HETEROPTERA: MIRIDAE) Problems
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TAXONOMIC STATUS, BIOLOGICAL ATTRIBUTES, species, contribute to the difficulty in AND RECOMMENDATIONS FOR FUTURE WORK ON accurately assessing important economic THE GENUS LYGUS (HETEROPTERA: MIRIDAE) problems. By Thomas J. Henry and John D. Lattinl/ ORIGIN OF PEST SPECIES Species of Lygus, frequently called lygus Pest status varies with the organism and bugs, are the most important agricultural resource being evaluated. Acceptable pests in the plant bug family Miridae population levels may be low when dealing (Heteroptera). The tarnished plant bug, with disease vectors (Burgess et al., Lygus lineolaris (Palisot), 1983) or much higher when considering occurring over most of North America, is certain defoliators. The pest problem a pest of cotton in the Southern United then is a relative state dependent on the States. In the Western United States, circumstances. Norton and Conway (1977, several species attack alfalfa and p. 205) provided a useful definition: "A cotton. Because of the severe damage pest problem is, therefore, characterized these plant bugs inflict, an extensive not only by the state of the pest search has been implemented by the U.S. population itself but, more importantly, Department of Agriculture and cooperating as the damage or illness it causes and agencies for natural enemies, especially the value placed on these consequences by nymphal parasites in the wasp family human society." Several Lygus spp. are Braconidae. This report is an overview considered pests because their feeding of the taxonomic status of the genus damage on certain crops is unacceptable Lygus to help biological control to those involved in crop production researchers better understand the (Graham et al., 1984). This damage often taxonomy of Lygus and the taxonomic is enhanced because lygus bugs frequently problems related to controlling this pest feed on reproductive parts of the plant group. and thus are frequently pests of seed crops (Burgess et al., 1983). The plant bug genus Lygus in North America contains several economically Some Lygus spp. have become pests because important species. They feed on a they are usually highly mobile, variety of crops and native plants. Some multivoltine, and oligophagous species are widely distributed and (r-selected), feeding on new growth and broadly overlap the ranges of others. reproductive parts. In contrast to many Great morphological variation in adults other insect pests in North America, often makes positive identification these species are native insects difficult. At least some species of attacking a large array of introduced Lygus are parasitized regularly by crops, except in a few instances, such as Braconidae, chiefly in the nymphal an Oregon species that feeds on native stages. Positive identification of Lygus Limnanthes (meadowfoam) (J.D. Lattin, nymphs except in rare instances is not pers. observ.) and a California species now possible. The variations caused by that attacks guayule (Romney, 1946; seasonal factors, the variety of host Bolton et al., 1972; Lattin and Oman, plants, and genetic factors, coupled with 1983). Price (1976), Southwood (1977), the problem of identifying nymphs to and others have pointed out that many pest herbivores are r-selected species, and in the disturbed environments typical of most agroecosystems, such species can liRespectively, Systematic Entomology often colonize faster than their Laboratory, Agricultural Research predators and parasites. Southwood Service, U.S. Department of (1977) stated that polyphagous predators Agriculture, c/o National Museum of are likely to be important in such Natural History, Washington DC 20560, habitats. The heteropteran family and Systematic Entomology Laboratory, Nabidae contains several very common, Department of Entomology, Oregon polyphagous predators found frequently in State University, Corvallis, OR 97331. such disturbed habitats in North 54 America, for example, Nabis alternatus 1975; Scott, 1977; Fye, 1980; Snodgrass Parshley and N. americoferus Carayon et al., 1984). Since not all such plant (Graham and Jackson, 1982). records represent hosts on which eggs are laid, care should be taken to distinguish Lygus bugs, highly mobile as adults, between hosts, where nymphs occur, and often move from disturbed habitats into resting or feeding hosts, where only crop fields (Fye, 1980). Some species adults are found. Often host plant migrate from wild to cultivated plants records in the literature cannot be and have been collected at elevations as associated positively with species of high as 5,000 ft (Glick, 1939; Johnson Lygus because of the difficulty in making and Southwood, 1949). Although most accurate specific identifications and the Lygus spp. are normally multivoltine, lack of voucher specimens, particularly single generations may occur in the in western North America. If voucher northern part of their ranges (Kelton, specimens have been retained from the 1975; Craig, 1983). At least one numerous economic studies (see Graham et species, Lygus unctuosus (Kelton), may al., 1984), some associations can be have only a single generation (Kelton, verified or corrected. Fortunately the 1975; Craig, 1983). Other univoltine widespread Lygus lineolaris is more species of Lygus, for example, L. easily recognized throughout much of its vanduzeei Knight, normally are not range than is the complex group of economically important (Reid et al., western taxa. 1976). Care should be taken in determining the actual number of Because legumes frequently are utilized generations (Shull, 1933; Salt, 1945; as hosts by North American species of Craig, 1983). Lygus, it is curious that L. lineolaris, with a large array of hosts (Snodgrass et Feeding occurs most often on growing tips al., 1984), is not a pest on soybeans, or reproductive parts of the plant, areas even though this species occurs of high nutrients. This is particularly throughout much of the crops range and damaging to plants grown for seed 453 insect species have been recorded (Jeppson and MacLeod, 1946; Craig, 1983; from this crop in North America (Kogan, Rice et al., 1985). According to Scott 1981). The establishment and subsequent (1970, 1983), such feeding might reduce spread of alfalfa in western North seed yield and germination in carrots, America likely affected significantly the tit the plants grown from damaged seed movement and colonization of western fed on by Lygus spp. may show accelerated species of Lygus bugs (Bolton et al., growth and larger plants and roots. 1972). The effect of such a crop on the Conversely, some evidence indicates that widely scattered, naturally occurring such feeding might stimulate increased populations of Lygus remains to be productivity in some crop species (Scott, studied. When positive species 1970). In some instances, the host plant identifications can be combined with may affect adult longevity and fertility accurate host associations, it will be In Lygus bugs (Al—Munshi et al., 1982). possible to examine the status and distribution of some western Lygus Lygus bugs also feed as scavengers and species. This may be an example of facultative predators of arthropods. " reverse" island biogeography, where a Predation on living insects probably crop plant provides a corridor for involves moribund individuals or those dispersal between naturally separated undergoing ecdysis (Wheeler, 1976). The populations. laboratory rearing of Lygus spp. may be enhanced by adding animal food to the DEFINITION AND PLACEMENT OF LYGUS IN THE diet (e.g., Bryan et al., 1976). FAMILY MIRIDAE Several pest species of Lygus are known The group of bugs commonly referred to as to occur on a wide variety of plants, lygus bugs belongs in the genus Lygus both introduced and native (Stitt, 1949; Hahn and to the plant bug family Miridae, 431colm, 1953; Parker and Hauschild, 55 the largest family in the true bug order the subgenus Exolygus to accommodate it Heteroptera (superorder Hemiptera). and allies. Leston (1952), in the first About one-third to one-halt of all attempt to evaluate internal Heteroptera are mirids. Although fewer relationships of the genus on a worldwide than 7,000 species were listed in the basis, recognized six subgenera, Carvalho world Miridae catalog (1957-60), including his new subgenus Taylorilygus, many have been described since the 1950s, which contained Lygus pallidulus probably bringing the total to 10,000 or (Blanchard) (as Lygus apicalis Fieber, a more. Most mirid workers predict that junior synonym) and the African "lygus the species inventory for the world will bugs." reach 20,000. Within the Miridae, Lygus is included in the largest subfamily, the Lestons work served as the basis for an Mirinae. According to Carvalho (1957-60), important revision by Kelton (1955a), more than 250 mirine genera were who, by utilizing male and female described by 1955. Of these, Lygus genitalia and external morphology, gave (sensu lato) contained the greatest strong evidence that most of the number of species, about 300. subgenera previously placed in Lygus deserved generic status. He recognized Currently, however, only 34 species are as valid genera Agnocoris Reuter (type- placed in Lygus in North America and species Lygaeus rubicundus Fallen), about 8 are known from the Old World Liocoris Fieber (type-species Cimex (Kelton, 1975). The reduced tripustulatus Fabricius) (with Exolygus number of species recognized in Lygus Wagner as a junior synonym (type-species deserves some explanation. Historically, Cimex pratensis Linnaeus)), Lygus Hahn the broad