Tomato trouble?

Greenhouse-grown tomato (Solanum lycopersicum) plants are susceptible to attack by a variety of insect and mite pests including aphids, greenhouse whitefly, tomato russet mite, twospotted spider mite, and western flower thrips. These insect and mite pests not only cause direct damage by feeding on the aboveground portions (e.g. leaves and flowers) of tomato parts, but may subsequently cause indirect damage by vectoring diseases (e.g. viruses). Both types of damage may result in a reduction in tomato yields. Below are descriptions of each of these insect and mite pests. Then the management strategies that may be used to alleviate problems with these pests on greenhouse-grown tomatoes.

INSECT AND MITE PESTS

Aphids

Aphids are soft-bodied insects, 0.10 to 0.25 cm in length with tubes or tail-pipes (cornicles) protruding from the end of the body. They vary in color from green, black, yellow, to pink. In greenhouses, all aphids are females (there are no eggs), which can give birth to 100 live female nymphs that can themselves give birth to live offspring. Their rapid reproductive rate may result in extensive population outbreaks within a short period of time. Aphids can develop and reproduce throughout the year in greenhouses under continuous tomato production.

Aphid species that feed on greenhouse-grown tomatoes include the green peach aphid (Myzus persicae) and potato aphid (Macrosiphum euphorbiae). Aphids feed on new terminal growth and leaf undersides, causing both direct and indirect damage to greenhouse-grown tomatoes. Direct damage is affiliated with aphids removing plant fluids using their piercing-sucking mouthparts. Aphids feeding on new growth may cause leaf yellowing and plant stunting, and leaves may appear distorted or curled upward or downward. In addition, aphids may vector particular viruses. Indirect damage is associated with the excretion of honeydew during feeding. Honeydew is a clear, sticky liquid that serves as a growing medium for certain black, sooty mold fungi, which may inhibit the ability of tomato plants to produce food by means of photosynthesis. Excessive aphid populations may result in the presence of white cast or molting skins.

Greenhouse whitefly

Greenhouse whitefly (Trialeurodes vaporariorum) life stages including the egg, nymphs, pupa and adult are located on the underside of tomato leaves. Adults are winged, approximately 4.2 mm long, with the body covered by a white, powdery wax. Adults hold their wings flat, parallel to the top of the body. A single female greenhouse whitefly adult can lay up to 20 eggs in small circles on the leaf underside; laying up to 300 eggs during its 30- to 45-day lifespan. Eggs are erect, spindle-shaped and attached to a short stalk. The eggs eventually turn gray, hatching in four days. Nymphs move short distances on plants before locating a suitable place to settle down and initiate feeding. The pupa, or fourth instar nymph, does not feed, and has distinct red eyes. Pupae possess elongated waxy filaments that encircle the body. The pupae are elevated in profile with vertical (perpendicular) sides; resembling “cakes” on the underside of leaves. Development from egg to adult takes 14 to 30 days to complete, although this depends on the ambient air temperature. Warmer temperatures will shorten the development time.

The nymphs and adult of the greenhouse whitefly having piercing-sucking mouthparts, which are used to withdraw plant fluids. The nymphs, in general, tend to cause greater plant damage because they feed more than adults. Direct feeding damage to tomatoes may result in leaf curling, leaf yellowing, chlorotic mottling, and stunting and wilting. Nymphs may also cause indirect damage by secreting copious amounts of honeydew.

Tomato russet mite

Tomato russet mite (Aculops lycopersicii) is an eriophyid mite. Adults are less than 0.3 mm in length and look-like most eriophyids; they are robust, wedge-shaped, and somewhat tapered, resembling cigars, with the head and legs located on one end of the body. They also possess two pairs of legs. Tomato russet mite varies in color from translucent, yellow, brown or tan, to pink. Under a microscope (100x power), heavy infestations of tomato russet mite may appear as dust or mold. Females lay eggs on leaf undersides, on leaf petioles, or on stems near the lower portions of plants. Nymphs that hatch from eggs resemble adults, and tend to aggregate on leaf edges. Nymphs eventually transition into adults. This mite prefers warm, dry environmental conditions with the life cycle, from egg to adult, usually completed in one-week under temperatures =26ºC or 80ºF. Lower plant portions that are heavily infested or damaged may cause mites (both adults and nymphs) to migrate to new, young leaves. Tomato russet mite populations may spread from plant-to-plant via wind currents or when plant leaves are touching. Both the adults and nymphs possess piercing-sucking mouthparts, and feed primarily on stems and the undersides of lower leaves, causing leaf bronzing. In addition, leaves may turn yellow, wither or curl upward, dry-up, and fall off tomato plants. The damage caused by tomato russet mite typically starts on the lower leaves, and then progresses upward. The type of damage caused by this mite may resemble a nutritional deficiency, plant disease or water stress.

Twospotted spider mite

Twospotted spider mite (Tetranychus urticae) adults are 0.3 to 0.4 mm in length, oval-shaped, and can vary in color from yellow-green to red-orange. Adults have distinct black markings on both sides of the body. Female adults live about 30 days; laying between 100 and 200 small, circular-shaped eggs during a two-week period. Eggs are deposited on the leaf underside along the mid-veins. Eggs hatch into yellow-green, six-legged larvae that mature into eight-legged nymphs, and then develop into adults. Development, from egg to adult, can be completed in one to three weeks; however, this depends on ambient air temperature. For example, the life cycle takes 14 days at 21ºC (70ºF) and seve days at 29ºC (84ºF). All life stages are located on the underside of tomato leaves because twospotted spider mites are very sensitive to ultra-violet light (sunlight).

Twospotted spider mites have piercing-sucking mouthparts, which allow them to feed on individual plant cells. Feeding by twospotted spider mites reduces the chlorophyll content in leaves and lowers the plants ability to manufacture food through the process of photosynthesis. Damaged leaves are bleached and stippled with small, silver-gray to yellow speckles. There can also be fine mottling on the upper leaf surface. Heavily infested leaves may appear bronzed, turn brown, and eventually fall off tomato plants. Excessive populations and feeding by twospotted spider mites can cause premature defoliation and webbing is usually present.

Western flower thrips

Western flower thrips (Frankliniella occidentalis) adults are approximately 2.0 mm long, slender, with two pairs of hairy wings. The larvae and adults vary in color from brown to yellow. The life cycle consists of an egg, two larval stages, two pupal stages, and an adult. The life cycle takes two to three weeks to complete depending on ambient air temperature with the optimum range between 26 and 29ºC (79 and 84ºF). At optimal temperatures, the life cycle can be completed in seven to 13 days. Adult females live approximately 45 days and can lay up to 300 eggs during their lifetime. The eggs are inserted into plant leaves.

Western flower thrips can cause direct damage by feeding on tomato leaves and even flowers. Western flower thrips have piercing-sucking mouthparts, which are used to feed within the leaves of plants. After inserting their mouthparts, western flower thrips lacerate and damage cell tissues. Western flower thrips then ingest fluids from the cells. Symptoms of feeding include leaf scarring, distorted growth and sunken tissues on leaf undersides. Leaves develop a “silvery” appearance because air fills the empty cells after plant fluids have been removed. Black fecal deposits may also be present on the underside of tomato leaves. Western flower thrips cause indirect damage to tomato plants by vectoring the tospovirus; tomato spotted wilt virus. The first and second instar larvae acquire the virus by feeding on infected weeds or plants, and then adults transmit the virus to tomato plants. Plants infected with a virus must be disposed of immediately. The direct and indirect damage caused by western flower thrips may result in an economic loss to greenhouse producers.

MANAGEMENT

One of the primary means of alleviating problems with insect and mite pests of greenhouse-grown tomatoes is to avoid over-fertilizing plants with high nitrogen-based water soluble fertilizers because this can lead to the production of soft, succulent growth that is easier for these pests to penetrate with their mouthparts. Weeds inside the greenhouse should be removed promptly, and deposited into containers with tight-sealing lids or placed into debris piles located away from the greenhouse. Many weeds serve as refuge sites for these pests, and may also act as reservoirs for the viruses vectored by certain pests. A random sample of tomato plants should be monitored regularly for the presence of pests in order to detect localized infestations. Contact pesticides (in this case, insecticides and miticides) labeled for use on greenhouse-grown tomatoes should be used early to prevent pest populations from building-up to damaging levels. Always obtain thorough coverage of all plant parts, especially leaf undersides, which will result in greater kill of pests. Some commercially available contact insecticides/miticides that may be used include potassium salts of fatty acids (insecticidal soap), and petroleum, mineral, and/or neem-based oils (horticultural oils). Multiple applications will be required due to the short residual activity of most insecticides/miticides, and because of the dispersal and reproductive capacity of pests within greenhouses; especially when environmental conditions such as temperature and light intensity are favorable. Heavily infested tomato plants should be discarded immediately. Biological control or releasing natural enemies such as parasitoids and/or predators is another option in dealing with these pests. There are a number of commercially-available parasitoids and predators that can be introduced to regulate pest populations. However, natural enemies must be released before pest populations are extensive and causing damage to tomato plants. Contact your local supplier/distributor to assess natural enemy availability for specific insect and mite pests.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.