Dealing with Hornworms on Tomatoes

 

A large tobacco hornworm. Note the fake eyes near the horn at the tail end.
Photo Pam Dawling

Two summers ago, I wrote about Hunting Hornworms on Tomato Plants.

Here we are in July again, and here are the hornworms again! Yesterday, in two 80 ft (24.4 m) rows of tomatoes, I found 53 inches (1.35m) of hornworms! There were 24, varying in length from 1” (2.5 cm) to 4” (10 cm). Today I found even more: 42 caterpillars totaling 85” (2.2m)! They are stripping leaves and munching on the green fruit.

In our hornworm photos, you might notice ours are not the same as yours. Ours are tobacco hornworms, not tomato hornworms, but both are bad news and both attack tomato plants. Before Twin Oaks Community started here in 1967, the land was a tobacco farm. Tobacco hornworms have a red (not black) horn, and diagonal white lines, not arrowhead vees.

Hawk moth (mother of hornworms) caught in a web of a zipper spider. Photo Pam Dawling

Hornworms hatch from eggs laid by the night-flying Carolina sphinx moth or  Tobacco hawk moth. This year I did catch one of the moths, and kill it, but we still have plenty of caterpillars. The moths hatch from strange coppery pupae with pipes or spouts attached, which overwinter in the soil. Even our most vigilant caterpillar-hunting seems to miss some, which then drop to the ground to pupate. Another way to break the lifecycle is to close the hoophouse at dusk every night (and open it promptly every morning before it gets too hot), but we’ve decided not to go that route.

Hornworm with parasitic wasp pupae.
Photo Pam Dawling

Hornworms often get parasitized outdoors by a tiny braconid wasp that lays eggs in the backs of the caterpillars. The larvae develop inside the caterpillar and then the pupae develop as white rice-grain-like cocoons sticking out of the back of the hornworm. Usually our friend the parasitic wasp doesn’t come inside the hoophouse and to get parasites into the hoophouse hornworms we have to bring in parasitized hornworms from outdoors. This doesn’t work so well, because the hoophouse tomatoes are a month earlier than the outdoor ones, and the hornworm cycle is well underway in the hoophouse by the time the parasitic wasps are in action outdoors. This year we’ve found several parasitized hornworms indoors in the past few days, and we are very happy.

Hornworms have stripped these tomato leaves.
Photo Pam Dawling

Meanwhile, we are conducting hunting raids every morning. To find where the hornworms are working, first look at the upper leaves of the tomatoes. If they are stripped bare down to the ribs, that’s a good place to look. Hornworms only like the tender upper leaves. If there are intact newer younger leaves, it might mean there was a hornworm, but it’s been removed already, and the plant is recovering. Another sign of hornworms in the area is chewed fruit. Another sign is “pineapple poop” – miniature brown pineapples or hand grenades. If you see fresh poop, look directly upwards – remember the law of gravity. The size of the poop is, naturally enough, in proportion to the size of the hornworm.

Hornworm poop on a tomato.
Photo Pam Dawling
Evidence of hornworms.
Photo Pam Dawling

Having determined there is a hornworm in the vicinity, the next task is to find it. You’d think it would be easy – a big striped caterpillar like that. Not so! They are the exact same shade of green as tomato leaves. Hornworms can look remarkably similar to curled tomato leaves. The white stripes mimic the veins on the undersides of the leaves.

When I find some signs, I gaze at the area, looking for discrepancies in the pattern – bare stems with lumps on them. Usually the caterpillar is on the underside of a chewed stem, and often (but not always) they have their heads raised. When you find one, get a firm grip, pull it off the plant (they have strong legs which hold on tight), drop it on the ground and stomp on it. The skins are quite thick.

Found it! Hornworm on a tomato leaf.
Photo Pam Dawling

If I still can’t see the worm, I stand still and sway a bit from side to side, viewing the plant from different perspectives. It helps if the top of the plant is back-lit, but I do always check both sides of the row, no matter where the sun is. Knowing the signs of hornworm grazing can save you time looking everywhere. Focus your attention on where you are most likely to find them, and you will get the most success in the least time.

A large hornworm eating a tomato.
Photo Pam Dawling

Bt is an organically-approved pesticide spray that kills small caterpillars, without killing other insects. I don’t expect it to work on big hornworms. Hunting seems to be the way to go!

Our goal – tasty ripe tomatoes! These are Five Star Grape.
Photo Pam Dawling

Summer pests and diseases

Hornworm on a tomato plant.
Photo Pam Dawling

It’s August, which for us is the peak time for pest insects as well as for fungal diseases.

This year, we have been collecting Japanese beetles from our okra plants, to prevent the leaves getting stripped off. It seems to be working. Many plants can take 30% defoliation without a loss of yield, but some years, our okra plants end up as just a bunch of stalks, and you know that means almost no photosynthesis can happen. Japanese beetles are one of the insects that have an aggregation pheromone, so those commercial traps are not only gathering up your own Japanese beetles, but also attracting neighbors’ beetles. Sometimes this means no net improvement, as the newcomers eat crops before going into the trap.

Another bad summer pest here is the Harlequin bug, a brightly colored stink bug. We try to have July be “No Visible Brassicas Month” aiming to disrupt their lifecycle. We use insect netting over the fall brassica seedlings we sow in June, and we clear our spring brassicas before the end of June. In July and July we use netting over any new brassica transplants, and sowings of turnips. During August we remove netting from older transplants and sowings, on to newer ones, such as kale and collards.

Leaf-footed bug, one of the stink bugs. Photo Kati Falger

Here’s a link to an article on Stink Bug Identification and Management in Vegetables, from the Alabama IPN Communicator Newsletter. Read More

I wrote about squash bugs and cucumber beetles on our hoophouse squash in June.

Squash bug nymphs and eggs
Photo Pam Dawling

Margaret Roach at A Way to Garden has a good post on distinguishing between the tobacco and the tomato hornworm.

The tobacco hornworms, Manduca sexta, is close cousins with the tomato hornworm,

Both species eat Solanaceous crops (nightshades) such as tobacco, tomato, peppers, eggplant and potatoes. The differences are in the stripes and the horns.

We have tobacco hornworms (this land was a tobacco farm before Twin Oaks bought it 51 years ago. This year, for the first time, we haven’t had any hornworms in the hoophouse. We’ve pulled up our tomato plants in there – we only grow the earlies in there, as it’s plenty warm enough to grow tomatoes outdoors here from May to October. I might be celebrating too soon – we still have a bed of peppers in the hoophouse and I think I might have seen evidence of a hornworm at work, although I could not find the culprit yesterday.

Tomato plant badly damaged by hornworm
Photo Pam Dawling

Pictures of Tomato Spotted Wilt Virus and other tomato diseases

We’re always a bit anxious that TSWV might show up one day, but luckily it hasn’t. Meanwhile we forget what it looks like. Here’s a few photos on the Cornell Vegetable MD Online site.

And here’s a good collection of tomato disease photos on the You Should grow site.

Spotty tomato leaves. I think this is Septoria Leaf Spot.
Photo Pam Dawling

All the news is not bad! Here’s a satisfying photo:

Hornworm mother Hawk Moth trapped by a zipper spider.
Photo Pam Dawling
Zipper spider.
Photo Wren Vile