The tobacco hornworm. Photo from Mississippi State University Extension Service.

By John E. Woodmansee
Agriculture/Natural Resources Extension Educator, Purdue Extension Whitley County

COLUMBIA CITY — In last week’s article, I focused on weeds as unwanted garden guests. This week, we’ll focus on insect pests, and next week, diseases.

As you know, many insect pests may affect garden plants. Since I cannot cover everything today, we’ll just scratch the surface on a few common insect pests.

John Woodmansee

Starting with the most common garden vegetable grown, the tomato, you may have seen fat, green caterpillars chewing on your tomato stems. These are either tobacco hornworm or tomato hornworm.

Besides being fat and up to 4 inches long, these caterpillars have diagonal white stripes on each side of their body, and what resembles a horn projecting from their posterior end (hence the name). Tobacco hornworm’s seven white marks go in one direction, and are edged in black, with a red horn. The tomato hornworm’s eight marks per side are each more like a chevron, or “V,” with no black edge, and a dark blue horn. Ironically, in Indiana the tobacco hornworm is more commonly found on tomatoes than the tomato hornworm.

In order to prevent the destruction of tomato plants, management of hornworms is usually needed. However, the best control methods do not involve insecticides. Simply pick them off with your hand. If you see one of these caterpillars with a bunch of white cocoons on its back, leave it! These are developing braconid wasps, a predator of hornworms. Let this natural control run its course. The affected caterpillar is pretty sick at this point and will consume very little, if any, of your tomato plant.

If you grow squash or pumpkins, you may have seen a mysterious vine collapse of your previously healthy plants. This may be due to squash vine borer. This clearwing moth is predominantly orange in color with small black dots on their back in a straight line. When they lay an egg on a stem, the hatched larva burrows into the vine and begins to feed. Eventually, the damage to the interior of the vine causes wilting and vine collapse, especially if the larva is in the main stem.

Purdue expert Laura Ingwell states, “SVB damage can be distinguished from other squash pests by the identification of the entry site of the larva into the stem and the presence of frass (insect poop) that looks like sawdust.” Ingwell recommends using resistant varieties, practicing crop rotation and sanitation (removal of infested vines), installing row covers, or using labeled insecticides close to the time of egg hatch.

A similar pest of cucurbits is the squash bug. Both nymphs and adults suck plant sap from leaves, vines, and fruit. Their saliva contains plant toxins, turning leaves black and crispy. The plant wilts and may eventually die. Squash bugs may also feed on the fruit, causing the fruit to collapse. Management options include sanitation and using labeled insecticides that target young nymphs.

We’ll conclude our short discussion with aphids. Many species of aphids affect a large number of plants. Aphids are small, soft-bodied insects that can be present in large numbers. Most species have a characteristic pair of points called cornicles protruding from the posterior ends of the abdomen in an exhaust pipe-like fashion. Aphids can cause damage by feeding on plant sap, leading to stunted growth, yellowed leaves, and distorted leaves. They can also spread plant diseases.

In the home garden, aphids may be held in check by a number of natural predators, including lady beetles, minute pirate bugs, lacewing larvae, and syrphid fly larvae. Insecticides may be considered as a last resort.

For more information, see “Managing Insects in the Home Vegetable Garden.”