When you've got hornworms (big fat green caterpillars with a red horn on one end) on your tomato plants you don't want the damage, however there are times when they are part of the process that makes them beneficial. Before you spray consider the hornworm's relationship to a beneficial wasp in the natural world.
Braconid wasps are considered beneficial bugs to your garden. They parasitize and kill aphids, flies, coddling moths, elm bark beetles, cabbageworms, hornworms, corn borers, armyworms, and other pest insects. Getting a garden in balance with nature working to protect your plants is a great way to garden in the rhythm of things. Avoiding all those chemicals makes you more healthy also.
These tiny wasps are not brightly colored like some other wasps, but generally come in black or brown with thin waists and long antennae. Their larvae are pale-colored grubs. They are rarely over inch long. The external cocoons they weave resemble insect eggs, but are actually made of silk.
Some attack the host internally, others feed from the outside of a host insect. Different species of braconids attack insect eggs, larvae, pupae, and adults.
With the tomato hornworm, the female wasps inject eggs into the caterpillar’s body. The larvae that hatch from the eggs feed inside the caterpillar until they mature and eat their way out through the caterpillar’s skin. They then spin the characteristic cocoons from which adult wasps later emerge.
Leave parasitized caterpillars with cocoons alone so the wasps can emerge to attack more hornworms.
Attract the adult braconid with nectar and/or water. It is particularly attracted to plants with small flowers, such as dill, fennel, parsley, Queen Anne's lace, clover, and yarrow. If you see caterpillars with attached cocoons, leave them so that the wasps can continue to develop.
Here's a great photo of a hornworm with the cocoons.
The die-off of bees and bats from the use of pesticides presents a challenge for our food supply being pollinated, so using natural methods to garden makes a big difference for the planet considering the dangers of pesticides.