Wednesday, September 14, 2011

tomato hornworms

This year I became acquainted with a new garden pest, the Tomato Hornworm. This large bug is actually a caterpillar that chomps away on tomato plants, in a perfectly green disguise. My first clue they existed was actually the frass (bug poop), because it is dark and stands out against the leaves. The ‘horn’ is fake, and meant to scare predators.

These bugs can defoliate an entire tomato plant overnight, so let’s discuss prevention and control. One way to prevent these pests is to cultivate the top few inches of soil wherever tomatoes have/are being grown. This will expose the pupa to predators, like birds. It’s helpful to have a birdbath in the area where your tomatoes are growing.

Another environmental control are parasitic wasps (Braconidae) that lay eggs, hatch, and spin cocoons on the backs of the caterpillars that end up looking like grains of rice. These tiny wasps slowly eat the caterpillar as they grow into adults. If you find a caterpillar covered in these cocoons, place the bug and some leaves in a jar with large-holed screening to enable the wasps to escape and infect more caterpillars.

I have only seen a few of these caterpillars (each with the parasistic wasps present), but should the issue become worse, the next phase of control is targeted insecticide. Bt (Bacillus thuringiensis kurstaki) is a naturally occurring bacterium which targets caterpillars specifically. After eating the Bt sprayed leaves, they lose the ability to feed and die within a few days.

Bt is one of the safest insecticides on the market, sold under the names ‘Dipel’, ‘Green Step’ etc. and is certainly preferable to broad-spectrum insecticides. Lady Bugs and other beneficial insects will not be affected, and neither will the birds that eat caterpillars who have consumed Bt.

Some plant damage on plants has to be acceptable when gardening in an eco-friendly way; the Tomato Hornworm is after all the larvae of the Five-Spotted Hawkmoth (Manduca quinquemaculata). But, if you are dependant on your tomato crop for food, spraying Bt every few weeks will prevent crop loss and is an environmentally sensitive means of pest control.

The image above is a Tomato Hornworm parasitized by wasp larvae.

1 comment:

  1. Hello! I am not quite sure how I have come to find your blog but it is very nice! I too have these hornworms. I take off the green ones with no cocoons and put them in a jar with some leaves to show customers at the garden center where I work. Many, many times, the cocoons appear in a few days so now I never kill these caterpillars for fear I may kill the wasp larvae. If they have no cocoons on their back, they come off the plants but go into a jar to insure the wasps survive. Nature is quite amazing!