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Lettuce IPM


David L. Kerns and John C. Palumbo
Department of Entomology
Yuma Valley Agricultural Center
University of Arizona
Yuma, AZ 85364

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There are approximately 75,000 acres of lettuce grown in the southwestern United States, 75% of which is grown in Yuma County, Arizona. Most of the lettuce grown in the Southwest is field packed, meaning that the product is harvested, packaged in the field, and shipped to market with no further processing. Because most lettuce undergoes so little processing, it is essential that the product be free of insect damage and contamination at harvest. Because any insect, including beneficial predators and parasitoids, can act as a contaminant in lettuce, there is a heavy reliance on chemical control of insect pests in lettuce.

In the desert Southwest, lettuce is planted beginning in late­August, and continues into December. Fall lettuce is planted in late­August through mid­October, while Spring lettuce is planted from late October to December. Depending on when the lettuce is grown, insect pests and pressures can vary. Fall lettuce is subjected to the most severe insect pressures. In the fall, temperatures often exceed 100° F, and insect pests are very active. During this time, cotton is being defoliated and harvested, melons are being picked and beginning senescence, and summer annual weeds are abundant. Many of the insect pests encountered in fall lettuce originate in local fields of cotton, melons and weeds. When these sources begin to mature, their insect populations move to alternative crops such as lettuce. Monitoring insect pests in cotton, melons and alfalfa close to lettuce plantings is an essential predictive tool for forecasting pest severity in lettuce.

Lettuce market prices vary wildly depending on demand and availability. Lettuce has a short shelf live and is sold at harvest. Thus, growers do not the luxury of waiting for a favorable price. The lettuce market ultimately determines how much insect damage and contamination a packer will accept. When the lettuce price in fair and there is an abundance of lettuce being harvested, packers are very discriminating and only high quality lettuce with no insect damage or contamination is accepted. However, when the price is good and lettuce availability is low, packers will often accept lettuce that is of poorer quality, or has some insect damage or contamination. Because lettuce growers cannot predict the price of lettuce at harvest, growers manage their crops as if only high quality lettuce will be accepted. Thus, insect pests are not tolerated in lettuce crops and insecticides are relied on heavily.

I. Ground Dwelling Pests

Field crickets (Gryllus spp.), Darkling Beetles (Blapstinus spp.), Ground Beetles (Carabids) and Rove Beetles (Staphylinids).

Description and Life History
These insects are annual pests in early planted sprinkler­irrigated lettuce fields in the low desert. When they occur, they can quickly destroy most of a field. Problems are usually in fields planted closely to cotton or sedan grass in August and September. Moving out of cotton, sedan grass and desert flora, large numbers will migrate to seedling lettuce or cole crops if available. Most damage occurs at night. They hide during the day in soil cracks, ditches, weeds, and under irrigation pipes.

Darkling beetles are shiny dark black or brown and approximately ¼ in. in length. They are similar in appearance to many ground beetles. Darkling beetles normally have the tips of their antennae slightly enlarged, while ground beetles antennae are not enlarged on the tips. Most ground beetles encountered in lettuce are about ¼ in. in length and are black, brown, or reddish in color. Ground beetles are predators feeding primarily on other insects.

Rove beetles are mostly small elongated beetles less than ¼ in. in length and shiny dark black or brown. They have very short elytra covering their wings, but their abdomen is not covered. Rove beetles are often confused with winged ants or termites. When disturbed they will elevate their abdomens similarly to a scorpion. Rove beetles are insect predators or scavengers feeding on debris in the field.

Damage
Cricket and darkling beetles will destroy a crop by eating the newly emerged seedlings. Although ground beetles and rove beetles do not feed on the plants and are usually considered beneficial insects, they often damage fall vegetable crops by digging and rooting up the seed and small seedlings.

Management and Control
These insects are difficult to monitor. Early planted lettuce in close proximity to cotton or sedan grass should be considered high risk fields and should probably be treated as soon as the plants begin germinating

Apply baits around field edges to control migrating populations and apply insecticides through the sprinkler pipe during germination and when the plants emerge. Scout the field by looking under the sprinkler pipe to determine if control was achieved or reapplication is necessary.

II. Foliage Feeders

A. Saltmarsh Caterpillar, Estigmene acrea (Drury)

Description and Life History
Saltmarsh caterpillars are not normally a pest of fall grown vegetables but will often migrate in as larvae from neighboring cotton or alfalfa. Large populations can be extremely damaging to seedling lettuce.

The larvae are usually yellowish brown in color and covered with long, dark black and red hair. Many people refer to them as wooly bear caterpillars. Full grown larvae may be 2 in. long. Adult moths have white to yellowish wings and are peppered with many black spots. Their wing span is approximately 2 in. Eggs are laid in clusters of 20 or more on the leaves

Damage
Most damage occurs to early planted seedling lettuce. Large populations of larvae will move out of newly defoliated cotton and devour the young plants. After thinning, saltmarsh caterpillars are generally not a problem. However, they should be included in counts for Lepidopterous larvae. On older plants damage is distinctive. They prefer to feed in groups and will completely skeletonize several adjacent plants.

Management and Control: Scout adjacent cotton fields prior to lettuce emergence. It is best to control saltmarsh caterpillars before they enter the field. If possible treat the population in the cotton field during or just before defoliation. Saltmarsh caterpillars are particularly sensitive to Bacillus thuringiensis (B.t.).

Physical barriers are effective at preventing larvae from entering a field. Saltmarsh caterpillars do not like to cross fence type barriers of aluminum sheeting or irrigation pipe. These devises can be used to herd populations into holes containing cups of oil. Ditches filled with water containing liquid detergent or oil are also effective. Carbaryl can be sprayed around cotton fields or along ditches to kill migrating populations.

B. Major Lepidopterous Pests

Lepidopterous pests are probably the most important group of insect pests affecting lettuce production. Although, there are several major Lepidopterous pests of lettuce, management and control tactics for these pests are similar. Therefore, the management and control guidlines for beet armyworm, cabbage looper and Heliothinae have been combined.

1. Beet Armyworm, Spodoptera exigua (Hübner)

Beet armywormDescription and Life History
Beet armyworm is a key pest of lettuce. In Arizona, it is most prevalent from August through November on fall­planted lettuce. However, when temperature are warm, this pest can be a problem season long. The larvae feed on many field crops, including cotton and alfalfa, and often migrate from these crops onto lettuce in the fall. Several summer annual weeds also serve as hosts.

Eggs are light green in color and are laid in irregular clumps or masses, usually on the under surface of leaves. One female will lay on average 500 to 600 eggs over a 4 to 10 day period. The female moth covers the eggs with white scales from her body, giving the egg masses a cottony appearance. Eggs will darken as they near hatching, and will hatch in 2 to 5 days. The young larvae will feed in groups and spin webs over the underside of the foliage where they are feeding. Larvae vary in color, but are usually olive green with light colored stripes down the back and a broader stripe along each side. Beet armyworms usually have a dark spot on the side of the body above the second true leg. Mature larvae vary in size but are usually about 1¼in. in length. Larvae will generally pass through five instars. The armyworm larvae disperse as they get older and move toward the center of the plant. Large larvae are quite mobile, and a single larva may attack several plants. Larvae reach maturity in about 2 to 3 weeks in warm weather and pupate in the soil. The moth has grayish brown forewings with a pale spot in the mid­front margin, and the hindwings are white with a dark anterior margin. The wingspan of an adult is approximately 1¼in. The entire life cycle from egg to adult requires approximately 36 days at 80°F.

Damage
Hatching larvae begin feeding on the leaf and may completely consume seedlings. Beet armyworms may severely stunt or kill seedlings lettuce plants. Damage to lettuce is usually not economically damaging between thinning and cupping unless populations are high. However, once cupping begins larvae may feed on the head, rendering it unmarketable. Armyworm larvae enter heads from the bottom working their way inward while feeding along the leaf margins. Often the damage cannot be seen without removing frame leaves and dissecting the head.

2. Cabbage Looper, Trichoplusia ni (Hübner)

Cabbage looperDescription and Life History
The cabbage looper is a very destructive pest on lettuce and will feed on many other crops including cole crops, leafy greens, melons, tomatoes, and cotton. Cabbage loopers occur year round in Arizona's central and southwestern desert areas. Populations are a especially a problem in the fall, when newly-planted winter vegetables are emerging.

Cabbage looper moths lay single, dome-shaped eggs on the under side of older leaves. A single female may lay 275 to 350 eggs. Eggs will darken as they age, and will hatch in 2 to 5 days. The larvae are light green in color and have a distinctive white stripe along each side of the body. The larvae have two sets of legs in the front of the body and three sets of fatter, unjointed prolegs at the rear. They move in a "looping" manner, arching the middle portion of the body as they move forward. Two to four weeks are required for full development to a 5th instar larvae. Cabbage looper pupae appear as greenish to brown pupa wrapped in a delicate white cocoon of fine threads usually attached to the underside of the leaf. Pupation usually takes 10 to 16 days. The moth is mottled brown in color, and has a small silvery spot (sometimes a figure 8) near the middle of its front wing. Cabbage loopers may have 3 to 5 generations per year.

Damage
Loopers damage plants by eating ragged holes in leaves, and sometimes working their way into heads. They also cause damage by contaminating marketable portions with their bodies and frass. High populations can chew seedlings severely enough to kill them or slow growth enough to inhibit uniform maturing of the crop, but most economic damage occurs after heading. Young plants between thinning and heading can tolerate substantial feeding by loopers and other caterpillars without loss of yield or quality. Heads contaminated with loopers, or tunneled into by loopers are not marketable.

3. Heliothinae
Corn earworm, Helicoverpa zea (Boddie) and Tobacco bubworm, Heliothis virescens (Fabricius)

Heliothinae (budworm)Description and Life History
Heliothinae are very destructive pests of many crops including corn, cotton, tomatoes, lettuce, soybeans, and grain sorghum. Heliothinae frequently move into lettuce from surrounding crops, particularly cotton. This pest occurs statewide but is most common in central and western Arizona. Although, the tobacco budworm is the predominant species in lettuce in the low desert areas, both corn earworm and tobacco budworm are very similar in appearance and biology, and their management strategies in lettuce are the same.

Corn earworm moths vary in color but most have light grayish brown front wings with irregular lines and dark areas towards the tip of the wings. The hindwings are white with irregular dark spots. The front wings of the tobacco budworm moth are pale olive in color with three narrow, dark, oblique bands. The hindwing is white with a reddish­brown border. The wingspans of both moths is approximately 1½ in. Female moths lay their eggs singly on lettuce leaves. Eggs are white when laid but develop a dark red or brown ring around the top within 24 hours. They darken before hatching as the larvae develop inside. Deeper ridges and a more hemispherical shape distinguish Heliothinae eggs from those of cabbage or alfalfa loopers. One female moth will lay 500 to 3000 eggs. Heliothinae prefer to lay their eggs towards the crown portion of the plant, in the terminal growth. Eggs hatch in 2 to 10 days.

Along the backs of newly hatched Heliothinae are discrete rows of tubercles with one or two hairs protruding from each. Heliothinae larvae are cannibalistic and will eat siblings and other Lepidopterous larvae. Thus, they are usually not found in close proximity to other worms. They prefer to feed on the terminal growth or heads of lettuce plants. Heliothinae larvae usually develop distinct stripes as they mature, but overall color of caterpillars is variable. The tubercles and hairs remain obvious on older larvae that are dark colored but are less visible on lighter ones. In addition to the larger hairs and tubercles, Heliothinae have tiny short spines covering large portions of the skin that can be seen with a 10X hand lens. These tiny spines distinguish the corn earworm and the closely related tobacco budworm from all caterpillars likely to be found on lettuce. Heliothinae larvae will feed 2 to 4 weeks and molt five time before pupating. They will pupate in the soil for10 to 25 days.

Damage
When early­season populations are high, Heliothinae can decimate seedling stands of lettuce. Damage to seedlings is similar to that caused by beet armyworms. Heliothinae are much more likely to bore into lettuce heads than other Lepidoptera larvae, rendering the heads unmarketable. Larvae feed in the plant's crown leaving holes and gouges in the midrib, sometimes killing the growing point. Potential for damage decreases as the seedlings grow; economic damage is not common between thinning and head formation.

Once heads form, large Heliothinae larvae will usually bore into the head. Larvae may enter the head from any point, although they usually burrow in from the top half. When burrows begin under or between the wrapper leaves, the infestation may not be noticed until the head is harvested. Once inside the head, Heliothinae are protected and difficult to control with insecticides.

Management and Control of Major Lepidopterous Pests
Cultural controls can help suppress Lepidoptera populations. Disc fields immediately following harvest to kill larvae and pupae. Sanitation along field borders is important; beet armyworms often migrate from weedy field edges into newly planted fields. Delaying lettuce planting until after nearby cotton is defoliated may help in reducing beet armyworm and Heliothinae pressure.

Control of Lepidoptera larvae on seedling lettuce is essential for stand establishment. Monitoring for all Lepidoptera pests on lettuce should begin before seedlings emerge. Check weeds on ditch banks and field borders for Lepidoptera larvae and eggs as fields are being seeded. Once seedlings emerge, sample at least 25 plants in each quadrant of the field twice a week for eggs and young larvae. If one larva per 100 seedling plants is present, an insecticide application is probably warranted. Target small larvae, they are easiest to control with insecticides. Timing insecticide applications at peak egg hatch will improve control and kill the larvae before they have time to enter protected areas within the head. Dark colored eggs are those near hatching. The action threshold for Lepidopterous larvae in lettuce between thinning and heading is one larva for every 50 plants.

During head formation, Lepidoptera larvae should be scouted for two to three times a week depending on insect pressure. Scouting should include pulling back wrapper leaves and dissecting some heads. Just before and after heading, treat if Lepidopterous larvae reach one larva per 25 plants. Repeated insecticide treatments are often required to maintain low population levels.

Correct species identification is essential in determining which insecticides to use. Insecticides effective against one species, are often ineffective against another. When several species are present in large numbers, tank mixes of insecticides is usually required. Many growers have reported difficulty controlling beet armyworms with insecticides. Good spray coverage and insecticide resistance management tactics should be practiced. Addition of a B.t. to conventional insecticides will usually increase control and aid in resistance management. Additionally, cabbage loopers are especially sensitive to B.t.s. Including a B.t. with insecticide applications targeting beet armyworms will usually control most cabbage loopers present.

C. Leafminers

Vegetable Leafminer, Liriomyza sativae Blanchard and Liriomyza trifolii (Burgess)

Liriomyza leafminerDescription and Life History
Liriomyza leafminers occasionally cause economic damage to lettuce. The principal leafminer species in Arizona include L. trifolii and the vegetable leafminer, L. sativae. Problems with leafminers are most often likened to nearby cotton or melon fields. On lettuce planted in August or September, L. sativae is usually the predominant species, but by February, L. trifolii usually predominates.

The leafminer adults are small, shiny black and yellow flies with a bright yellow triangular spot on the upper thorax between the wings. Subtle differences in color exist between adult L. sativae and L. trifolii. The latter species has developed resistance to many insecticides. Females puncture young leaves and oviposit eggs within the leaf. Both male and female flies often feed at puncture sites. After 2 to 4 days, larvae hatch and begin feeding on plant mesophyll tissue just below the upper surface of the leaf. The winding tunnels that result are initially small and narrow, but increase in size as the larvae grow. Larvae emerge from the mines after completing three instars in 4 to 20 days, drop to the soil and pupate. Pupation takes 7 to 25 days. At temperatures of 50°F or lower, development ceases. On lettuce, leafminers sometimes complete the pupal stage on the plant near the base of the leaf.

Adult flies emerge from the pupae after about 7 to 11 days. The entire life cycle can be completed in less than 3 weeks when the temperatures are warm. Many generations are produced each year in Arizona.

Damage
Mining of leaves by the larvae is the principal cause of plant injury. The mines reduce plant photosynthesis, render harvestable portions unmarketable, and provide an access for pathogens. When populations are high, plants may be killed or stressed to the point where pathogen can easily infect the plant. Leafminers can also cause damage after harvest. Leafminers that cut out of the leaf tissue after harvest will sometimes pupate in between the leaves. These pupae not only act as contaminates, but will often die and rot, providing a substrate for post­harvest pathogens to infect the lettuce.

Management and Control
Monitor young seedlings regularly for the presence of leafminers. In lettuce, most mines occur on the cotyledons and first true leaves. After thinning, sample leaves from the middle portion of the plant. If leafminer populations build to high levels when seedlings have only four or five leaves, chemical treatment may be necessary. The threshold for leafminers in lettuce is an average of one or more active mines per leaf except on the marketable portions where damage cannoat be tolerated.

Sticky traps can assist in determining when early migrations take place, and also help in determining species composition. It is important to identify the predominant leafminer species, L. trifolii is much harder to control with insecticides than L. sativae.

Natural enemies, primarily parasitic wasps in the Diglyphus, Opius and Chrysocharis genera, usually maintain leafminer population below economic injury levels. Parasitoids are often killed by insecticides applied to control other pests such as beet armyworm. This results in a secondary outbreak of leafminers. Use of selective insecticides for control of worms will often preserve leafminer parasitoids so that treatment will not be necessary.

III. Sucking Pests

A. Sweetpotato Whitefly, Bemisia tabaci (Gennadius) B­Strain a.k.a Silverleaf Whitefly, Bemisia agentifolii Bellows and Perring

Sweetpotato whiteflyDescription and Life Cycle
On a worldwide basis the sweetpotato whitefly is one of the principal pests of crops grown for food and fiber. It was not considered an important pest until 1981, when extremely large populations became common on melons and lettuce throughout the Southwest. In ten years, sweetpotato whitefly has shifted from a position as a secondary pest to being the primary pest for fall vegetables and cotton in the Southwest. This shift is pest status is thought to have occurred due to the development of a new strain of sweetpotato whitefly (B­strain). The B­strain is also known as the silverleaf whitefly. The host range of the B­strain is thought to be much greater than the old strain, and it is better at tolerating insecticides.

The eggs are deposited mainly on the underside of leaves. The eggs are minute (0.2 mm), pointed, oblong, and yellow. One female will lay 50 to 400 eggs. Near hatching, the apex of the egg will darken. Eggs hatch in 2 to 5 days into crawlers with limited powers of mobility. Crawlers (first instar nymphs) are yellowish in color and are oval and flattened in appearance. They are 0.2 to 0.3 mm in length, and will move about until they locate a minor vein. Once they locate an acceptable feeding site, they become immobile and remain so through four nymphal instars. They feed by inserting their tubular mouthparts into the vein and extracting phloem sap. Late third and fourth instar nymphs have distinctive red eye spots and are termed red­eyed nymphs. At the end of the fourth instar they enter what is called the pupal stage. Their pupal cases are dome shaped and oval in their outline, and are 0.7 to 0.8 mm in length. The size of the whitefly population in a field of fall lettuce is related, in a large part, to the proximity of the crop to cotton or melon fields.

Damage
Damage by large whitefly populations can result in crop injury through reduced head size, delayed harvest, and leaf chlorosis associated with whitefly feeding. Whiteflies also cause economic damage through contamination associated with the insect themselves, and honeydew and sooty mold accumulation. Total destruction of early fall lettuce plantings has been observed because whiteflies have extracted such large amounts of phloem sap from seedlings.

Management and Control
Lettuce planted in high risk situations (August and September plantings, or later plantings near a significant whitefly source) should be treated prophylactically with a soil­applied systemic insecticide. Lettuce planted in October, or later when temperatures have receded and there is no significant source of whitflies in a one mile radius, should be treated as needed with foilar adulticides. Best control is usually achieved from tank mixing insecticides. Good coverage is essential for adequate control. If possible use a high pressure ground applicator delivering approximately 60 gallons per acre at 300 p.s.i.

Lettuce should be monitored as soon as the plants emerge. Whitefly populations will build in cotton and alfalfa, so growers should pay particularly close attention to lettuce planted downwind or adjacent to these fields. Once whitefly adults appear in a field in sufficient numbers, treatments should begin. Whiteflies are best controlled by preventing colonization; do not allow adults to build and lay eggs. Monitor for whiteflies early in the morning when the adults are sedentary. Once temperatures begin to increase, the adults will begin to stir and move, and they will become difficult to count. Mid­morning, monitor movement by looking for dispersing swarms.

Delaying plantings of fall lettuce until after most cotton has been defoliated and harvested will avoid major whitefly population flights. Destruction of crops post harvest is a valuable tool for preventing whitefly population escalation. Once temperatures begin to cool, whiteflies are generally not a problem on lettuce. Thus, spring lettuce is generally not affected by whiteflies.

B. Green Peach Aphid, Myzus persicae (Sulzer)

Green peach aphidDescription and Life History
The green peach aphid is considered the most economically important aphid pest on lettuce in the southwestern United States. The green peach aphid is generally considered to be a pest in the spring

Winged green peach aphid adults have a black/brown head and thorax. Their abdomen is light green or red with a black/brown mottling. At the base of each antenna of many aphids is a small bump called a tubercle. In green peach aphids these tubercles are pronounced and converging inwardly, while similar species tubercles are less pronounced or diverging. Wingless adults are light green or red with the same antennal bumps. Nymphs appear as smaller versions of the wingless adults.

The life cycle of the green peach aphid is typical for aphids. In southern climates it reproduces asexually. Populations consist entirely of female aphids giving live birth to female progeny. This type of reproduction gives aphids a tremendous reproductive capacity. One female can easily give birth to 80 to 100 young in her lifetime of about 30 days. In colder climates green peach aphids overwinter as eggs laid on peach trees (or occasionally on apricot or prune).

In response to crowding by other aphids or declining host plant quality, migratory (winged) forms are produced that move to new hosts (weeds or crops). This ordinarily occurs in the early spring.

Damage
Green peach aphid damages lettuce primarily by acting as a contaminant. Packers will accept very little aphid contamination. Additionally, green peach aphids serve as vectors for alfalfa mosaic virus, lettuce mosaic virus, and beet western yellows virus that affects lettuce. The viruses that attack lettuce have not been a problem in recent years. Predicting virus problems is virtually impossible as it is dependent upon when aphid flights take place and levels of inoculum. Little if anything can be done to prevent the disease even if we could make accurate predictions.

Management and Control
Lettuce planted so that harvest will occur during February and March, should be prophylactically treated with a soil­applied systemic insecticide at planting. Other plantings can rely on foliar materials for control. Check fields twice weekly, but most intensely beginning mid­January. Green peach aphid prefers the underside of the older leaves. After thinning and before heading, colonies of ten or fewer aphids, can be tolerated. Once lettuce nears head formation, green peach aphids cannot be tolerated. The key to controlling green peach aphid is to prevent the formation of large colonies. Adequate control is often difficult to achieve with foliar sprays, and follow­up scouting should be preformed to determine if another insecticide application is necessary. Green peach aphids are often most numerous in fields containing weedy mustards and members of the goosefoot family. Control of these weeds may help prevent buildup of green peach aphid.

C. Thrips

Western Flower Thrips, Frankliniella occidentalis (Pergande) and Onion Thrips, Thrips tabaci Lindeman

Description and Life History
Thrips are present season long in most lettuce fields, but are usually most abundant during the spring after temperatures have warmed. Thrips can build up in weedy areas, onions, wheat or non­irrigated pastures, moving to lettuce in large numbers when other hosts begin to dry down. Thrips are minute slender elongated insects. They have feathery strap­like wings. They are very active and will rapidly scurry around on the plant tissue. Because of their difference in ease of control, it is important to be able to distinguish between western flower and onion thrips. The western flower thrips is very difficult to define, because it has many color forms. The females range in color from light yellow, yellow with brown blotches on the body, to dark brown. Male western flower thrips are light yellow. Adult western flower thrips are about 1/20 in. (1.5 mm) in length. Immatures are generally light yellow in color. Western flower thrips have reddish­orange ocellar pigmentation and eight­segmented antennae. In the United States, the onion thrips reproduce asexually through parthenogenisis. There are no male onion thrips in the U.S. The female onion thrips is slightly smaller than female western flower thrips, being only 1/25 in. (1.2 mm) long. The body is yellow with brown blotches on the thorax and abdominal terga. The legs are yellowish­brown and the antennal segment I and the base of segments III to V are brownish­white, the rest of the antenna is brown. Their ocellar pigment is gray, and they have seven­segmented antennae. Immature onion thrips are generally light yellow in color. Adult light colored forms are predominant during warm temperatures while brown forms occur when it is cool.

Thrips metamorphosis is between simple and complete. The stages are: egg, two larval stages, prepupae, pupa, and adult. The small white bean shaped eggs are laid in the plant tissue and hatch in 2 to 7 days. Nymphs resemble adults except for their smaller size, lack of wings, and are paler in color. Nymphs will pass through four instars in 15 to 30 days. The two later instars do not feed and are passed in the soil or litter as pupae beneath the plants.

Damage
Thrips feed by making a hole in the epidermis with a single mandible, puncturing cells with the maxillary stylets, and sucking the sap that oozes out. Thrips feeding causes the leaves of slow growing seedlings to become wrinkled and distorted. However, this injury may be of little importance if temperatures are warm enough for the plants to outgrow it. Feeding also causes brown scarring of leaves; giving them a speckled or scorched appearance. This type of damage is often confused with windburn or blown sand damage, but can distinguished by numerous black flecks scattered over the discolored area; these are the feces of thrips. On young plants, severely damaged leaves dry out and drop. Damage is often not noticed until the thrips population has declined or disappeared. Near harvest, thrips can act as contaminates in harvestable portions.

Management and Control
Thrips outbreaks are often associated with fields heavily infested with weedy mustards or fields near commercial plantings of mustard or wheat. Many thrips populations, particularly western flower thrips, are resistant to some insecticides and adequate control is often difficult to achieve. Chemical control should only be used when thrips populations are extremely high or product contamination is of concern. Pyrethroids should be avoided for control of western flower thrips because they are suspected of causing thrips population flares, but have been shown to be effective against onion thrips.

References

Anonymous 1987. Integrated pest management for cole crops and lettuce. Univ. of Calif. Statewide IPM Proj., Publication 3307. 112 pp.

Kerns, D. L. and J. C. Palumbo. 1995. Using Admire on desert vegetable crops. Univ. of Ariz., Coop. Extn. 195017, Tucson. IPM Series No. 5.

Kerns, D. L., J. C. Palumbo and D. N. Byrne. 1995. 1995 Insect pest management guidelines for cole crops, cucurbits, lettuce, and leafy green vegetables. Univ. of Ariz., Coop. Extn. Publ. 195007, 34 pp.

Palumbo, J. C., D. L. Kerns, C. E. Engle, C. A. Sanchez and M. Wilcox. 1996. Imidacloprid formulation and soil placement effects on colonization by sweetpotato whitefly (Homoptera: Aleyrodidae): head size and incidence of chlorosis in lettuce. J. Econ. Entomol. 89: In press.

Palumbo, J. C. and D. L. Kerns. 1995. Effects of imidacloprid as a soil treatment on colonization of green peach aphid and marketability of lettuce. Southwest. Entomol. 18: 339­346.

Palumbo, J. C. C. H. Mullis, Jr. and F. J. Reyes. 1994. Composition, seasonal abundance. and parasitism of Liriomyza (Diptera: Agromyzidae) species on lettuce in Arizona. J. Econ. Entomol. 87: 1070­1077.