Distribution. Aster leafhopper is a native to North America, where it is found in nearly all states and provinces. It is most common, however, in the central states and provinces. Also, it overwinters poorly in cold areas. Most areas with aster leafhopper problems are invaded annually by leafhoppers originating in the southern Great Plains. In the mild-climate northwest, however, leafhoppers are able to overwinter successfully, and long-distance dispersal is not an important factor.
Host Plants. Aster leafhopper has a wide host range, but the plants suitable for maintenance of adults are not always suitable for reproduction and the development of nymphs. It tends to overwinter on grains such as wheat and barley, and on grasses, clover, and weeds, then dispersing to vegetables in the summer months. The vegetable crops damaged by aster leafhopper include carrot, celeriac, celery, corn, lettuce, parsley, potato, and radish, but among vegetables only lettuce is consistently suitable for leaf-hopper reproduction. Other crops fed upon are barley, clover, dill, field corn, flax, oat, rice, rye, sugarbeet, and wheat.
In Washington, Hagel et al. (1973) indicated that the most important breeding areas were mixtures of clover and pasture grasses, followed by clover, sweet corn, oats, carrots, lawn grasses, rye, field corn, and various weeds. Among the weeds favored were flea-bane, Erigeron spp.; ragweed, Ambrosia spp.; dandelion, Taraxacum officinale; wild lettuce, Lactuca canadensis; tumble mustard, Sisymbrium altissimum; and lambsquarters, Chenopodium album. Low, sparse, and young vegetation provided the best habitat. Meade and Peterson (1967) indicated that large crab-grass, Digitaria sanguinalis; horseweed, Conyza cana-densis; barnyardgrass, Echinochloa crusgalli; fowl-meadow grass, Poa palustris; and barley, wheat, and oats were especially suitable for reproduction in Minnesota. Carrot, dill, potato, and radish were important adult food plants, but were not good breeding hosts. Based on laboratory choice tests, McClanahan (1963) ranked flax and wheat among the most preferred host plants.
Natural Enemies. Natural enemies seem not to be well studied, nor very important in the population ecology of aster leafhopper. The most important enemies are the parasitoids Pachygonatopus minimus Fen-ton, Neogonatopus ombrodes Perkins, and Epigonatopus plesius Fen ton (all Hymenoptera: Dryinidae). The best known among these is P. minimus, which caused up to 37% parasitism. The wingless female of this dryinid is also predatory on leafhoppers (Miller and DeLyzer, 1960, Barrett et al., 1965).
Life History and Description. This insect overwinters in the egg stage in northern locations, and in the adult stage in warmer climates. In Manitoba and Maine there are three generations per year, whereas three to four are reported from Washington, and up to five generations may occur in more favorable midwestern locations (Westdal et al., 1961). As the generations overlap and are initiated by both overwintering eggs and migrating leafhoppers, it is difficult to discern the generations. Total generation time requires about 27-34 days.
Good documentation of the northward dispersal by aster leafhopper was provided by Wallis (1962), who worked in the western Great Plains, the western edge of the migration path. He reported overwintering of the adult leafhoppers only in Texas, though north of Texas eggs may overwinter. By May, northward movement was evident, with adults present in Kansas, Nebraska, and South Dakota where no nymphs were found previously. By June, the leafhoppers progressed northward into Montana and North Dakota, and westward into Colorado and Wyoming.
The biology of aster leafhopper is available from many sources, but some of the more important contributions were provided by Hagel and Landis (1967), Nielson (1968), and Hoy et al. (1992). Osborn (1916) described the nymphal stages. Hagel and Landis (1967) and Hou and Brooks (1975) gave rearing procedures. A key for Macrosteles spp. was given by Beirne (1952).
Leafhoppers pierce leaf tissue of plants and remove the sap. The feeding punctures cause death and discoloration of individual plant cells, resulting in a yellow, speckled appearance in affected plants. This feeding damage, while unsightly, is minor in comparison to the damage caused to numerous vegetable crops by transmission of aster yellows by leafhoppers.
Aster yellows is a plant disease caused by a phyto-plasma, and is transmitted almost exclusively by aster leafhopper. Such crops as carrot, celery, cucumber, lettuce, potato, pumpkin, and squash are affected. Losses of 50-100% are reported due to this disease. Phytoplasma-infected plants are discolored, stunted, and deformed. On carrots, for example, the symptoms are red or yellow foliage and excessively hairy, bitter-
Adult aster leafhopper.
tasting roots. On lettuce, symptoms are chlorosis, stunting, and lack of head formation.
Sampling. Leafhoppers are easily collected with sweep nets, especially from grasses and grain fields. Sequential sampling protocols have been developed for sweep net sampling in carrot (O'Rourke et al., 1998). Yellow-sticky traps are also useful and easy to use. Light traps equipped with fans for suction also have been used effectively to capture leafhoppers. Cool and wet weather limit leafhopper activity and decrease the ability to sample effectively. Wind also interferes with population assessment (Durant, 1973).
In addition to sampling for leafhopper abundance, it also is desirable to determine the proportion of leaf-hoppers that harbor the phytoplasma. Formulas based on both insect number and disease incidence, the aster yellows index, have been developed to trigger control measures before the pathogen is widely transmitted to susceptible crops. Leafhoppers are collected before they enter an area, fed on aster plants, and the plants read for disease. This works effectively to alert large areas, such as entire states, but is not useful for local prediction. Disease incidence varies both spatially and temporally (Mahr et al., 1993). Further information on the aster yellows index, and action thresholds for several crops, were given in Foster and Flood (1995).
Insecticides. Insecticides commonly are used to kill leafhoppers, and thereby to minimize disease transmission. As there are protracted acquisition and incubation times associated with this disease, chemical-based disease suppression is feasible (Eckenrode, 1973; Koinzan and Pruess, 1975). Insecticides are especially effective in the absence of long-distance dispersal by leafhoppers. Systemic insecticides are often favored due to their persistence, but contact insecticides can be effective also (Thompson, 1967; Henne, 1970). Insecticides are often applied at 5-7 day intervals. As it takes 10-15 days for infected plants to show signs of infection, it is not necessary to treat plants just before harvest.
Cultural Practices. Crop varieties differ in their susceptibility to infection with aster yellows; this is well-studied both for carrots and lettuce, two of the more susceptible crops. Cultural manipulations can also enhance resistance. In studies conducted in Minnesota, Zalom (1981b) demonstrated very significant reductions in disease incidence where aluminum foil mulch was used. Straw mulch was equally effective (Setiawan and Ragsdale, 1987). Row covers, where economically feasible, should provide good protection against leafhoppers and disease transmission (Lee and
Robinson, 1958). Destruction of weed species known to harbor aster yellows is desirable.
Disease Transmission. Aster leafhopper acquires the phytoplasma by feeding on infected perennial and biennial weeds, or crop plants. Acquisition requires a prolonged period of feeding, usually at least two hours, before the leafhopper is infected. Usually less than 2% of dispersant leafhoppers become infected. There is evidence that the phytoplasma multiplies in the body of the leafhopper, and there is an incubation period of about two weeks in nymphs and 6-10 days in adults before the insects are capable of transmitting aster yellows. Leafhoppers remain infective for the duration of their life, but the phytoplasma is not transmitted between generations through the egg stage. Nielsen (1968) provided a good summary of aster yellows from an entomological perspective.
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