University of Illinois Extension

Vegetable Garden Pests

Vegetable garden pests usually come in a couple of versions, those that eat vegetable plant parts for food and those that use the sap for their source of food, said a University of Illinois Extension horticulture specialist.

"Insects that chew on plant parts are more likely found out since having holes in the leaves and vegetable fruits are quickly spotted," said Richard Hentschel. "Chewing insects in the vegetable garden could be the tomato hornworm, bean leaf beetle, flea beetles (on eggplant), potato beetles, cabbage worm, corn earworm, plus a few other miscellaneous beetles and worms."

The type of chewing insect in your garden you will influence how you manage that pest.

"For example, it is nearly impossible to grow cole crops - cabbage, broccoli, cauliflower - without protecting those plants from the cabbage worms," he said.

"The same thing holds true for sweet corn and the corn rootworm beetle. Flea beetles can destroy eggplant foliage, leaving the fruit alone. You can avoid a big problem by planting your snap beans after the farm fields have been planted with soybeans."

Control can be simple like removing egg masses before they hatch or removing the very young caterpillars while their appetite is small. For certain kinds of caterpillars, an organic material like Bacillus thuringiensis can be used.

"If a chemical spray is absolutely necessary, consider using the one that is least harmful to the environment that will control the pest insect," Hentschel said. "Plan on treatments when the insects are smaller rather than larger, and direct the sprays where the insects are feeding or hiding during non feeding periods during the day.

"Chewing insects are typically treated with products that are ingested as the insect feeds, they will eat a little more, and then be no more."

The other versions of garden pests are those that are sneaky, and harder to see the damage going on. These insects use their mouthparts to remove the plant juice or sap from leaves and stems, nothing as obvious as holes in the leaves.

"They do their damage over time, taking away food that would otherwise be going to produce larger or more fruits," he said. "The most common ones are aphids, mites and thrips, each one operating a bit differently."

Aphids are the biggest of these small insects. Aphids exude a sticky wet substance called "honeydew" as they feed. This sticky material is shiny and pretty easily seen. The aphids themselves may be nearly the same color as the plant or could be hiding under the foliage.

Mites feed on our vegetable plants by injecting a digestive fluid through their mouthparts, breaking down the plant cells, leaving minute yellowing spots on the foliage. Mites favor hot weather and younger plant parts. A telltale sign of mites besides the yellowing foliage is a mass of very small spider webbing in the area where they are feeding and on the undersides of the foliage. Mites are about 1/100 of an inch in size, and you must use a hand lens if you want to see them.

Thrips are also small, about 1/16 of an inch, and favor the growing points and flowers, distorting and preventing the plants from growing properly. Thrips can also discolor small fruits like strawberries. You would need a good hand lens to see these, so the symptom you are looking for will be the distorted growing points and general lack of new growth.

Insects that feed on our vegetable plants in this manner can be managed by using a forceful stream of water and dislodging the pests.

"You will need to repeat this regularly until you see normal growth resume for thrips, when the sticky shiny honeydew goes away for aphids, or the leaves return to a normal green color for mites," he said.

Since these insects do not actually eat any plant parts, a contact pesticide is needed to properly manage the pest populations. Aphids and mites are easier to treat than thrips. If you have thrips, be sure to get the spray down and into the distorted areas on the vegetable plant.

"A bonus of controlling the vegetable garden insects is you will also be controlling those plant diseases that are spread by your vegetable garden insect pests," Hentschel said.