Insects attack vine crops from seeding until harvest. They can reduce the stand, defoliate the leaves, feed on roots or flowers, transmit bacterial and viral diseases, and create wounds that help fungal pathogens enter the plant. Major insect pests include seedcorn maggot, two species of cucumber beetles, squash vine borer, squash bug, two species of aphids, two-spotted spider mites, and whiteflies. Growers need to protect the crop against insect damage while concurrently ensure pollination by bees. Row covers are effective against beetles, vine borers, and squash bugs, but they must be removed during flowering to allow for pollination.
Vine crops have 2 types of flowers: cucumbers, squash, pumpkin and watermelon have separate male and female flowers, while muskmelon has male and hermaphroditic (bisexual) flowers. Growers must support pollination by bees. The dense and sticky pollen needs to be transferred to the female flowers to ensure fruit that is well shaped, and to optimize yield. Fruit size and shape is related to the numbers of seeds produced, and each seed requires 1 or more pollen grains. Flowers are usually open and attractive to bees for only 1 day, and pollination must take place on the day that the flower is open. Ensuring the presence of 1 to 2 honeybee colonies per acre, and up to 3 hives per acre, has a reliable method of pollination in the mid-Atlantic. Colonies should be strong: at least 1,200 square inches of brood per colony and enough adults to care for them.
Vine crops are not especially attractive to honeybees. Moving colonies into fields after blooming has started helps ensure that the bees work the vine crops. When bees are in the field, insecticides should only be applied near the evening, when the bees have returned to the colony. Notifying beekeepers and written contracts are good practices. Finally, we need to encourage our native ground-nesting bees. Several species exist which may be very useful cucurbit pollinators.
Seedcorn maggots invade vine crops, beans, and sweet corn. Maggots pupate inside a dark brown capsule-like puparium that resembles a grain of wheat. Seed corn maggot puparia can be found in soil throughout the year, and they overwinter in these puparia. The adult flies emerge from puparia during late April and early May. The adults are brownish-gray flies that closely resemble common houseflies except that they are about half the size. Tiny, white, elongated eggs are deposited among debris and around plant stems near the soil surface. Eggs hatch in a few days and the maggots work their way into the soil in search of food. Maggots are dirty white with a yellowish tinge, legless, cylindrical, and tapered; full-grown maggots reach 1/5- to 1/4-inch in length. Maggots feed in the seed or on the underground parts of seedlings. Damaged seed may germinate, but there may not be enough food reserves left in the seed for the plant to survive. The time required to grow from egg to adult is between 3 to 4 weeks. There are 3 to 5 generations each year. Populations tend to decline during dry months of the summer.
Seedcorn maggots tend to cause greater losses during cool, wet years, and in fields with an abundance of decaying organic matter, such as manure or a recently plowed cover crop. Incorporate organic matter well before planting, and cover transplant root balls with soil. Any cultural practice that will speed up germination, plant emergence, and early plant growth will help reduce crop losses from maggots - this allows the plant to "outgrow" the feeding damage. Planting in warm soils significantly helps plants outgrow maggot injury. If significant damage occurs, replant those areas of the field. Look to see if the maggots are still present. If maggots are small (<3/8 inch), then wait 7-10 days for those maggots to begin to pupate before replanting. Most chemically treated seed is designed to help prevent damage during seed storage. Additional treatments applied at-planting are effective but typically are for commercial use. Sprays applied to soil after damage is seen are not effective.
Striped and Spotted Cucumber Beetles
Two species of "cucumber beetles" [striped cucumber beetle, and spotted cucumber beetle] feed on cucurbits. Two additional species [northern corn rootworm, and western corn rootworm] invade late in the season. All four can be found in vine crop flowers later in the season. Of these species, the striped cucumber beetle is present in the highest density and over the longest time span. The striped cucumber beetle overwinters as an adult both inside and outside of cucurbit fields. Striped cucumber beetle adults are the overwintering life stage. They invade fields soon after transplanting, and lay eggs at the base of plants. The hatching larvae feed on the roots of vine crops, pupate, and then emerge as new adults in about 25-30 days. There are 2-3 generations per year in Pennsylvania.
In contrast, spotted cucumber beetles do not overwinter well in Pennsylvania, and immigrating adults from the south will occur later in the season. And the two additional "corn rootworm" species overwinter as eggs, hence they appear as adults in vine crops - typically in the flowers - later in the season; the corn rootworms have a single generation per year.
Adults are easy to distinguish. All are about 1/5 inch long. Striped cucumber beetles are yellowish-green with 3 black stripes on the wing cover, and a black abdomen; they also have a black head and yellow thorax. Western corn rootworm also has stripes, but the abdomen is yellow. Spotteds are also yellowish-green, but have 12 black spots. Northern corn rootworms are a solid green to green-yellow.
Adult feeding during early plant growth can cause stand reduction and rind-feeding by adults or larvae later in the season renders crops unmarketable and may serve as routes of entry for pathogens. Larval feeding also impacts root development. Most importantly, the striped cucumber beetle vectors bacterial and viral pathogens. The major pathogen is the causal agent of bacterial wilt. Disease management currently relies on vector management. Even a low beetle density on young plants can result in significant plant disease. Bacterial wilt is most severe on melons and cucumbers, but this disease seems to be infesting some squash and pumpkin plantings.
Row covers can be effective. Put the covers on right at transplanting. The row covers effectively exclude the beetles for as long as you have the crop covered. If you can find a self-pollinating cultivar (there are some for cucumbers), then you can leave the floating row cover on until harvest. Otherwise, you need to remove the cover to allow bees to pollinate the crop. Use of insecticides labeled for cucumber beetle control can be effective at stopping significant plant feeding, but to also slow transmission of the bacterial pathogen the insecticide must be applied quickly. Protecting the young plants is especially important.
Squash Vine Borer
The squash vine borer is an important pest of squash, pumpkins and gourds. Cucumbers and melons rarely become infested. Small plantings and home gardens are the most seriously infested: small plantings result in eggs being concentrated onto fewer vines. Squash vine borer overwinters in as a pupa inside a tough, silk-lined cocoon in the soil. The moths begin to emerge in late June; the day-flying adults are active during July and early August. The adult is a beautiful, clear-winged moth with a wing spread of l-l/4 inches. The front wings are a metallic olive-green and the hind wings are transparent with a bluish reflection and black scales along the margin and on the veins. The body is strikingly colored with orange and black markings. The hind legs are long and ornamented with tufts of long orange, black, and white hairs. When in flight, the moth may easily be mistaken for a wasp. Their eggs are reddish-brown, l/25 inch in length, usually laid singly or in small groups on the stem near soil surface. Eggs hatch in a week to ten days and the tiny borer enters the stem.
When fully grown, the borer is one inch long, and white with a brown head. It takes approximately forty days for the borer to reach maturity. Toward the end of the season when vines become woody, the borer will attack the fruit. Borers leave the plant in August and September, enter the soil, and form overwintering cocoons. There is only 1 generation per year. Injury is first manifested by sudden wilting of a runner or the entire plant and yellow pellets of frass on the soil surface near the plant base. The vine will be hollowed out and partially filled with moist, shiny frass, in the midst of which is the squash vine borer. Decay sets in and the stem is frequently severed, shutting off the sap supply after which the vines wither and die. It is most destructive to late squash.
Home gardeners may try excluding the pest with sleeves or collars on the vine stems at the base of the plants, or removing borers from vines. When adults are flying, insecticides can be repeatedly applied to the base of plants.
Squash bug adults are hard, brown to coppery-brown, flattened, one-half inch long, and give off a distinct odor when handled. These adults overwinter in sites protected from wind and cold, and emerge in late spring. Females lay shiny, orange to coppery-brown spindle-shaped eggs in clusters containing four to forty eggs in more or less regular rows on the undersides of leaves in the forks of veins. Eggs hatch in one to two weeks, with nymphs developing and feeding on plants for five weeks before becoming adults. Young nymphs are green with rose-colored appendages for a short time, and then molt to a grayish-white. Younger nymphs are often near the eggs. There is only one generation per year. Adults and nymphs pierce veins, feed on plant juices, and may inject toxins or possibly pathogens. Infested vines quickly dry out.
Placing boards on the ground near vines can help trap adults overnight, and let you know when they are moving into a planting. Trapped adults can be removed. Scouting is then best directed at the eggs - begin at early flowering. Home gardeners can achieve control by trapping out adults and repeatedly inspecting lower leaf surfaces and removing eggs and nymphs. Chemical control requires thorough coverage underneath leaves. The insects are well protected under the large leaves of vine crops. Make sure you are not moving into canopy closure with a population of squash bugs.
Both melon aphids and green peach aphids infest vine crops. Aphids are soft-bodied insects, round to oblong, about 1/8th inch long, that extract plant sap. Both winged and wingless forms can be present. Colonies develop on undersides of leaves or on plant terminals. Direct damage by aphids is minimal until populations build to high levels, but feeding causes distorted, stunted plants, and molds grow on a sticky excretion emitted by aphids. Most importantly, they transmit the several viruses. In home gardens, remove plants that show signs of virus infection. Aphids are often controlled by natural parasites and predators which rely on these slow-moving insects as a host resource. High populations can be reduced with insecticides labeled for aphid control.
Adult whiteflies are are small (1.5 mm long) insects that look like tiny white moths. Eggs are depositied on undersides of leaves, which hatch into tiny, flattened translucent green to yellow nymphs that do not move far from the eggs. Nymphs molt into flattened, scale-like pupa, which then become adults. Adults tend to fly toward younger plant tissue, and repeat the cycle. Whiteflies do not overwinter in Pennsylvania, but can infest plantings from transplants or near greenhouses, and where they occur they can quickly build up very high densities. Adults and nymphs extract plant sap through needle-like mouthparts. Infested leaves dry out and drop, and sooty mold grows on honeydew excreted by whiteflies. Recognizing early infestations and removing infested plant parts will slow population growth. Chemical control is difficult - the chemical must contact whiteflies under the leaf surface. The eggs will probably survive, thus repeat applications may need to be applied to contact newly hatched nymphs.
Two-Spotted Spider Mites
Spider mites are tiny, eight-legged animals most closely related to spiders. They appear as 1/25th of a inch round specks on the undersides of leaves. They require large populations to cause serious damage, but their populations build up very quickly when temperatures are hot (>80F). Dry weather (<50% RH) also is correlated to mite build-up. They can complete development in only 5-7 days under these conditions, which is 2 to 3 times faster than many of our other vegetable pests. Often mites move in from nearby crops or weeds, and initial densities are high near field edges. Mites pierce the epidermal cells of plants and extract plant sap. Damage appears as leaves that are stippled, yellowing, and dirty. Leaves may dry and drop. There may be webbing between leaves or on the lower surfaces of the leaves. Removing damaged leaves may slow the spread of mites in a planting. Spot-treat with a chemical labeled for mites when white stippling along veins on underside of leaves is first noticed and 20 mites per leaflet are present.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Shelby Fleischer, Professor