground life cycle. This is a multi-stage process that is completed within a month.
At the end of the month, they burrow beneath the soil and become larvae. This stage lasts for two-to-four weeks. At the end of the larval stage, adults emerge to mate and lay eggs.
This cycle is completed twice annually, so you will see new adults emerging in late spring and again in the mid-summer.
How Can You Find These Large Caterpillars?
If you notice Sphinx Moths around your garden or your porch light at night, you should begin looking for their eggs and small caterpillars amongst your crops. Catching them early is key to control.
Signs of infestation include black caterpillar droppings (frass) on the ground around your plants and the leaves.
How Can You Get Rid Of Hornworm Caterpillars?
A single hornworm can consume all of a plant’s leaves in a very short period of time. This large green caterpillar has voracious appetite and eats constantly, so they grow very large very quickly. It is possible for them to grow as large as six inches in length and one inch in girth.
Follow these tips to eradicate hornworms:
1. Check your plants every day for signs of these pests. These signs include:
- Chewed leaves
Note that when hornworms eat leaves, they usually leave the veins and only eat the flesh of the leaf. When you see this, you can be sure you are dealing with hornworms.
2. Be thorough! Scan both sides of leaves and look around on the ground for signs. Be aware that hornworm droppings are quite large. In fact, you may even mistake them for rabbit droppings.
3. At the early stage, you can just give the foliage a good, vigorous spraying with plain water to knock small caterpillars off and drown them. This goes a long way toward solving your problem.
4. In addition to a visual inspection and spraying with water, try spraying your plants with a solution of dish soap and water. This spray mixture causes the caterpillars quite a bit of distress. They’ll move around trying to get away, and you will be able to see them and catch them. Just drop them into a bucket of soapy water to finish the job.
TIP: Wear gloves when picking off hornworms by hand because they will attempt to defend themselves by spitting “tobacco juice” (dark brown liquid) onto your hand.
You can prevent hornworms returning to your garden by taking a few simple precautions.
1. When you till up your garden soil, keep an eye out for hornworm larvae. They look like small, brown torpedoes. Pick them out and drop them in a bucket of soapy water.
2. Don’t plant the same crops in the same place year after year. Crop rotation helps keep all pests off guard. Never plant any type of nightshade varieties in the same place from one year to the next. For example, when you rotate your tomato crops (for example) you should not replace them with potatoes or another type of nightshade.
3. Cover the ground around your crops in the nightshade family with black plastic mulch. The plastic helps block the emergence of the adults from the soil in the springtime.
4. Spray your garden with a natural mixture of water, cayenne powder, insecticidal soap and garlic to prevent Sphinx Moths from laying eggs on your plants.
5. Make your yard a haven for caterpillar eating birds. Be sure to set up bird baths and feeders in your yard and garden to enlist the help of these natural predators. Mockingbirds, Robins and other larger, carnivorous and omnivorous birds are very fond of big juicy hornworms.
6. Decoy hornworms with a trap crop. Planting a crop of dill may help keep hornworms off your hemlock crops. They prefer this plant, and having them gathered around it will make it easier for you to find them and eliminate them.
7. Encourage the presence of parasitic wasps a natural enemy. These wasps lay eggs on hornworms’ backs. When the eggs hatch, the wasp larvae eat the hornworm.
8. Place pop-up bird netting around individual plants. It is supposed to keep birds away from your fruit, but hornworms are so big that they cannot get through the mesh either. This solution is a bit cumbersome because these nets are rather expensive. Also, they do nothing to prevent pupae emerging from the soil in the form of moths.
9. Make wise use of Bacillus thuringiensis bt. This organic pesticide consists of a form of bacteria that is very effective in poisoning young caterpillars but has no effect on other garden fauna. The strain of Bt that is known as Thuricide is an excellent choice for controlling hornworms.
You simply mix the liquid or powdered concentrate with water and spray your tomato plants with it early on to catch hornworm caterpillars before they can do much damage.
Because this solution is caterpillar-specific, remember that it will also kill butterfly caterpillars, so apply it carefully and keep it away from your butterfly garden and individual plants that butterfly caterpillars enjoy.
10. Attract beneficial insects. In addition to predator wasps, a number of other beneficial insects lay waste to hornworms by eating the eggs. Be sure to protect and encourage lady-bugs and lacewings, as well as trichogramma and braconid wasps.
Vary Your Pest Control Methods
It is important to use a wide variety of ways to control and eliminate hornworms because they can build up resistance and learn workarounds for just about anything you do if you only do one thing. Keep them on their toes by varying your methods throughout the growing season.
A good technique is to use Bacillus thuringiensis bt early on. Give it 48 hours to dissipate and then introduce predatory insects to your garden to keep your hornworm numbers low. At the end of the growing season, be sure to rototill your garden.
This disrupts the soil and interferes with the pupae underground. Some studies show that tilling can kill as much as 90% of buried larvae. As mentioned, if you see hornworm pupae when you till, be sure to collect and destroy it.
When you see hornworms with parasitic wasp cocoons on board, collect them and keep them in an area of discarded or volunteer hemlock crops or dill. Allow the wasps to hatch and eat the hosts. Then you will have a beautiful, healthy crop of predator wasps to continue helping you year after year.