Tobacco Hornworm, Vol. 9, No. 08
Your Extension Experts
September 5, 1997
July 25, 1997
June 23, 1997
May 12, 1997
March 17, 1997
“These big green caterpillars get our tomatoes every year. They are almost as big as hotdogs! First, they eat all the leaves and then they start chomping on green tomatoes! Is there anything we can do to avoid them? We don’t like spray insecticides.”
Tobacco hornworms are the scourge of backyard tomato gardeners throughout the South. The moths have a very good sense of smell, which allows them to find small, isolated plantings of tomatoes. Young caterpillars are a bit odd-looking because at this point in their life that horn on their rear is almost as big as the rest of the caterpillar, but they grow quickly and develop into a three-inch-long caterpillar in about 20 days.
Even large caterpillars are well-camouflaged and difficult to spot, though their feeding damage and fecal droppings often give them away. But young caterpillars and the relatively small amount of defoliation they cause often go unnoticed. These caterpillars do 90% of their feeding in the last five to six days of their larval life. This is why a tomato plant that looked fine on Saturday can be nearly stripped of leaves by Tuesday morning. Even one big hornworm can eat a lot of leaves, and if there are several caterpillars on a plant, the damage can be dramatic. When ready to pupate the caterpillars crawl to the ground, burrow in and form a “naked” pupa (no cocoon). The pupal stage last 14 days or so in the summer, but they also overwinter as pupae in the soil.
Big caterpillars grow up to be big moths, and the adults, known as Carolina sphinx moths, can have wing spans of over 4.5 inches. The wings are drab, mottled gray, but the top of the abdomen has a row of six orange or yellow spots on either side. Because they are nocturnal, the moths are seldom seen, but these and other sphinx moths are sometimes seen sipping nectar from flowers near dusk or on overcast days.
“But I always heard these called tomato hornworms?” There is a tomato hornworm, Manduca quinquemaculata, but it is more common up North. In the South we mostly have tobacco hornworms. Biology and control are similar; so are the caterpillars, but there are subtle differences. If the horn is tobacco-colored, it is a tomato hornworm; if the horn is tomato-colored, it is a tobacco hornworm. Also, tobacco hornworms have seven diagonal white strips on their sides, while tomato hornworms have white “V” shapes.
Have you ever seen a hornworm with what appear to be white eggs on their back and wonder what is going on? Those are not eggs; they are cocoons of a tiny parasitic wasp.
Control: Home gardeners are often surprised to learn that hornworms are not major pests of commercially grown tomatoes. The insecticides used to control fruitworms, stink bugs and other more important pests usually provide excellent preventive control of hornworms. Home gardeners who spray for these other pests won’t have hornworms either. Insecticides such as zeta-cypermethrin, permethrin, and Spinosad are all very effective. Some formulations of Spinosad are “organic,” but Spinosad will not control stink bugs or leaffooted bugs. See BEV No. 10 of 2015 for information on tomato fruitworms, and BEV No. 8 of 2022 provides specific recommendations on insect and disease control for home gardeners.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
The information given here is for educational purposes only. Always read and follow current label directions. Specific commercial products are mentioned as examples only and reference to specific products or trade names is made with the understanding that no discrimination is intended to other products that may also be suitable and appropriately labeled.
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