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Beneficial Insects at Work!

September 3, 2010

What you are looking at below is a tomato hornworm (notice the reddish green “horn” protruding from the end of the caterpillar) being devoured by braconid wasps.  More precisely, braconid larvae have spun miniature cocoons right on the hornworm’s body!  The braconids were laid as eggs and will feed on the hornworm until they emerge from the cocoons as adults, eventually killing the hornworm.  This is good news for gardeners, as tomato hornworms feed on members of the Solanacea family (nightshades), especially tomatoes.  Hornworms can quickly devour leaves and kill entire plants!

All of this is very exciting to me as it is more evidence of an ecological garden, organic gardening where plant predator populations are reduced by other insects rather than harmful chemicals.  In this example, the adult braconid wasps are parasitoids, meaning that they kill their animal hosts.  Braconid wasps feed on tomato hornworms, as well as, caterpillars, beetles, aphids, squash bugs and stink bugs.  These small wasps, only about a half inch long, are great garden allies.  You can attract these beneficial insects to your garden by providing nectar producing host plants.  Braconids are attracted to plants with small flowers, such as German chamomile or fleabane.  Members of the Umbelliferae family  are also great host plants for these killer wasps.  For all of the gory details and great pictures of braconid wasps parasitizing caterpillars please visit this website

In addition to waiting for braconid wasps (or lady beetles and green lacewings) to feed on your tomato hornworms, you can control these pests by hand picking them then squashing them or removing them from your garden.  Monitor your tomatoes for these pests.   As you harvest each day, search for any signs of these predators.  Hornworms will feed on leaves, stems and fruit of your nightshades, leaving dark green or black droppings behind.  Please note that you should only hand pick the tomato hornworm if they are free of the Braconid wasp cocoons!  If you see a hornworm covered in small white cocoons, like the one shown above, it is best to leave the critter alone and allow the wasps to do their work killing the caterpillar!   

Adult Hornworm, photo by Paul g. Weigman

Adult hornworms are quite unusual.  They are known as “sphinx” or “hummingbird” moths, as they feed on nectar and buzz around just like hummingbirds!  If you notice these interesting creatures on your flowers, begin checking your tomatoes for their hungry offspring.  For more information and close up photos of the tomato hornworm please visit this site.

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