Yes, this is the title of a 1978 comedy horror film (sans the hornworm part, that was added to provide context to the article). I have a small garden at home and last week I noticed foliage and fruit chewed along with fairly large frass pellets laying around so I had all the clues needed that hornworms had indeed found at least two of my tomato plants. I began searching diligently but could only find one hornworm, I’m sure there were several more hidden among the foliage digesting their lunch.
Finding these worms at home made me think about scouting the much larger tomato research project at the Western Ag Research Station in South Charleston. Unfortunately, there is no pheromone trap to detect the moth so periodic monitoring of the plants for feeding damage or caterpillars is required. I started looking at the plants and soon discovered there were hornworms infesting about 5% of the plants.
Hornworm damage to tomato plant
Hornworm damage
While there are many types of hornworm moths, commonly referred to as hawkmoths or sphingid moths, there are only two economically important species found in Ohio that feed on tomato plants, the tomato and tobacco hornworm. The key characteristic I use to identify these two species is the shape of the markings on the side of the caterpillar; the tomato hornworm has a sideways “V” called a chevron on each segment while the tobacco hornworm has a slanted white slash that looks like a cigarette (to me). There are other characters to separate the species such as the color of the “horn” on the last segment (black vs. red) but these tend to be less reliable. The tobacco hornworm is usually more common in Ohio tomato fields than the tomato hornworm. The length of the worms found at the research station was up to 3 inches, which is near their maximum size.
Tobacco hornworm, note white slashes on segments
Tomato hornworm, note “V” shape on segments
Hornworms are often found with many small white cocoons stuck on their body. The cocoons show that biological control by natural enemies is in progress. A hornworm covered with cocoons has been parasitized by Cotesia congregata, a small braconid wasp. By the time the cocoons emerge from the larva, the hornworm is close to death and will not reach its pupal stage. One wasp will emerge from each cocoon.
Parasitized hornworm
The threshold for treating this pest is two or more hornworms or fresh damage per 40 plants scouted of any stage, from seedling to fruiting. Whether parasitoid cocoons are seen or not, the chances of biocontrol contributing to the management of this pest are increased greatly by avoiding the use of broad-spectrum insecticides. The best insecticide to use is one of the BTs such as Dipel or Javelin or Xentari, especially if worms are small. If insect pests in addition to hornworns are found in the tomato field, then conventional insecticides will be needed; most insecticides are very toxic to hornworms so the selection of the product should be based on the other pest(s). Products such as carbamates (Lannate, Sevin) and pyrethroids (Asana, Baythroid, Pounce, Warrior) are toxic to hornworms but disruptive to natural enemies like the Cotesia parasitoid. Products such as Radiant, Intrepid, or Neemix are toxic to hornworms but have a more gentle impact on beneficial insects. More insecticide options are listed in the Midwest Vegetable Production Guide (https://mwveguide.org).
Part of this article was excerpted from a previous VegNet article posted in 2009.