Tomato Plants and Green Worms
- The oldest hornworm is a caterpillar that is green with V-shaped markings along its sides. They are very large larvae that can measure up to 4 inches in length and can wreak havoc on a tomato plant. Hornworms go through five to six instars, or stages. The first instar after hatching is a much smaller worm with yellow and white and no markings. The caterpillar has a raised bump or horn on the last section of its body, which gives the worm its name.
- The hummingbird moth is a large brown furry insect that lays its eggs in late spring on the host plant. The oval, light green eggs are attached singly to both the top and bottom of the tomato plant leaves. The eggs only need six to eight days before they hatch. The young caterpillars begin feeding immediately and will go through all the instars and be full grown in only three to four weeks. The mature caterpillar burrows into the soil to pupate, and moths emerge two weeks later. The species creates two generations each season so the process starts anew.
- Hornworms can severely defoliate a tomato plant with their feeding. It doesn't take too long in the first generation for the 4-inch worm to eat off the majority of the leaves on a young tomato plant. The damage will be noticed first in the upper leaves of the tomato plant. Hornworms leave behind dark green or black droppings but otherwise blend in far too well with the tomato foliage. The last instar of development can consume as much plant material as all the previous instars combined. A large mature hornworm will even eat the fruit.
- Check your tomato plant twice per week for hornworms. You can manually pick these large worms off of your plant and destroy them. The second generation of pupae overwinter in soil so tilling well after harvest will kill any pupae or caterpillars in the soil. You can order some prey insects such as lady beetles that will eat the hornworm. There are plenty of biological chemicals that work such as spinosad and bacillus thuringiensis, which is a naturally occurring bacteria that is a stomach poison to the worm when ingested. Whichever method you prefer, it needs to be applied early when the hornworm is young.