GOSHEN, Ind. — If you were an insect, you would be looking over your shoulder at every turn. There is always something out there that wants to feed on you. For those who grow tomatoes, you probably have witnessed this first hand when you find a parasitized tomato hornworm on your plants.
It is not unusual to find tomato hornworms in September covered with white-like masses. These objects are actually cocoons of a braconid wasp. There are many species of braconid wasps, some of which feed on aphids, beetles, flies and the larva of butterflies and moths.
This is not a wasp to be afraid, for they do not sting people. Instead, the female wasp lays her eggs inside the host hornworm. Once the affected hornworms are being fed upon, they significantly cut down on their feeding. When the eggs hatch, the braconid larvae begin to eat the insides of the host. After a few days, the larva chews its way out of the hornworm and spins a small silk cocoon on the caterpillar’s body. A few days later, a tiny wasp emerges, ready to find new hornworms to lay eggs on.
In recent years, scientists have learned that there is more going on behind the scenes between the braconid wasps and hornworms. The wasps, it seems, have co-evolved with a virus, which the wasp carries in their body, and inject into the host, along with the eggs. The virus has no negative effect on the wasp, but once inside the host, it disables the hornworm’s immune system. Without the virus, the wasp eggs would be destroyed by the hornworms immune response.
Tomato hornworms can be very difficult to find on the plants because they are camouflaged so well. Ironically, a pupa covered hornworm sticks out like a sore thumb against the green background of your tomato plants. Your first instinct may be to squash the poor little thing before it eats more leaves. However, I would suggest a different strategy. Allow the infested hornworm and its prey to finish out the drama. By doing so, you will be encouraging the completion of the braconid wasps’ life cycle, and potentially reducing the number of hornworms in your crop.
— Jeff Burbrink, Extension Educator, Purdue Extension Elkhart County
For more news from Indiana, click here.