Leptinotarsa decemlineata Say Coleoptera Chrysomelidae

Natural History

Distribution. The origin of Colorado potato beetle probably is Mexico. This insect apparently dispersed northward into the Great Plains region, feeding on weeds, before introduction of extensive agriculture in the 1800s. The introduction of potato to the Great Plains provided this species with an abundant food supply, resulting in crop damage beginning about 1859 in Nebraska. Once Colorado potato beetle accepted potato as a food plant, it spread rapidly, crossing the Mississippi River by 1864, and reaching the east coast from Connecticut to Virginia by 1874. By 1878, all of eastern Canada was infested. It spread southward more slowly, not reaching Florida and Louisiana until by 1900. Colorado potato beetle is now found throughout the United States except for California, Nevada, Alaska and Hawaii. It also occurs throughout southern Canada. In recent times the Colorado potato beetle has become a pest in most of Europe, Mexico, and portions of Central America.

Host Plants. Colorado potato beetle feeds entirely on plants of the family Solanaceae. The most important vegetable host, as the common name suggests, is potato. Colorado potato beetle can sometimes damage tomato and eggplant, and rarely pepper, in the eastern United States. Among the weeds often accepted by adults, and also generally supporting larval growth, are bittersweet, Solanum dulcamara; buffalo bur, S. rostratum; horsenettle, S. carolinense; hairy nightshade, S. sarachoides; silverleaf nightshade, S. elaeagnifolium; S. dimidiatum; and henbit, Hyoscyamus niger; and possibly some other Solanum spp. (Hsiao 1988; Weber et al.,

1995). Buffalo bur is perhaps the original host of Colorado potato beetle, and though being a good host, potato usually is superior (Hsiao and Fraenkel, 1968). For example, Brown et al. (1980) compared the growth of Colorado potato beetles fed potato or hairy nightshade. Larvae fed nightshade grew more slowly, required longer to commence oviposition, and produced fewer eggs than those fed potato. Solanaceous plants that are sometimes accepted as poor hosts include petunia; tobacco; wild tobacco, Nicotiana rustica; jimsonweed, Datura stramonium; apple of Peru, Nicandra physalodes; ground cherry, Physalis hetero-phylla; and matrimony vine, Lycium halimifolium. Colorado potato beetle has repeatedly displayed the ability to adjust its host range to accept locally abundant Solanum species (Horton et al., 1988; Mena-Covarrubias et al., 1996), so its host range varies geographically and continues to expand.

Colorado potato beetles have been observed feeding on non-solanaceous plants under ield conditions, and the ability of larvae to feed on these and to display short-term growth have been confirmed under laboratory conditions. Among these casual hosts are milkweed, Asclepias syriaca; butterfly weed, A. tuberosa (both family Asclepiadaceae); pea (family Legumino-sae); cabbage; shepherdspurse, Capsella bursa-pastoris (both Cruciferae); Romaine lettuce; and bullthistle, Cirsium vulgare (both Compositae) (Hsiao and Fraenkel 1968). However, such feeding is rare and these plants should not be considered normal hosts. Bongers (1970) conducted extensive tests on host selection and provided an extensive review of the literature. Hsiao (1988) also addressed host specificity, including the semiochemicals that affect acceptance of plants by beetles.

Natural Enemies. Many natural enemies have been identified, but they usually are unable to keep the Colorado potato beetle population at low levels of abundance. Harcourt (1971), for example, concluded that there were no effective natural enemies in Canada and that populations were regulated by starvation and dispersal. Not every investigator is so pessimistic about natural enemies, but clearly there is a shortage of effective naturally occurring enemies. Among common predators are green lacewings, Chrysoperla spp. (Neuroptera: Chrysopidae); several stink bugs but especially twospotted stink bug, Perillus bioculatus (Fabricius) and spined soldier bug, Podisus maculiventris (Say) (both Hemiptera: Pentatomidae); damsel bugs, Nabis spp. (Hemiptera: Nabidae); many lady beetles but particularly Coleomegilla maculata (De Geer) and Hippodamia convergens Guerin-Meneville (both Coleoptera: Coccinellidae); and ground beetles, Lebia grandis Hentz and Pterostichus spp. (both Coleop-

tera: Carabidae). The aforementioned insects are general predators, feeding on a number of soft-bodied insects, and so their occurrence is not tightly linked to the abundance of Colorado potato beetle.

Parasitoids of Colorado potato beetle are more specific in their host associations. The best known is the fly Myiopharus doryphorae (Riley) (Diptera: Tachini-dae), which characteristically builds to high densities in autumn, inflicting high levels of parasitism on the final generation of beetles. Unfortunately, despite the ability of this tachinid to cause high levels of parasitism, it has poor overwintering ability, and begins the year with low population densities and poor ability to suppress early season potato beetle populations. An exception seems to be in Colorado, where parasitism rates are high early in the season, and where Colorado potato beetles are not usually a serious pest (Horton and Capinera, 1987a). Other tachinid parasi-toids of Colorado potato beetle include Myiopharus aberrans (Townsend), M. australis Reinhard, M. macella Reinhard, and Winthemia spp.

Pathogens seem to be relatively unimportant in the survival of Colorado potato beetle. The nematodes Mesodiplogaster lheritieri and Pristionchus uniformis (both Nematoda: Diplogasteridae) have been recovered from Colorado potato beetle (Poinar, 1979). A mollicute, Spiroplasma leptinotarsae, inhabits the gut lumen of both adults and larvae, but does not induce disease (Hackett et al., 1996).

An interesting life-table study was conducted on tomato, a sub-optimal host, by Latheef and Harcourt (1974). Not surprisingly, the major mortality factor was physiological; i.e., the tomato plant was a fairly unsuitable host. However, these authors also suggested that cannibalism was the major mortality factor during the egg stage, and that rainfall affected the larvae.

Life Cycle and Description. The winter is passed in the adult stage, which emerges from the soil in the spring at the time potatoes are emerging. A complete generation can occur in about 30 days. The northernmost portion of its range usually allows only a single generation per year, but two generations occur widely, even in Canada. Southern areas such as Maryland and Virginia can support three annual generations, but not all the beetles go on to form a third generation; the third generation often develops on weeds rather than potatoes, so it is overlooked ( Johnson and Ballinger, 1916). Diapause is induced by a combination of photo-period, temperature, and host quality. Long day lengths normally promote continued reproduction, whereas short day lengths promote diapause, but diapause induction varies widely among populations. For example, populations in Washington and Utah do not reproduce when the photoperiod is less than 15 h, whereas beetles from Arizona reproduce at a 13 h photophase. Beetles from southern Texas and Mexico are relatively insensitive to photoperiod and reproduce at photoperiods of 10-18 h (Hsiao, 1988). The overwintering population may be comprised of individuals from more than one generation. In Massachusetts, egg hatch begins in late May and first generation larvae are present until July. First generation adults, the progeny of overwintered beetles, begin to emerge in July. Those emerging before August 1 normally go on to reproduce, whereas those emerging later usually enter diapause (Ferro et al., 1991). Some beetles remain in diapause for more than a year.

  1. Overwintering adults usually feed for 5-10 days before mating and producing eggs, though some beetles mate in the autumn and can oviposit without mating in the spring. The eggs are orange and elongate oval, measuring about 1.7-1.8 mm long and 0.8 mm wide. They are deposited on end in clusters of about 5-100 eggs, but 20-60 eggs per cluster is normal. The eggs are deposited on the lower surface of foliage, and anchored with a small amount of yellowish adhesive. The eggs do not change markedly in appearance until about 12 h before hatching, when the embryo becomes visible. Mean development time of eggs is 10.7, 6.2, 3.4, and 4.6 days when held at 15°, 20°, 24°, and 30°C, respectively.
  2. The larvae are reddish and black, and easily observed and recognized. The larvae are very plump, with the abdomen strongly convex in shape. The larvae bear a terminal proleg at the tip of the abdomen, in addition to three pairs of thoracic legs. Young larvae are dark red with a black head, thoracic plate, and legs. Two rows of black spots occur along each side of the abdomen. The larger larvae are lighter red, with the black coloration of the thoracic shield reduced to the posterior margin. There are four instars. Head capsule widths are about 0.65,1.09,1.67, and 2.5 mm for instars 1-4 respectively. During these instars body length increases from 1.5-2.6, 2.8-5.3, 5.5-8.5, and 9-15 mm, respectively. Developmental thresholds vary geographically, but 8-12°C is common. Rapid development and low mortality occur between 25-33°C, with 28°C optimal. Mean development time of first instars is about 6.1, 3.7, 2.1, and 1.4 days at 15°, 20°, 24°, and 28°C, respectively. For the second instar, mean development times are about 5.0, 3.8, 2.2, and 1.6 days; for the third instar 2.8, 2.5, 2.3, and 1.7 days; and for the fourth instar 9.5, 6.6, 3.3, and 2.4 days, respectively, when reared at 15°, 20°, 24°, and 28°C. (See color figure 107.)
Colorado potato beetle larva.
  1. At maturity, larvae drop to the soil and burrow to depths of 2-5 cm where they form a small cell. After about two days they develop into pupae. The pupae are oval and orangish in color. They measure about 9.2 mm long and 6.4 mm wide. The form of the adult beetle is recognizable in the pupal stage, though the wings and antennae are twisted ventrally. Mean development time of the pupal stage, exclusive of the period spent in the soil prior to actual pupation, is about 5.8 days. Ferro et al. (1985) report the below-ground combined prepupal, pupal and post-pupal period to average about 22.3, 14.9, 11.7, and 8.8 days at 15°, 20°, 24°, and 28°C, respectively.
  2. After transforming from the pupal to the adult stage, the beetle remains in the soil for 3-4 days before digging to the surface. Adults are robust in form, and oval in shape when viewed from above. The dorsal surface of adults is principally yellow, but each forewing is marked with five longitudinal black lines. The head bears a triangular black spot and the thorax is dotted with about ten irregular dark markings. The underside of the beetle and legs also are mostly dark. (See color figure 106.) Beetles produce eggs over a 4-10 week period, with most of the eggs produced during weeks 1-5. Fecundity of beetles under field conditions has been estimated about 200500, but this is likely an underestimate. Under labora-
Leptinotarsa Decemlineata Say
Adult Colorado potato beetle.

tory conditions, mean fecundity was 3348 eggs perfe-male when fed potato, and 2094 when fed hairy nightshade (Brown et al., 1980). In the autumn, newly emerged beetles feed for a time, and then dig into the soil, normally to a depth of 7-13 cm, to pass the winter. When beetles emerge in the spring they are not highly dispersive, mostly seeking out hosts by walking, though they are capable of flight. Voss and Ferro (1990) reported on dispersal behavior under field conditions in Massachusetts. Male beetles also walk upwind toward females, suggesting the presence of a pheromone that operates up to a distance of at least 50 cm (Edwards and Seabrook, 1997).

A good description of Colorado potato beetle was given by Girault and Rosenfeld (1907), general behavior and ecology by Chittenden (1907), and an overall review was published by Hare (1990). Thermal relations were presented by Walgenbach and Wyman (1984), Ferro et al. (1985), and Logan et al. (1985). Diapause in potato beetle, particularly the physiological aspects, was reviewed by de Kort (1990).

Damage

Colorado potato beetle is the major defoliator of potato throughout most of North America. Both adults and larvae feed on leaves, and when foliage has been consumed they will gnaw on stem tissue, and even tubers. First instars are responsible for about 3% of total leaf consumption, and second, third, and fourth instars for 5% 15%, and 77%, respectively. Total leaf consumption is estimated at 35-45 sq cm for larvae; adults consume foliage at a rate of 7-10 sq cm per day.

Most plant protection programs attempt to protect the early and mid-season growth of the plant. Potatoes are most susceptible to injury during bloom and shortly thereafter when tubers are rapidly expanding. Generally, defoliation should not exceed 10-25% during this period. Late season defoliation is much less damaging (Hare, 1980; Ferro et al., 1983; Zehnder et al., 1995). Tomato is readily damaged by early season defoliation, and mean population densities as low as 0.5 beetles per plant reduce tomato yields (Schalk and Stoner, 1979).

Management

  1. Potato beetle populations initially are aggregated, but as larvae mature they disperse and aggregation is decreased but not eliminated. Sampling is usually accomplished by visual examination of plants. Harcourt (1964) recommended that entire plants be examined for above-ground life stages, and samples of 50-200 plants be examined to establish accurate population estimates. Estimates based on visual examination are highly correlated with whole plant samples (Senanayake and Holliday, 1988). Several researchers have developed sequential sampling plans for potato beetles; Martel et al. (1986) described a sampling protocol for Colorado potato beetle on potato, and Hamilton et al. (1998) proposed separate sequential sampling plans for Colorado potato beetle populations on eggplant, one for egg clusters and another for motile stages. The adults are attracted to yellow, and can be captured with traps (Zehnder and Speese, 1987), but this is not a common practice because the various life stages of this insect can so easily be detected on plants in the field.
  2. In the late 1800s and early 1900s, the depredations of Colorado potato beetle were so great that this insect largely was responsible for the development of arsenical insecticides. Following the development of modern insecticides in the 1940s and 1950s, the status of Colorado potato beetle as a major pest was reduced, but it periodically resurges as a concern owing to development of insecticide resistance (Forgash 1981; Gauthier et al., 1981; Heim et al., 1990). Application of insecticides to protect potatoes, and to a lesser degree tomato and eggplant, is a common practice in many areas. Foliar applications are most common, but systemic insecticides are sometimes applied at planting for early season protection (McClanahan 1975b; Boiteau et al., 1997). Some strains of the bacterium Bacillus thuringiensis, particularly B.t. tenebrionis, are effective against these beetles. It is usually necessary to apply B. thuringiensis products to the irst two instars to attain high levels of suppression, though even a single application can provide enhanced potato yield (Nault and Kennedy, 1999). Some botanical insecticides are effective if applied to larvae (Zehnder 1986), but formulations may benefit from inclusion of a synergist (Zehnder and Warthen, 1988).

Cultural Practices. Cultural techniques have been studied extensively, initially because damage by this serious pest predates insecticides, and more recently because potato beetles evolved resistance to nearly all insecticides. Perhaps the most valuable cultural practice is crop rotation. Overwintered beetles initially are not highly dispersive, so moving the location of potato fields from year to year is beneficial. However, distances of at least 0.5 km are necessary to provide isolation (Hough-Goldstein and Whalen, 1996; Weisz et al., 1996). Such rotation significantly delays the invasion of fields by beetles, eliminating the need for one or two early-season insecticide applications. Also, because beetles initially disperse principally by walking, trenches lined with plastic can be used to capture beetles and impede colonization of fields. Trenches with walls that slope at an angle greater than 45° capture 50% or more of the beetles, often resulting in high, but variable, reductions in beetle damage (Boiteau et al, 1994).

The copper- and tin-based fungicides have useful anti-feedant properties, but are not directly toxic to beetles. Regular application of these fungicides reduces feeding and oviposition by Colorado potato beetle, particularly on less preferred hosts such as tomato (Hare et al., 1983; Hare, 1984). Failure to cause mortality apparently limits the commercial acceptance of such anti-feedants.

Cultural manipulations such as application of straw mulch and compost have been studied to determine their effects on potato beetles. Application of mulch causes decreased numbers of beetles, whereas compost seemingly has no effect (Zehnder and Hough-Goldstein, 1990; Stoner et al., 1996). The decrease in beetle abundance that commences within two weeks of application of mulch is attributed to increased abundance of soil-dwelling predators, and results in less defoliation and increased tuber yields (Brust, 1994). Application of nitrogen has only a slight, but negative, effect on potato beetle development rates (Jansson and Smilowitz, 1985). Intercropping and weed management also affect beetle densities. Potato monoculture is highly beneficial for potato beetle. Both beetle densities and fecundity tend to be higher under monoculture conditions (Horton and Capinera, 1987b).

Special equipment like propane flamers and crop vacuums have been used to destroy potato beetles. Flamers direct heat to the soil surface at the beginning of the season before the crop emerges, or soon after plant emergence (Pelletier et al., 1995). Such treatment can kill 50-90% of the beetles, with slight damage to plants. This approach works best with young plants, and is limited by both weather constraints and cost, so it is infrequently used. Crop vacuums can remove up to 70% of small larvae and adults, but are also limited to use early in the season, and by the considerable expense of equipment procurement (Boiteau et al., 1992).

Biological Control. None of the naturally occur-ing beneficial organisms display the ability to regulate potato beetle populations consistently, though the tachinid, Myiopharus doryphorae, and the stink bugs, Perillus bioculatus, and Podisus maculiventris, have been cultured and released to suppress beetle damage (Tamaki and Butt, 1978; Tamaki et al., 1983a; Biever and Chauvin, 1992; Cloutier and Bauduin, 1995; Hough-Goldstein and McPherson, 1996). Stink bugs are effective at destroying eggs and larvae, if a favorable ratio of predatory bugs to beetle egg clusters can be established—a ratio of one bug per about 100 beetle eggs can reduce injury by 80%. Efforts have been made to identify new biotic agents, the most promising of which is the egg parasitoid, Edovum puttleri (Hyme-noptera: Eulophidae). This wasp can kill more than 80% of the eggs in an egg cluster, but the effectiveness of E. puttleri is limited by its requirement for warm temperatures in order to be active, and its inability to overwinter in temperate climates. Repeated release of E. puttleri into eggplant fields in New Jersey was reported to be an important element in producing high-quality fruit with minimal pesticide use and high financial returns (Hamilton and Lashomb, 1996). This parasitoid was described by many authors, including Grissell (1981), and Sears and Boiteau (1989).

Pathogens have been investigated extensively for Colorado potato beetle suppression and strains of Bacillus thuringiensis, mentioned above under "insecticides," are most promising. However, the fungal pathogen Beauveria bassiana has been used in Europe with modest success (Roberts et al., 1981). It is limited mostly by the economics of potato production, and the incompatibility of the entomopathogenic fungus with fungicides that must be applied to control foliar plant diseases. Entomopathogenic nematodes can be applied to the foliage to suppress feeding larvae or to the soil to kill larvae as they pupate (MacVean et al., 1982; Toba et al., 1983; Nickle et al., 1994; Berry et al., 1997).

Host-Plant Resistance. Although cultivars of potato and tomato differ slightly in their susceptibility to potato beetle, effective levels of plant resistance have never been available in commercial varieties. However, incorporation of Bacillus thuringiensis toxin into transgenic potato plants (Wierenga et al., 1996) appears to be an effective basis for protection against the feeding by young larvae, at least for the short term.

0 0

Post a comment