Description and Life History: These insects are annual pests in early planted sprinkler-irrigated 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 Sudangrass in August and September. Moving out of cotton, Sudangrass and desert flora, large numbers of these pests will migrate to seedling lettuce 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 1/4 inch 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 ¬ inch 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 ¬ inch 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 Sudangrass should be considered high risk fields and should probably be treated as soon as the seed begins 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 if re-application is necessary.
A) Darkling Beetle
B) Field Cricket
Description and Life History: Saltmarsh caterpillars are not normally a pest of fall grown vegetables but will often migrate 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 inches long. Adult moths have white to yellowish wings and are peppered with many black spots. Their wing span is approximately 2 inches 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.
Description and Life Cycle: In the Southwest, the sweetpotato whitefly is one of the principal pests of crops (Fig. 5). It was not considered an important pest until the early 1980's, when extremely large populations became common on cotton, melons and lettuce throughout the Southwest. In a short period of time, the sweetpotato whitefly shifted from a position as a secondary pest to being the primary pest for fall vegetables. This shift in pest status is thought to have occurred due to the introduction of a new strain of sweetpotato whitefly (B-strain). The B-strain is also known as the silverleaf whitefly. The host range of the B-strain is much greater than the old strain.
Figure 5. Sweetpotato whitefly adults, nymphs and eggs.
The eggs are deposited mainly on the underside of leaves. The eggs are minute, pointed, oblong, and yellow. One female will lay numerous eggs. Near hatching, the apex of the egg will darken. Eggs hatch in 2 to 5 days into crawlers with limited mobility. Crawlers (first instar nymphs) are yellowish in color and are oval and flattened in appearance. They are less than 1/10 inch 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 red-eyed 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 quite small. Following the pupal stage, fairly mobile adults emerge. These are capable of easily moving as far as 1 « mile from where they originated. As a consequence 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 by reducing head size, delaying harvest, and leaf chlorosis associated with whitefly feeding. Whiteflies also cause economic damage through contamination associated with the insect themselves, honeydew and sooty mold accumulation. Total destruction of early fall lettuce plantings has been observed because whiteflies have extracted 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 soil-applied systemic insecticide. Lettuce planted in October, or later when temperatures have declined, should be treated as needed with foliar adulticides. Best control is usually achieved from tank mixing insecticides. Good coverage is essential for adequate control.
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. During mid-morning, 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 practice 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.
Description 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 (Fig. 6). Problems with leafminers are most often prevalent in lettuce planted near 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.
Figure 6. Adult vegetable leafminer fly.
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. After completing three instars in 4 to 20 days, larvae emerge from the mines and drop to the soil to 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 secondary pathogens. When populations are high, plants may be killed or stressed to the point where pathogens 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 post-harvest 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 cannot 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. trifoliiis 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 for leafminers will not be necessary.
Description and Life History: Beet armyworm is a key pest of lettuce (Fig. 7). In Arizona, it is usually most prevalent from August through November on fall-planted lettuce. However, when temperatures are warm, this pest can be a problem season-long, particularly if alfalfa is nearby. 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 1¬ inches 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 mid-front margin, and the hindwings are white with a dark anterior margin. The wingspan of an adult is approximately 1 1/4 inches 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 seedling lettuce plants. Damage to lettuce is usually not economically damaging between thinning and cupping stages 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.
Management and Control: Cultural controls can help suppress armyworm populations. Disk fields immediately following harvest to kill larvae and pupae. Sanitation along field borders is important; armyworms often migrate from weedy field edges into newly planted fields.
Monitoring for beet armyworm on lettuce should begin before seedlings emerge. Control of beet armyworms on seedling lettuce is essential for stand establishment. Check weeds on ditch banks and field borders for larvae and egg masses as fields are being seeded. Once seedlings emerge, sample at least 25 plants in each quadrant of the field twice a week for armyworm egg masses and young larvae. The action threshold for all Lepidopterous larvae in fall lettuce between thinning and heading is one first or second instar larva for every 50 plants. Many growers have reported difficulty controlling beet armyworms with insecticides, and resistance to Lannate (methomyl) has been documented in Yuma County. Good spray coverage and insecticide resistance management tactics should be practiced. Target small larvae which are easier to control with insecticides. Timing insecticide applications at peak egg hatch will improve control. Addition of a B.t. to conventional insecticides will usually increase control and aid in resistance management. Just before and after heading, treat if Lepidopterous larvae reach one worm per 25 plants.
Description 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 (Fig. 8). Cabbage loopers occur year round in Arizona's central and southwestern desert areas. Populations are especially a problem in the fall, when newly-planted winter vegetables are emerging.
Figure 8. Cabbage looper larva.
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 larva. Cabbage looper pupae appear as greenish to brown pupas 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.
Management and Control: Monitoring for cabbage looper and other Lepidopterous pests should involve sampling plants twice a week once seedling emergence begins. When populations appear to be increasing, check more often. Follow the guidelines used for monitoring beet armyworms. On lettuce, monitor for eggs and larvae of loopers while checking for other caterpillar pests that feed on leaves and heads. Action thresholds are similar to those of beet armyworm: treat seedlings or small plants when populations of small loopers are large enough to stunt growth. If other Lepidopterous species are present, also include them in this total. Between thinning and heading, treat if the worm population reaches one larva per 50 plants. During head formation, treat if sample counts exceed one larva per 25 plants. Cabbage loopers are especially sensitive to B.t.s. Including a B.t. with insecticide applications targeting beet armyworms will usually control any cabbage loopers present.
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 corn and 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 toward 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 reddish-brown border. The wingspan of both moths is approximately 1 1/2 inches. 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 3,000 eggs. Heliothinae prefer to lay their eggs toward 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 or small bumps, 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 times before pupating. They will pupate in the soil for 10 to 25 days.
Damage: When early-season 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: Delaying lettuce planting until after nearby cotton is defoliated may help in reducing Heliothinae pressure. Follow the guidelines for monitoring other Lepidopterous larvae. Lettuce seedlings are very susceptible to Heliothinae damage. As soon as seedlings emerge, check for Heliothinae eggs. Be particularly attentive if the lettuce field borders cotton. Dark-colored eggs will be those near hatching. It is best to time insecticide applications at, or just after peak egg hatch. Lettuce should be treated if a significant number of Lepidoptera eggs and larvae are present.
Between thinning and heading, lettuce plants can tolerate up to one Lepidopterous larva per 50 plants. Be sure to correctly identify the species in your field. Biology and management guidelines for the tobacco budworm are essential the same as for corn earworm, except that the range of available insecticides is more limited for the budworm. Repeated insecticide treatments are often required to maintain low population levels.
During head formation, check for Heliothinae larvae every time you visit the field by pulling back the wrapper leaves or even cross sectioning some heads. Once heads form, keep Heliothinae populations as low as possible. One larva boring into a head will cause that head to be unmarketable. Treat when Lepidopterous larvae average one per 25 plants. Time applications to control the caterpillars during hatching and before they enter the protected areas within the head.
Description and Life History: The green peach aphid is considered the most economically important aphid pest on lettuce in the southwestern United States (Fig. 9). The green peach aphid is generally considered to be a pest in the spring.
Figure 9. Green peach aphid adult.
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 soil-applied systemic insecticide at planting. Other plantings can rely on foliar materials for control. Check fields twice weekly, but most intensely beginning mid-January. 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 follow-up scouting should be performed to determine if another insecticide application is necessary. Green peach aphids are often most numerous in fields containing mustards weeds, cheeseweed and members of the goosefoot family. Control of these weeds may help prevent the buildup of green peach aphid.
Description and Life History: Thrips are present season long in most lettuce fields, they are usually most abundant during the spring after temperatures have warmed. Thrips can build up in weedy areas, onions, wheat or non-irrigated pastures, moving to lettuce in large numbers when other hosts begin to dry down. Most thrips are slender, light-colored yellowish-brown insects not more than 1/16 inch long as an adult. They are very active and will rapidly scurry around on the plant tissue. The adults usually have distinctive red eyes and feathery wings. The small white bean shaped eggs are laid in the plant tissue. The eggs will 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.
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, while the male western flower thrips are light yellow (Fig. 10). Adult western flower thrips are about 1/20 inch in length. The immature stages are generally light yellow in color. Western flower thrips have reddish orange ocellar pigmentation and eight segmented antennae. Additionally, western flower thrips have long black setae at the front and back of the pronotum (Fig. 11).
Figure 10. Adult western flower thrips.
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 inch long. Its body is yellow with brown blotches on the thorax and abdominal terga. The legs of the onion thrips are yellowish brown, while the antennae appear striped. Antennal segment I and the base of segments III to V are brownish~white. The rest of the antenna is brown. The onion thrips has seven-segmented antennae and their ocellar pigment is generally gray. 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. The onion thrips has long back setae at the back of the pronotum, but lack them at the front (Fig. 11).
Figure 11. Onion thrips have seven segmented antannae and long black setae only at the back of the pronotum, while the western flower thrips have eight segmented antennae and long black setae at the front and back of the pronotum.
Damage: 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 wind burn or blown sand damage, but can be 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. Romaine lettuce is especially susceptible to thrips damage and should be closely monitored for thrips. Near harvest, thrips can act as contaminates to harvestable portions.
Management and Control: Thrips outbreaks are often associated with fields heavily infested with weedy mustard weeds or fields near commercial plantings of mustard, alfalfa 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 used alone should be avoided for control of western flower thrips, because they are suspected of causing populations to flare, but have been shown to be effective against onion thrips.