By Will Summers, Washington County Master Gardener
There are lots of interesting but damaging insects in our gardens this time of year. One of the most fascinating is the tomato hornworm (Manduca quinquemaculata). There is a similar, more common species that also attacks tomato plants, the tobacco hornworm (Manduca sexta). These relatively enormous insect larvae are highly destructive, but also one of the most misunderstood garden insects. At full-size, this large, bright-green caterpillar reaches a length of about four inches long before completing its life cycle and becoming a reproductive adult moth. Usually, tomato hornworms complete two life cycles each summer. The hornworm gets its name from the large pointed posterior spear. This “horn” is entirely harmless but no less intimidating to most new gardeners.
Hornworms not only feed on tomato plants, but all related species such as pepper, potato, eggplant, and a few local weeds belonging to the nightshade family. The tobacco hornworm is more common in our area. The tobacco hornworms have a curved, red horn with seven, white, diagonal stripes on its sides. The tomato hornworm horn is straight and dark blue or black and has eight lines on its sides which form the shape of a “V”.
The mottled, striped, green caterpillar is difficult to spot in healthy tomato plants. Most gardeners do not see the caterpillar until a large amount of damage is done. Hornworm damage is hard to miss. Large patches of tomato leaf ends will be missing. Hornworms defoliate tomato stems as well as eat the fruit. If you suspect that you have tomato hornworms and may be unsure if actual grazing has occurred, look for wrinkled, black or greenish fecal pellets on the ground or lower leaves. The larger the pellets – the larger the hornworm. At a maximum, these wrinkled pellets reach the size of a small pea.
The best way to control hornworm infestation is to remain vigilant and hand-pick the caterpillars when damage first appears. Remember, hornworms start small and hide under the leaves, but mature to full size in only three weeks. Be prepared for a hard pull to remove the worm from a tomato stem when you find one, but the spiked horn will never harm you. Expect that the insect will also defecate and/or regurgitate a thick, slimy paste as its worst and only defense. The tobacco hornworm absorbs nicotine toxins, if feeding on tobacco, which makes its slime toxic, and birds even avoid eating the worm.
The tiny Braconidae wasp is the natural enemy of all hornworms. This wasp lays its egg in the hornworm and upon hatching, feeds and paralyzes the hornworm. Upon maturity, the wasp larvae hatch and exit the hornworm body in large numbers and attach themselves to the outside of the hornworm as small, white, oblong cocoons. The cocoon hatch into many more braconid wasps. It is not a good idea to remove any hornworm showing these white braconid wasp larvae.
Another effective hornworm control is to encourage birds. Locate your bird bath adjacent to your tomato garden. Try to have at least two bird houses in the nearby. Both the house wren and the Carolina wren are aggressive caterpillar hunters, especially when feeding their young.
Hornworms are difficult to see against a background of tomato vines and leaves. An ultraviolet light causes hornworms to glow in the dark. Hornworms show up iridescent under the glow of a black-light if any are present. Be sure to look for the small, early caterpillar instars of the larval caterpillar.
However, accept that you may have missed one or more hornworms and that the mature caterpillar will drop and burrow into the ground, form a cocoon, and return to feed again on your tomatoes. This is additional justification for not growing tomatoes in the same location next year.
Few people realize that the tobacco hornworm is the juvenile offspring of the Carolina sphinx moth, and the tomato hornworm is the juvenile of the five-spotted hawk moth. These familiar, early morning – late evening, pollinating garden insects, hover over our garden’s flowers and vegetable blossoms. Consider sacrificing some tomato crop for the benefit of providing more pollinating services by the adults.
Please take the opportunity to educate children on this strange and peculiar insect animal that can be fun to learn about. For more information on any garden pest, please visit your local public library or ask your local Master Gardener or visit the office of your nearby University of Illinois Extension Service.
