Leafminers

Available natural enemies and their potential for control

A few parasites are effective against the species of leafminers commonly found in greenhouses.The potential for successful biological control is high on vegetable crops and moderate on ornamentals.

Order Diptera: Flies

Family Agromyzidae: Leafminer flies

Liriomyza trifolii

Vegetable leafminer,

Liriomyza sativae

Chrysanthemum leafminer,

Chromatomyia syngenesiae

Pea leafminer,

Liriomyza huidobrensis

Several different species of leafminers attack greenhouse crops.The most common are Liriomyza trifolii and the vegetable leafminer.L. trifolii is a cosmopolitan species, originally from the Caribbean,that attacks numerous plant species. It is a problem on vegetable crops, including tomato, cucumber, lettuce, and pepper, and ornamentals such as alstroemeria, cineraria, chrysanthemum, gerbera, gypsophila, and snap-dragon.The vegetable leafminer is a problem on a wide range of vegetables. Other important species include the chrysanthemum leafminer which is a pest on chrysanthemum and cineraria, and the pea leafminer, a pest of aster, carnation, gypsophila, and some vegetable crops, especially in the coastal valleys of California, Mexico, and South America.Liriomyza bryoniae,a European species that attacks vegetable crops, mainly tomatoes, is not known in North America.

Damage

Leafminer larvae tunnel within leaves between the upper and lower surfaces, making unsightly white blotches or twisting lines. Different species produce different types of mines that may vary with the host plant.The adult female flies also make numerous punctures in the leaves that show up as white spots on the upper surface.The mines and punctures interfere with photosynthesis and, if numerous, can reduce crop yields. Young seedlings may be destroyed. Infestations on edible portions of vegetable crops and on ornamentals make the plants unmarketable. Although tomato plants can tolerate high levels of damage with little effect on yield, leafminer feeding accelerates leaf drop above the developing tomato fruits, making them vulnerable to sunburn. In addition, punctured and mined tissue is more susceptible to diseases, such as bacterial leaf spot on chrysanthemum.

Description and life cycle

Adult leafminers are shiny black flies with yellow markings and are slightly smaller than fruit flies, at 1/16 inch (1.3-2.5 mm) long. Most species appear very similar, except chrysanthemum leafminer, which is slightly larger and dark gray.The female punctures leaves with the ovipositor, leaving a small white spot at the point of entry. Both female and male flies feed on the sap that oozes from the punctures. Pale, oval eggs are inserted into about 15% of these punctures. Each female produces an average of 60 eggs in her 2- to 3-week life span.The area of the plant preferred for oviposition depends on the leafminer species and the host plant. The white or yellow maggots hatch in a few days and tunnel through the leaf tissue for up to 2 weeks.This produces the narrow, winding, white mines visible on leaves.The width of the mines increases as the larvae grow. Once larval development is complete, the maggots drop out of the leaf and pupate inside their hardened skins, or puparia, in cracks in the soil, on benches, or within cupped leaf surfaces.The barrel-shaped pupae change from bright yellow to brown. Adults emerge in about 10 days.

Monitoring

Begin weekly monitoring of susceptible greenhouse crops after transplanting them. Monitor leafminers in two ways. Use yellow sticky traps to detect leafminers moving into the greenhouse and to keep track of adult populations. Examine the plants visually for both adult and larval damage. In fast-growing crops the damage of the first generation may be hidden by new foliage.You may not observe the infestation until the second generation attacks the upper leaves of mature plants.

Yellow sticky traps are easier to handle and assess than other sampling techniques, such as sweep nets or pupal trays.The traps are commercially available or can be made by the grower. Place the sticky traps just above the crop canopy and adjust the height of the traps as the crop grows. Also place traps near doors, air-intake vents, among newly arriving plants, and near more susceptible varieties to detect new infestations.The number of traps needed depends on the crop, but on average they should be spaced 18-23 meters (20-25 yards) apart. Count the leafminer adults on the traps once or twice per week.

Even when sticky traps are used it is still important to check the crop visually. Inspect plants regularly for flies and for white spots on the upper surfaces of leaves, which are the oviposition and feeding punctures of the adults. Sample leaves weekly to estimate larval leafminer densities. Select leaves randomly from plants throughout the greenhouse or mark plants with colored tape for continual monitoring of the same plants through the season. Record the number of mines per leaf on three leaves from either the middle or bottom of each plant, wherever most of the leafminers are found.Sample a minimum of 10 plants per greenhouse.

Leaf samples can also provide information on parasite activity, although many growers do not have the time or expertise to do this. Learn to recognize adult parasites and mines containing parasites. Carefully cut open the mines on sampled leaves and examine them for the presence of parasites or leafminer larvae that have been killed by adult parasite feeding. A 10X-15X magnifier or hand lens, or a dissecting microscope will help you observe the insects in small mines. Parasite pupae and prepupae can easily be seen through the leaf by holding it up to a light. Record parasitism and mortality rates weekly. Parasitism can also be checked by collecting leafminer pupae that fall into trays placed beneath the plants. Make sure trays have small drain holes so they won't fill with water. Place the pupae in clear plastic containers and keep the samples at room temperature, out of direct sunlight, until parasites or flies emerge.

A sequential sampling plan for L. trifolii in chrysanthemum or gerbera crops has been developed in California. An estimate of adult leafminer populations is determined from the number of flies caught on yellow sticky traps spaced uniformly throughout a uniform crop. Larval densities are estimated from leaf samples taken during crop growth or at harvest. Treatments should be applied when larval and adult populations reach certain thresholds established in the plan.

The methods that have been developed for sampling leafminers provide fairly accurate assessments of leafminer populations, but the infestation level does not always correlate well with plant damage or aesthetic injury, especially on ornamentals. Although it is usually acceptable to have a maximum of 10% mined leaves, this maximum varies with the crop, the time of year, and the market for the product. For example, cut chrysanthemums can have mining damage on the lower leaves (which are removed at harvest), while the same plants grown for sale as potted plants cannot have any mines on the mature foliage. Vegetable crops, such as tomatoes, can tolerate fairly heavy leafminer damage without any effect on yield. Information on the population levels present will provide a basis for making pest management decisions, especially for determining how many parasites to introduce.

Natural enemies

Numerous wasp species parasitize leafminers, but few predators or pathogens attack leafminers.

Parasites

All of the parasites of leafminers parasitize the larval and/or pupal stages. Only those that have been studied as possible biological control agents in greenhouses are mentioned below. These wasps are in the families Braconidae, Eulophidae,and Eucoilidae, for which there are no common names.

Dacnusa sibirica. This European braconid wasp is a solitary endoparasite of all instars of L. bryoniae,L. trifolii, and the chrysanthemum leafminer.The adults are black and 1/8 inch (2-3 mm) long, with extended,flexible antennae. Using their antennae and ovipositors, the females locate leafminer larvae within the leaf and deposit single eggs in the larvae.They are best at finding late-instar larvae and prefer these for oviposition. Each female lays up to 90 eggs during a 2-week life span.The eggs hatch in 4 days, and the larvae complete their development in about 2 weeks in the living leafminer larvae. Adult wasps emerge from the leafminer puparium. The adults do not host feed.This wasp prefers cool conditions (65°-75°F). It is commercially available.

The European braconid wasp Dacnusa sibirica.

Diglyphus isaea. This Eurasian eulophid wasp parasitizes L. trifolii, L. bryoniae, and chrysanthemum leafminer.The V16-inch (1-2 mm) adults are black with short antennae. Females detect leafminer larvae by drumming their antennae along the mines.When a larva is found, the female drills through the leaf and into the leafminer body with the ovipositor, paralyzing the larvae.The female then either feeds on the body fluids of the leafminer larva or lays one to five eggs on the body of the host, depending on its size.The wasp larvae feed inside the leafminer body for 3-5 days, then pupate inside the mine. Feeding by the wasps stops leafminer and mine development.The adults emerge about a week later.The females live for 3-4 weeks and lay approximately 60 eggs. Over half of the leafminer mortality is caused by adult host-feeding, which is necessary for egg production.This wasp prefers warm conditions (75°-90°F). It is commercially available.

Other Diglyphus species. Several other species of Diglyphus have been studied as possible biological control agents for leafminers in greenhouses.The appearance, life cycle, and biology of these species are similar to that of D. isaea. None of these wasps are commercially available in the United States:

■ D. begini,a facultative—or environmentally adaptable—gregarious wasp, attacks all leafminer species. It develops much faster than L. trifolii.

■ D.intermedius host feeds on all instars of L. trifolii but prefers the third instar for oviposition. It also attacks the vegetable leafminer and chrysanthemum leafminer.

■ D.pulchripes parasitizes the vegetable leafminer.

The Eurasian eulophid wasp Diglyphus isaea.Adult females feed on leafminer larvae or lay eggs in them.

Ganaspidium utilis. This eucoilid wasp, native to subtropical areas of North America, is a larval-pupal parasite of vegetable leafminer and L. trifolii.The robust V20-inch (1.2-mm) adults are black with slender antennae. Females oviposit in leafminer larvae.The eggs hatch after the leafminer pupates, and the wasp larvae feed on the pupa within the host puparium. It is not commercially available.

The adult wasp Diglyphus beginiattacks all leafminer species. It is not yet commercially available in the United States.

The Eurasian eulophid wasp Diglyphus isaea.Adult females feed on leafminer larvae or lay eggs in them.

Opius dimidiatus. This American braconid wasp is one of the most abundant larval-pupal parasites of the vegetable leafminer and L. trifolii,b oth outdoors and in greenhouses in the United States and in southern Ontario, Canada. It will also parasitize L. bryoniae. It is not commercially available.

Opius dissitus. This braconid wasp is a native parasite of the vegetable leafminer in Florida that will also parasitize L.trifolii.The 1/16-inch (1.5-mm) females oviposit in leafminer larvae of any instar. After eggs hatch the wasp larvae consume the leafminers within their puparium. Adult wasps emerge about 2 weeks after the eggs are laid. This wasp is not commercially available.

Opius pallipes. This European braconid wasp is a solitary endoparasite of all instars of L. bryoniae and chrysanthemum leafminer larvae. It will lay eggs in L. trifolii, but the leafminer kills the eggs. Females locate larvae by drumming their antennae and inserting the ovipositor into the mine.They lay single eggs into larvae. After the wasp completes development in the leafminer, the host dies and the wasp emerges from the mine to pupate. Females do not host feed.This wasp is commercially available in Europe.

Oscinidius (=Chrysocharis) parksi. This American eulophid wasp is a solitary, larval-pupal endoparasite of L.trifolii and the vegetable leafminer.The 1/25-inch (1-mm) adults are metallic black with white legs. Females lay eggs in third-instar leafminers.The wasp larvae hatch and begin development in the leafminer larvae. Leafminers complete their larval development, then drop to the ground. They die when the wasp larvae start to pupate.The adult wasps emerge from the host puparium. By host-feeding, the females kill more leafminers than they need for reproduction.This wasp is not commercially available in the United States.

Predators

A few general predators, such as spiders and ants, will feed on adult or larval leafminers, but none are specific to leafminers.The tomato bug, Crytopeltis modestus,a facultative mirid predator of Liriomyza spp., has been investigated in California for leafminer control on chrysanthemum and tomato. Older nymphs and adults suck out the body contents of leafminer larvae within the mine and larvae that have emerged to pupate. However, because C.modestus feeds on tomato stems and only the older stages eat leafminers, this bug is considered a pest. Much more research is necessary before it can be recommended as a biological control agent.

Pathogens

Steinernema (=Neoaplectana) carpocapsae. This nematode is a common species with considerable potential as a biological control agent because of its rapid infectious action and environmental safety. (See "Fungus Gnats and Shore Flies"for a description and life cycle information.) The nematodes enter the mines through small tears in the leaf or punctures made by the adult leafminer, then infect second- and third-instar leafminer larvae through body openings. Nematodes may also be able to infect leafminer pupae in or on the growing medium. Foliar applications of nematodes may be effective against leafminer larvae in leaves, but very high relative humidity is required.The need for constant moisture is a major limitation in their effectiveness.

Possibilities for effective biological control

Biological control of leafminers is possible on many crops. Natural populations of parasites that often enter unscreened greenhouses from outdoors in early summer may provide some control. Releases of both larval and larval-pupal parasites can maintain leafminer populations at acceptable levels. However, the high cost of these parasites and uncertain availability may detract from their usefulness. Also, adequate biological control of leafminers on floral or foliage crops is difficult, because even a low population of leafminers may produce cosmetic damage.

Dacnusa sibirica is often used to control the first generation of leafminers, while Diglyphus spp. is used against later generations, but appropriate wasp selection depends on your specific circumstances. D.sibirica prefers cool temperatures, while Diglyphus isaea favors warmer conditions. Diglyphus may control subsequent generations more effectively if temperatures are around 80°F. Because D.sibirica attacks late-instar leafminers when mines are already well developed, this wasp is best for use on crops with little aesthetic value, such as cut flowers where the foliage is not harvested, or on young plants that will outgrow any early damage before harvest. Diglyphus attacks young larvae before mining damage is extensive, and therefore is particularly suitable for use on flower crops. Diglyphus can parasitize leafminers already parasitized by D.sibirica,so it usually becomes the dominant parasite under warm conditions.

Although your best guides are personal experience and the supplier's instructions, results of recent research using leafminer parasites on specific crops can help in selecting the proper natural enemies. Dacnusa sibirica, Diglyphus begini, D.pulchripes, and Oscinidius parksi have all been shown to be successful as biological control agents on greenhouse tomatoes in various countries. D.pulchripes and Opius dimidiatus nearly eradicated leafminers on tomato in an experimental greenhouse in Ohio within 8 weeks. Dacnusa sibirica moved from open rearing units to a lettuce crop to control nearly 100% of pea leafminer in the Netherlands. D.intermedius effectively controlled L. trifolii on cut chrysanthemums in experimental greenhouses in California.When D. begini was introduced early in the crop cycle it virtually eliminated leafminers on marigolds within 2 months and maintained control for another 2 months, even at high leafminer densities.The lower, older leaves were damaged, but the upper portions were free of injury.

Leafminer parasites are shipped either as pupae in small cardboard boxes or as adults in plastic tubes. Open the parasite shipment in the greenhouse in case some wasps have already emerged from the puparia. Distribute the parasites evenly throughout the greenhouse, but place extra wasps in leafminer hot spots or near especially susceptible cultivars.

Proper release rates depend on the leafminer species, the crop, the growth stage of the crop,and the time of year.A single inoculative release of parasites may succeed if made early in the growing season. However, multiple releases are more likely to provide effective control. Suppliers should provide detailed instructions on the use of natural enemies.Introduce the parasites as soon as the first leafminers are detected on yellow sticky traps or when feeding punctures are observed. It is difficult to control high populations of leafminers even with very high release rates if the releases are not made early. Provide the optimal environmental conditions of 60°-85°F and 50-90% relative humidity. Do not use residual insecticides within a month of parasite release. If the leafminer population is high,you can use a nonresidual spray, such as horticultural oil,azadirachtin,or natural pyrethrins (not pyrethroids),to reduce the number of leafminers before releasing the wasps.

Leafminers egg adult pupa

Dacnusa spp.

Diglyphus

Ganaspidium

Opius spp.

Oscinidius

Nematodes adult egg pupa

Tomato crops can tolerate relatively high leafminer populations without yield loss.Weekly releases of one parasite for every 10 new mines on the crop are suggested for the first 6 weeks of an infestation on tomato. On chrysanthemum, the recommendation is to release three adult Dacnusa sibirica for every 1,000 plants (500 per acre) in the week after planting. Make a second parasite release at the same rate in the sixth week after planting to control the progeny of any surviving leafminers. Diglyphus isaea is often preferred for the second introduction. Releases of 500 D. sibirica adults per acre every 2 weeks can be made instead. Either parasite should control an infestation of one mine per six plants.

Monitor carefully after the releases to be sure that parasitism is high. Even if 90% of the first leafminer generation is parasitized, the population can still increase from one mine per 100 plants to about one mine per plant in the next generation. Make weekly introductions of wasps until a well-established population is present.Weekly introductions take into account natural mortality of the wasps. Cool, cloudy weather and the use of pesticides will reduce the effectiveness of the parasites. Introduce up to 400 parasites per acre each week if less than 90% of the leafminer larvae are parasitized. Even when introductions are done properly the results may vary, and extra parasite releases or chemical control may be necessary in some cases.

Scientists at the University of CaliforniaDavis have developed grower-friendly software to help simplify biological control efforts against L. trifolii infesting cut chrysanthemums.The program determines the number of Diglyphus to release each week for the duration of the crop to eradicate L. trifolii before marketable foliage develops. A salable cut crop was produced without using any pesticides following the program's recommended release rate.

Currently parasites are the best option for biological control of leafminers. More research must be conducted on predators and pathogens before they can be used to control leafminers. Nematodes, however, may be useful for suppressing leafminer populations. If constant moisture can be maintained for at least 24 hours, which may be difficult to achieve in a commercial greenhouse, foliar applications of the nematode S. carpocapsae are about as effective as abamectin, but neither treatment eliminates severe leafminer infestations. Make applications in late afternoon or evening because the nematodes may be killed by sunlight or desiccation. Allow several hours of drying time to give the nematode time to move to the leafminer larvae.The optimal temperature for infection is about 70°F.

0 0

Post a comment