Insects play a vital role in our world. Most food producing plants on our planet are dependent on insect pollinators. In addition, insects serve as a primary food source for many species higher up on the food chain. Of the over one million insect species in the world, less than one percent inflict an economical impact on plants. In other words, most insects are beneficial.
Gardeners everywhere are challenged with managing pests in the vegetable garden. Good management practices should be implemented to reduce pests and to select plants that have been breed for resistance to insects and disease. Periodical scouting for pests will also help to identify problems early before they get out of control. Every effort should be made to choose pesticides as a last resort.
Most damage causing insects can be broken down into two types based on their mouthparts: piercing/sucking and chewing. This characteristic is helpful not only in narrowing down which insect it is, but also identifying which growth stage it is in. Remember, insects go through metamorphous. Some insects do most of their damage to plants when they are in their juvenile phase, while others are most damaging as adults.
Aphids – are soft-bodied insects that suck out juices from the plant and excrete a sweet, sticky, honeydew residue on the plants that can turn into a black sooty mold. Many times this sweet residue will attract ants. Keep in mind aphids are the primary food source for ladybugs, so before treating look for evidence of both juvenile and adult ladybugs. Aphids generally congregate in large numbers on the undersides of leaves and stems. They are generally stationary and don’t move fast. That means that if you wear gloves you can squash them with your hands. If their numbers make that task difficult, a high-pressure water rise can sometimes do the trick. Insecticidal soap or malathion are effective methods for managing this pest.
Armyworms, Cutworms – are larva of a moth. The moths lay their eggs in the stems of plants near the soil line or directly in the soil. The eggs overwinter and pupate underground until they hatch when the temperature is around 65 degrees Fahrenheit. The cutworm larva generally cut the stem of the plant near its base. Some species will crawl up the plant and do damage to other parts. This insect feeds at night. Cutworms can be effectively and safely managed with Bacillus thuringiensis (BT), spinosad and esfenvalerate.
White Flies – are difficult to control because they have a wide range of host plants. They are very small with rounded white wings. The eggs of this insect are laid on the underside of leaves and are so small they are hard to spot without magnification. They are piercing sucking insects that secrete a honeydew similar to aphids and encourage the growth of sooty mold. Insecticide soap, malathion, and pyrethrum products can be used to manage this insect. Multiple applications may be necessary.
Cabbageworms – are small pale green caterpillars that turn into small white moths. They lay their eggs on the underside of leaves of plants in the Brassica family including cabbage, broccoli, and spinach. They chew irregular shaped holes in the foliage and leave excrement in their wake. If you do not treat proactively for this pest, expect damage. Better to treat with a BT product, which must be reapplied after rain.
Tomato/Tobacco Hornworm – are really caterpillars that turn into moths when they are adults. They feed on all plants in the Solanacae family. They are large and green with easy to distinguish white marks located on the side of the body. They are so well camouflaged with the color of the plant leaves that they can easily go unnoticed until their damage is apparent. They are ferocious eaters and can quickly defoliate an entire plant. Many deer have taken the blame for damage done by this insect. The difference is this insect leaves a clean cut, while deer don’t. Look carefully at your plants. If you see one just pull it off – they don’t move fast and won’t get away. If you are overwhelmed by them, you can apply BT. Just know that if this insect is covered in small egg sacks, the caterpillar will soon be dead anyway as the tomato Hornworm parasitic wasp has laid its eggs in the caterpillar and will be feeding off its flesh. The wasp is a predator we want to encourage.
Squash bug – This insect only attacks cucurbits (members of the pumpkin and squash family). This insect can do damage to plants when it is in both the juvenile and adult stage. Small tiny reddish brown eggs are laid on the underside of leaves in careful rows that come together on an angle. Each cluster contains 15 eggs or more. The eggs hatch into a tiny wingless nymph. This insect only has one generation a year. Scouting for eggs and squashing them before they hatch can be a very effective way to manage these pests, in addition to removal and destruction of the plants immediately after harvest. If the squash bug eggs are allowed to hatch, they quickly become destructive. They can be managed with malathion.
Spider mites – are tiny mite pests that are closely related to insects. They cause damage to multiple vegetable species, doing more damage when plants are stressed due to hot, dry weather. They are hard to see without magnification. Scout for them using a white piece of paper while shaking the plant over the paper. If present they will appear to be moving flecks of pepper. In severe infestations you will see webbing, curled leaves with brown edges or pre-mature leaf drop. They suck the plant juices leaving the leaves looking speckled with white dots. Insecticidal soap can be applied.
Flea beetles – are tiny black beetles with brown legs that measure about 1/18-inch. Their hind legs are slightly larger and used for jumping, which is how they got their name. They feed on a wide range of vegetable plants doing the most damage to plants in the Solanaceae family (tomatoes, peppers, eggplant, and potatoes). This insect overwinters in the soil and can have multiple generations. It chews small round holes in leaves, which in some small plants can lead to death by providing ways for other pathogens to enter the plant. Flea beetles can be managed with carbaryl and malathion.
Fire Ants – are invasive exotic ants that have slowly moved their way north from South America. They can establish a mound in a vegetable bed after mating in the spring or summer, which may show up in the fall after a good rain. While the ants don’t generally harm the plants, they make managing the garden difficult as many people have an allergic reaction to their bite. They are aggressive ants that can only be killed when the queen, who is deep in the mound, is killed. She is laying eggs at a rate of 200 eggs per day and can lay as many as 350,000 eggs in her lifetime. If left untreated, the mound will grow, and new mounds will appear. Managing fire ants in a vegetable garden is tricky. The only pesticide registered for use for fire ant management in a vegetable garden is spinosad. It can take several weeks to kill a mound.
Squashvine Borer – is a caterpillar whose larvae overwinters in the top two inches of the soil and turns into a moth in its adult phase. It is a very destructive insect to cucurbits (pumpkins and squash). The female moth lays eggs near the base of plant steams. The caterpillars bore to the center of the stem near the base of the plant soon after hatching. There are several options for cultural control. Actively scout and remove the unwanted pest from the inside of the stem with a wire or a razor. Carefully mound the exposed area with soil to prevent drying and encourage roots to grow.
Encouraging roots to grow every five feet along the vine will help ensure continued growth. There are a variety of pesticides that can be used to control this insect, but timing is critical. Begin treating towards the end of May. Using spinosad, neem, pyrethrin or carbaryl. Repeat applications will be required throughout the growing season of this crop at a frequency that will depend on the pesticide you choose. Crop rotation can reduce this pest population.
It is important to properly identify a pest before treatment. Misidentification can result in application of the wrong treatment, resulting in poor to no control of the problem. Individuals should also take care to read the label before using a pesticide. Always, when possible, choose the safest and most environmentally friendly method of treatment to manage a pest.
For more information, contact a Durham County Extension Master Gardener volunteer at 919-560-0528 or [email protected]
Michelle Wallace is the Consumer Horticulture Agent for the N.C. Cooperative Extension Service in Durham Co.