Garden Villain: Tomato Hornworm

End of the Tomato Season

I’m so sad.  I returned from a Labor Day weekend trip to discover that tomato hornworms had decimated my tomato plants.  In the space of five days, they ate virtually every leaf on eight mature tomato plants.  Only one plant, in a separate raised bed, escaped with minimal damage.  What are these terrible creatures and is there anything to do to control them?

What are Tomato Hornworms?

Tomato hornworms are the caterpillar stage of the hawk moth (Manduca quinquemaculata).  The latin name is as dangerous to spell as the hornworm is to the survival of tomato plants.  They feed on members of the Solanaceae plant family:  tomato, pepper, eggplant, potato, and tobacco.  The caterpillar gets the name ‘hornworm’ from the nasty looking spike or horn on the rear of its body.

Life Cycle

The adult moths lay eggs on the leaves of the host plant (my cherished tomatoes, boo!) which hatch in 2 to 8 days.  The larvae, the tomato hornworm caterpillar, then feast on the leaves of the plant.  They completely defoliate plants and even consume parts of the fruit.  It takes about 3-4 weeks for the larvae to completely mature.  Then they fall to the ground and make cocoons in the soil.  This stage can last for a few weeks to months, depending on the time of year.  They can overwinter in the soil and in the spring the moths will appear and start laying eggs again.

How to Control

Prevention:

1. Rotate where you plant your tomatoes – Because the larvae make their cocoons in the soil, it’s a good idea not to plant tomatoes in the same place every year.  Rotating crops also helps prevent some diseases and doesn’t deplete soil nutrients.
2. Rototill or cultivate soil well before planting – By chopping up the soil before planting, hopefully any cocoons in the soil will be destroyed preventing moths from appearing.

Once they appear:

1. Handpick – Pick them off when you first see signs of damage, which can appear as leaves that are completely eaten or as black waste droppings (also known as frass) laying on the ground.  Destroy the tomato hornworms by cutting in half (kids love to do this, use safety scissors), or stomp on them.  Be aware that they sort of explode green gunk all over!  Note they are hard to spot because they are the same color as the leaves and stems of the tomato plant.
2. Spray with Bt or Spinosad – Bt (Bacillus thuringensis) is a natural bacteria that controls leaf eating insects.  Note that Bt kills all caterpillars, including those which become butterflies.  So use only on plants where damage from hornworms is occurring.  Spinosad is another organic pesticide which can be use to control tomato hornworms.  With all pesticides, follow directions exactly, wear protective gear when using, store and dispose of properly.
3. Encourage natural enemies – If you see a tomato hornworm that has what looks like grains of white rice sticking out of its body, do not destroy.  The white things are the eggs of a parasitic wasp which will feed on the hornworm when they hatch.  How cool is that?!

So for this year, my yummy tomatoes are done.  Until next summer, I’ll just remember fondly the bruschetta I made on the Labor Day vacation!