Lygus elisus Van Duzee Hemiptera Miridae

Natural History

Distribution. These insects are native to North America. Both in appearance and biology, these species are quite similar, and have long been plagued with taxonomic problems. Therefore, they are best considered as a group.

Tarnished plant bug, Lygus lineolaris (Palisot de Beauvois) is widely distributed and is considered damaging wherever it occurs. It is found throughout the United States, including Alaska. In Canada, it is found from Newfoundland to British Columbia, including the Yukon.

Western tarnished plant bug, Lygus hesperus Knight, and pale legume bug, Lygus elisus Van Duzee, are more common in western North America. Western tarnished plant bug is found only as far east as Alberta in Canada, and Wyoming and western Texas in the United States. Pale legume bug occurs as far east as Manitoba in Canada, and Illinois in the United States. The western tarnished plant bug and pale legume bug are considered to be damaging principally west of the Rocky Mountains.

Host Plants. These Lygus species have a very wide range of hosts. The host range of tarnished plant bug is greatest because it occurs over a wider geographic range and is well-studied; Young (1986) listed over 300 species. Over half of the cultivated-plant species grown in the United States are listed as host plants for tarnished plant bug. However, all three species are similar in their damage potential. The favored habitat seems to be crops, weeds associated with crops, and other early successional plant communities. Barlow et al. (1999) reported that adults of L. hesperus survived better on dicot weeds than on monocots.

Among vegetable crops injured are artichoke, asparagus, broccoli, cabbage, carrot, celery, corn, chard, Chinese cabbage, coriander, cowpea, cucumber, eggplant, endive, escarole, faba bean, fennel, horseradish, lettuce, lima bean, mustard, onion, parsnip, parsley, pea, pepper, potato, radish, salsify, snap bean, spinach, squash, sweet potato, Swiss chard, tomato, turnip, and watermelon. Field crops such as alfalfa, cotton, and safflower, as well as various fruit and flower crops frequently are damaged by Lygus plant bugs. Numerous weeds serve as hosts; among those frequently cited are aster, Aster spp.; curly dock, Rumex crispus; fleabane, Erigeron spp.; goldenrod, Soli-dago altissima; lambsquarters, Chenopodium album; pigweed, Amaranthus spp., ragweed, Ambrosia spp.; and shepherdspurse, Capsella bursa-pastoris. Although Lygus plant bugs generally are considered to be herbivores, they sometimes engage in predation of other insects, particularly eggs and young larvae (Cleveland, 1987), or even in cannibalism.

Natural Enemies. Several parasitoids of Lygus spp. are known, but the egg parasite Anaphes iole Gir-ault (Hymenoptera: Mymaridae), and the nymphal parasites Leiophron uniformis (Gahan), Peristenus pal-lipes (Curtis), and P. pseudopallipes (Loan) (all Hymen-optera: Braconidae) are thought to be relatively important (Clancy and Pierce, 1966; Scales, 1973; Lim and Stewart, 1976; Jackson and Graham, 1983; Graham et al., 1986; Day, 1987; Sohati et al., 1989, 1992; Snod-grass and Fayad, 1991; Al-Ghamdi et al., 1995). Other parasitoids (Hymenoptera: Ichneumonidae, Mymaridae, Scelionidae; Diptera: Tachinidae; Nematoda: Mermithidae) occur occasionally. The native parasi-toids seem to be more effective at parasitizing Lygus on weeds than on crops. Several Peristenus species have been imported from Europe and released in the United States (Day et al., 1990). The imported parasi-toid Peristenus digoneutis Loan (Hymenoptera: Braco-nidae) is reported to have decreased tarnished plant bug abundance by 75% in the New Jersey. In addition, it seems not to have affected native parasitoids (Day, 1996). A key for identification of parasitoids affecting these plant bugs was provided by Loan and Shaw (1987).

The predators of Lygus spp. are reported to include Geocoris spp. (Hemiptera: Lygaeidae), Orius spp. (Hemiptera: Anthocoridae), Nabis spp. (Hemiptera: Nabidae), Podisus maculiventris (Say) (Hemiptera: Pen-tatomidae), Sinea diadema (Fabricius) (Hemiptera: Reduviidae), and lady beetles (Coleoptera: Coccinelli-dae) (Leigh and Gonzalez, 1976; Whalon and Parker, 1978; Fleischer and Gaylor, 1987; Arnoldi et al., 1991). Probably many other predators feed on Lygus bugs, particularly the nymphs.

Life Cycle and Description. These insects overwinter as adults. In cold climates they may remain on crop residues or weeds. Khattat and Stewart (1980) speculated that when the food plant became frozen or dry the insects resorted to predation and cannibalism. Emergence from overwintering is protracted, but adults can be found feeding and ovipositing on weeds as soon as the plants are available in the spring. Even in Canada, two generations are common, with the first generation adults produced in July and the second in August and September. In Quebec, a third generation sometimes is observed, with adults produced in late September and early October (Stewart and Khoury, 1976). There seems to be no accurate determination of generation number in warmer climates, but five generations have been suggested for North Carolina.

  1. The egg is inserted into the plant tissue. The whitish egg is cylindrical, and slightly curved. The top of the egg, where it meets the surface of the plant tissue, is flattened and bears a small elliptical opening through which the hatching nymph escapes. The egg length is about 1.7 mm, and width is about 0.5 mm. They often are deposited in leaf petioles or at the base of the leaf blade, but the preferred location varies with the crop attacked. They usually are deposited singly, but sometimes more than one egg will be found in a single oviposition slit. Duration of the egg stage of L. lineolaris is reported to be 14.6, 7.6, and 6.6 days when incubated at 20°, 25°, and 30°C, respectively. In L. hesperus and L. elisus, incubation duration at uncontrolled temperature is reported to be 9.6 and 9.4 days, respectively. Khattat and Stewart (1977) estimated egg production at four to five eggs per day, and total production at nearly 100 eggs, based on laboratory rearing. However, Gerber (1995) reported mean (range) progeny production of 239 (25-532) for overwintered females, and 303 (6-704) for summer females, based on field collections of insects. The latter data, though higher than reported by most authors, is likely to be more reflective of normal fecundity.
  2. There are five instars. The young nymphs are greenish, with red on the antennae. In both L. line-olaris and L. hesperus, the nymphal color tends to be yellow-green, while in L. elisus the color is bluish-green. The early stages, especially instars 3-5, bear a pair of circular black spots dorsally on the first and second thoracic segments, and a single spot near the base of the abdomen. When reared at 25°C, mean duration (range) of the five instars in L. lineolaris was 4.8 (4-6), 3.1 (2-5), 3.3 (2-5), 3.3 (2-5), and 5.2 (4-6) days, respectively. Total nymphal development time was 31.5, 19.7, and 14.9 days at 20°, 25°, and 30°C, respectively (Ridgway and Gyrisco, 1960). In contrast, mean nymphal development times at uncontrolled temperature was 3.9, 3.2, 2.5, 3.2, and 4.9 days for L. hesperus, and 4.2, 2.9, 2.7, 3.7, and 5.1 days for L. elisus (Shull, 1933).
  3. The adults measure about 4.0-6.0 mm long. Summer adults tend to be pale, mostly pale-green or yellow with brown or black markings. The level of dark pigmentation present in an adult is positively correlated with rearing temperature, and it increases with physiological age (Wilborn and Ellington, 1984). Overwintered adults are much darker than summer adults. The pronotum and scutellum are quite variable in color, varying from mostly yellow or light-green to black, but usually there is at least some light pigmentation at the tip (posterior) of the scutellum, often resembling a heart in shape. The front wings (hemely-tra) may be light or dark, but they usually are lighter apically. Legs are yellowish, often marked with reddish-brown. Antennae are yellowish-brown to brownish-black. Females commence oviposition 7-10 days after molting to the adult stage. The adult longevity is estimated at about 30-40 days for males and about 40-60 days for females.
0viposition
Tarnished plant bug nymph.

The other common species of plant bugs, Adelpho-coris spp., tend to be fairly elongate in general body form (see sections on alfalfa plant bug and rapid plant bug). When viewed from above, Adelphocoris spp. are about three times as long as wide. In contrast, the Lygus spp. tend to be fairly oval, about twice as long as wide. (See color figure 140.)

Differentiation of the Lygus species is very difficult, and probably is better left to a specialist; Kelton (1975) and Schwartz and Foottit (1998) provided keys. Knight (1941) provided some readily observable morphological characteristics to distinguish among the species, but they seemed not to be definitive. Briefly, Knight suggested that the body of western tarnished plant bug and pale legume bug were chiefly pale or green, sometimes with dark markings. In contrast, the body of tarnished plant bug is said to be yellowish-brown to reddish or black. The western tarnished plant bug and pale legume bug are further distinguished by the color of the abdomen; it is uniformly green in L. elisus and marked with black in L. hesperus.

A comprehensive treatment of L. lineolaris biology was given by Crosby and Leonard (1914), and of L. hesperus and L. elisus by Shull (1933). Developmental biology for L. lineolaris was provided by Ridgway and Gyrisco (1960), L. hesperus by Champlain and Butler (1967), and L. elisus by Butler (1970). Examples of rearing techniques based on plant parts were presented by Khattat and Stewart (1977) and Slaymaker and Tug-well (1982); an artificial diet was given by Debolt (1982).

Damage

Various types of injury are inflicted by Lygus plant bugs. Examples include sunken or darkened lesions, tip dieback, gall formation, shedding of blossoms,

Picture Lygus Damage Celery
Adult tarnished plant bug.

inhibition of seed germination, and leaf distortion. On celery, the foliar discoloration and tissue destruction is sometimes called "blackheart." The Lygus plant bugs tend to prefer feeding on meristematic tissue, floral buds and immature blossoms, and immature fruit relative to leaves, stems, and mature fruit. The term "blasting" is sometimes applied to the destruction by Lygus bugs of floral tissue, which turns brown, shrivels, or drops from the plant. Wukasch and Sears

  • 1981) working in Ontario, and Grafius and Morrow
  • 1982) in Michigan, both studied the effects of plant bugs on asparagus. Bug feeding caused collapse of spears and distortion of ferns, and the amount of injury was proportional to bug density. The plants were not permanently injured, tending to recover unless plant pathogens invaded the feeding sites. In Idaho, seed production by carrots was affected by both western tarnished plant bug and pale legume bug (Scott, 1970). Production of seed by cabbage plants in Washington was found to be affected severely; a single pale legume bug could destroy 1.7 fruiting structures per day (Getzin, 1983). Research conducted in Quebec by Khattat and Stewart (1975) showed that flower buds, blossoms and setting pods of beans could be damaged by L. lineolaris, but that it was principally late-instar nymphs and adults that inflicted damage.

Management

Sampling. Plant bug populations in crops tend to be aggregated or clumped. This necessitates large plant samples if population assessments are to be accurate. In celery, for example, Boivin et al. (1991) used visual examination and fixed samples of about 75-125 plants per field in decision-making. However, they were able to reduce sampling intensity by developing sequential sampling protocols. They estimated treatment thresholds at 0.5-1.0 insect per plant depending on the stage of plant growth.

Sampling efficiency on snap beans was studied by Stewart and Khattat (1980), who compared sweep net and suction sampling. They found that the two methods were highly correlated, but that sweep net samples of nymphs tended to underestimate the population. Multiplication by a correction factor of 1.8 was suggested. Their data suggest that sweep net catches of 25 insects per 50 sweeps is equivalent to vacuum samples of one insect per 10 plants. These authors suggested that economic injury occurred when bug densities reached about two insects per 10 plants in the blossom or pod set stage, and four insects per 10 plants in the flower bud stage. In California, treatment thresholds for beans tend to be about 0.5-1.0 bugs per sweep for blossom and other early fruiting stages, increasing to 1-2 bugs per sweep later in plant phenology.

Similar sampling studies have been conducted in lentil. Sweepnet, vaccuum, and fumigation sampling produced similar estimates, though sweepnet sampling tended to underestimate nymphal populations. Afternoon sampling produced more accurate population estimates than morning sampling (Schotzko and O'Keeffe 1986). To obtain accurate estimates in this crop, 10 single-sweep samples are required for nymphs and five 25-sweep samples are required for adults (Schotzko and O'Keeffe 1989b).

Only limited work on trapping has been conducted. Plant bugs can be captured in light traps, but this is not considered to be an efficient monitoring technique. Slaymaker and Tugwell (1984) reported the capture of plant bugs, principally males, in virgin female-baited sticky traps. White sticky traps placed at a height of about 0.5 m are useful for detecting tarnished plant bugs in apple orchards (Boivin et al., 1982), and reportedly work well in vegetable crops also (Boivin pers. comm.).

Insecticides. Chemical insecticides are commonly applied to the foliage of plants to protect them from plant bugs. The plant growth stages that especially require protection are the blossoms and young fruit, but owing to asynchronous blossoming and the desire to prevent damage to mature tissue, season-long protection is not uncommon. Also, the adults are highly mobile, often changing plants after each meal. Thus, there is continuous risk of invasion from weeds or untreated crops. Although numerous insecticides are effective for plant bug control, Lygus spp. are moderately difficult to kill with some insecticides (Martel et al., 1986).

Cultural Practices. Vegetation management is very important in reducing damage potential by these highly mobile insects. Plant bug sources, both weeds and alternate crops, should be monitored and perhaps be managed. Lygus spp. may attain high numbers in crops such as alfalfa and cotton, and at maturity or harvest the insects disperse and invade susceptible vegetable crops. Thus, it may be desirable to avoid planting vegetables near vegetation that harbors plant bugs unless the source crop remains attractive to the plant bugs, or the source crop can be treated with insecticide.

Tractor-mounted vacuum devices reportedly have been used to reduce the number of plant bugs in some crops. Both adults and large nymphs are easily dislodged from vegetation, and therefore are good targets for such technology. However, population reductions seem to be incomplete and transient, so such equip-

ment is generally viewed as unsatisfactory (Vincent and LaChance, 1993).

Biological Control. Mass releases of the egg para-sitoid Anaphes iole have been evaluated for suppression of Lygus hesperus. Parasitoid releases can result in increase in egg parasitism, reduction in nymph abundance, and less fruit damage. The benefits are temporary, however, and frequent parasitoid releases at high rates are necessary to achieve effective damage suppression (Norton and Welter, 1996).

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