Hornworms can be found throughout North America. This particular insect does not usually reach economical damaging levels in crop fields. That being said, a large number can spontaneously appear in home gardens.
Reproduction and Life cycle
During late spring the eggs of hornworms are deposited on both the upper and lower surfaces of host plants leaves, taking 6 to 8 days to hatch after being laid. During the larvae/ caterpillar stage, hornworms undergo 5 to 6 instar or molting stages. It takes about 3 to 4 weeks for the caterpillar to reach the final instar stage and full maturity. At this point the fully-grown larvae drop off the plant and burrow into the soil to pupate, also known as completing their metamorphosis process to become a moth. Throughout the summer months, moths will start to emerge from the pupae stage, mate, and begin laying the next generation of eggs on host plants. If there are still hornworms in the pupae stage, they will remain their all winter and emerge the following spring.
Hornworms Identification and Habitat
The eggs are smooth, oval, and yellow to light green in colour. The hornworm larvae/ caterpillars are pale green with black and white markings on them. In the first instar stage they are yellow to white with no markings and in later instars stages they develop 8 white, lateral “V-shaped” markings. Finally they have a black “horn” sitting at the end of the last abdominal segment of their body. During the last instar stage the caterpillar is about 3 ½ to 4 inches at full maturity. When the adult hornworm moths emerge from their cocoon, they have large bodies that are gray-brown in colour with yellow dots on the side of their abdomen. The have a wing span of 4 to 5 inches long with narrow front wings.
- -Tobacco hornworm (Manduca sexta)
- -Tomato hornworm (Manduca quinquemaculata)
Hornworm insects only feed on plants in the solanaceous family. This can comprise of eggplant, peppers, potatoes, and most commonly, tobacco and tomatoes. There are a few solonaceaus weeds that hornworms can be found on including: jimsonweed, horse nettle, and nightshade. The larva/caterpillars damage plants by feeding on the upper surface of leaves, leaving behind black or dark green droppings. It is hard to see the larvae as they blend into the canopy, and consequently go undetected until damage has already been done. Larvae in the late instar stage have the capability to destroy numerous fruit and leaves of host plants. As the larvae grow and mature, the amount of defoliation drastically increases. When they are in the last instar stage hornworms are able to consume over 90% of the total combined foliage that is consumed by all instar stages. Adult hornworm moths can be found hovering around many varieties of flowers at dusk.
Hornworms Control Methods
If you are looking to control of hornworms, examine plants regularly through the beginning of July into August. Dealing with a small infestation in a controlled area can be done by handpicking hornworms as an effective and safe management practice. Rototilling after harvest will destroy many of the larvae that are burrowing. For larger areas, tilling has been shown to cause up to 90% mortality of hornworms. There are a lot of natural enemies that can control hornworms. During the egg and early instar stage of the hornworms life they are often eaten by insects such as green lacewings, lady beetles and wasps.
Be sure to carefully read the label for cautions and proper application. It is extremely important to never spray on days that are windy. There are chemical insecticides that are available to control hornworms including carbaryl, permethrin, and spinosad. If there is more than 0.5 young larvae per plant, applying one of the insecticides listed above is recommended. For the most effective treatment, apply insecticides when the larvae are still in the early stages of instars. Once they have entered into the later instars it is much more difficult to eradicate them.