Last week we discussed tomato diseases. Now we’ll cover something just as destructive: insects.
• The Colorado potato beetle feeds exclusively on solanaceous (nightshade and potato family) plants and can be a significant pest of potatoes, tomatoes and eggplant. Adult beetles have cream and black stripes across the back. Females deposit orange eggs in clusters of 20 to 45 on the underside of leaves. Larvae are crimson in the early instars, with black legs and two rows of black spots on the sides of their body. Large larvae are orange and appear bloated and humpbacked. Pupae are located in the soil. Both adults and larvae feed on foliage and may skeletonize the crop. New adult beetles can fly into gardens so be sure to check plants regularly.
• Flea beetles are tiny — from 2.5 to 4.5 mm long — with a solid color body or a black body with a pale yellow stripe on each wing cover. They cause tiny round holes in the foliage of a wide range of vegetable plants. The insects derive their name from their well-developed hind legs; when disturbed, they jump like fleas. Flea beetles can overwinter on weed hosts surrounding the field, on residues of a previous tomato crop or in the soil if the previous crop was a flea beetle host. Flea beetles are common pests of seedling tomatoes. Adult beetles chew small holes in leaves, giving them a sieve-like appearance. Young plants can withstand some flea beetle injury but may be killed if the weather is dry and windy. The percentage of plants affected and forecasted weather conditions will indicate the need to treat. Once established, plants can overcome moderate flea beetle feeding.
• Aphids are common visitors to home vegetable gardens and infest a wide range of host plants. Some important cultivated hosts include potato, tomato, eggplant, sunflower, pepper, pea, bean, apple, turnip, corn, sweet potato, squash, pumpkin, asparagus, clover and rose. The soft-bodied, pear-shaped insects may be solid pink, green and pink mottled, or light green with a dark stripe. Usually wingless, they are about ⅛-inch long. Aphids pierce veins, stems, growing tips and blossoms with their needlelike mouthparts. As a result, blossoms are shed and yield is reduced. Aphids tend to spread rapidly from field to field, transmitting a number of viral diseases, including various mosaics, leaf roll, spindle tuber and unmottled curly dwarf.
• Hornworms feed primarily on solanaceous plants such as tobacco, tomato, eggplant, pepper and some weedy plants. Tobacco and tomato plants are preferred. Hornworm eggs are smooth, spherical and about 1/16 inch in diameter. Light green at first, they turn white before hatching. Tomato hornworms have eight V-shaped markings on each side; the horn on the back end is straight and black. Species are about 3 to 3½ inches long when fully grown. Hornworms strip leaves from tomato vines as they feed. If a heavy infestation develops, the caterpillars also feed on developing fruit. Rather than bore into the fruit, they feed on the surface, leaving large, open scars. Fruit damage, however, is much less common than loss of leaves. Hornworm damage usually begins to occur in midsummer and continues throughout the remainder of the growing season. They are especially attracted to plants under drought stress.
• Stink bugs feed on over 52 plants, including native and ornamental trees, shrubs, vines, weeds and many cultivated crops. The preferred hosts are nearly all wild plants. Stink bugs build up their numbers on these hosts and move to cultivated hosts as their preferred food becomes overly mature. Among vegetable crops, stink bugs attack bean, okra pods, ripening tomato fruit and stems of melons and asparagus. Bean, corn, sorghum, eggplant, potato, tomato, peach, strawberry, okra and watermelon are only a few of the many host plants.
The brown marmorated stink bug has become a serious pests of fruit, vegetables and farm crops in Pennsylvania. Adults are about 17 mm long and are shades of brown on both the upper and lower body surfaces. They are the typical “shield” shape of other stink bugs, almost as wide as they are long. Eggs often are laid on the underside of leaves and are a light green in color. They are elliptical in shape and are often deposited in a mass of about 28 eggs. This insect is becoming an important agricultural pest in Pennsylvania. I have seen the damage they cause on apples, peaches and sweet corn.
• Cutworms can be quite distinct from one another and their coloring can vary from brown or tan, or gray and black. Some cutworms are a uniform color while others are spotted or striped. It is a fat, basically dark caterpillar, 40 to 50 mm long when fully grown, with three pairs of legs near the head and five pairs of fleshy prolegs, and it is active at night. A young caterpillar climbs on leaves, while older caterpillars sever seedling stems near the ground and hides during the day in soil at the bases of plants. Cutworms curl up into a tight “C” when disturbed. Most cutworm damage occurs on vegetable seedlings early in the season when plants are small and have tender tissue. Although cutworms are active throughout the summer, they are rarely a problem after spring.
Cultural practices are helpful in avoiding many insect infestations. Tomatoes should be planted in well-prepared, fertile beds, mulched and properly watered to promote vigorous growth. Stressed plants tend to attract more insect pests than healthy plants.
In a home garden, handpicking and destroying many pests is an effective control measure. Drop adults and larvae in a pail filled with soapy water and remove or crush eggs on the underside of leaves.
Beneficial insects also are very helpful in controlling insects such as aphids, leafminers and hornworms. To avoid killing beneficial species, use insecticides only when necessary.
Natural parasitism often occurs on hornworms when tiny braconid wasps lay eggs in them. The larvae feed inside and then pupate on the backs of the hornworms. The pupal cases are seen as white projections, like rice grains, on the back of the hornworm. If parasitized hornworms are found on the plant, feeding will have ceased, so leave it for the next generation of beneficial wasps to hatch.
There are no viable strategies for control of the brown marmorated stink bug. The use of insecticides has very short-lived effect and there is evidence of resistance development.
Pollinating insects, such as honey bees and bumblebees, can be adversely affected by the use of pesticides. Avoid the use of spray pesticides (both insecticides and fungicides) as well as soil-applied, systemic insecticides unless absolutely necessary.
If spraying is required, always spray late in the evening to reduce the direct impact on pollinating insects. Always try less-toxic alternative sprays first for the control of insect pests and diseases. For example, sprays with insecticidal soap, horticultural oil, neem oil extract, spinosad, Bacillus thuringiensis (Bt) or botanical oils can help control many small insect pests and mites that affect garden and landscape plants.