Empoasca fabae (potato leafhopper)

The IPM described here is based on Radcliffe et al. (1993).The size of the leafhopper population required to cause economic damage varies according to cultivar, the stage of plant growth and environmental circumstances. Early-maturing cultivars are generally assumed to be more susceptible to leafhoppers, but these cultivars bulk more rapidly, and their yield may actually be affected less. Another common assumption is that the potato crop is more susceptible to leafhopper injury if it is under stress and hence that it is more important to control leafhoppers under those conditions. Some studies, however, have shown that the combined effects of leafhopper damage and water stress or certain diseases are less than additive. If leafhopper populations exceed locally accepted action thresholds, insecticides provide the only effective means of controlling these pests on potato. Soil systemics applied infurrow at planting or side-dressed at plant emergence give 6-8 weeks of control and can essentially prevent the transmission of leafhopper-borne pathogens. However, for reasons of cost, systemic insecticides are probably seldom used specifically for this purpose. The standard industry practice for leafhopper control on fresh-market potatoes is to apply foliar sprays.Since both the insect and the initial stages of plant injury are inconspicuous, it is common practice in some production areas to spray on a routine schedule. This approach usually results in unnecessary insecticide applications, which may induce an outbreak in aphid populations and increases selective pressure for insecticide resistance in other pests. Growers in central Minnesota, USA, typically spray every 10 days, but equivalent leafhopper control can be achieved with as few as two applications for the season. As integrated pest management comes into more common usage in potato production, spray schedules should be based on appropriate action thresholds and actual leafhopper populations determined from field scouting (Radcliffe et al., 1993).Most leafhoppers seem to have few effective natural enemies. An insect-pathogenic fungus, Erynia radicans infects the leafhopper in Wisconsin and Minnesota, but it is rarely found in Illinois. Temperatures may be a limiting factor there, since the fungus does not germinate above 30°C. Some other leafhoppers are also susceptible to this pathogen. This fungus appears to be very diverse and may actually comprise several species. E. radicans seems to be a pathogen new to North America, and it will be interesting to see whether it ultimately proves to be a valuable biological control agent. Most fungal pathogens operate under the limitation that they have strict temperature requirements, need high humidity for spore germination and infection, and are readily disseminated only when the host has a high population density. At the present time biological control is not a viable management option for leafhoppers.Effective controls for E. fabae should be applied before toxigenic symptoms are evident on the plants. Insecticides provide good control of the nymphs and adults. Early season leafhoppers can be controlled with systemic soil insecticides applied at planting time. However, due to the high cost and short protection period of such a treatment as well as the uncertainty of leafhopper infestations occurring, foliar treatments based on economic thresholds may be a preferred management strategy. Leafhopper populations may persist until the first frost, and fields should be continuously monitored in areas where substantial numbers have been determined. More than one application may be necessary if season-long control is desired (Radcliffe et al., 1993).Adequate screening of window and open areas, as well as proper sealing of door edges can reduce infestation by leafhoppers in a greenhouse. Chemical insecticides can provide adequate control in these situations.

Many crops have some levels of physiological resistance to leafhopper that can be classified into one of three categories: tolerance, antixenosis or antibiosis. Resistance has been documented in Medicago (Brewer et al., 1986a) and potato (Sanford and Ladd, 1987). The causes for resistance (glycoalkaloids) were examined extensively in potato (Schalk et al., 1975; Raman et al., 1979).Plant resistance workers have long noted an association or susceptibility to this leafhopper species with lack of pubescence and other physical characteristics in original germplasm lines of soyabean, potato and lucerne (Broersma et al., 1972; Robbins et al., 1979; Brewer et al., 1986a, b). In part, this physical resistance is because trichomes impede the normal attachment of individuals to the plant surface. Studies on glandular trichomes have been well documented by Mackenzie et al. (1977), Tingey and Gibson (1978), Tingey et al. (1978), Tingey and Laubengayer (1981, 1986), Tingey and Sinden (1982), Tingey (1984) and Sanford et al. (1992, 1994).A chemical basis for resistance to feeding is less frequently cited compared with a physical basis. For example, comparisons of closely related Solanum species suggest that trichome presence and not glycoalkaloid content is associated with resistance. Alternatively, Raman et al. (1979) found interrupted feeding behaviour in response to feeding on glycoalkaloids within an artificial diet in the absence of physical resistance factors. Increased glycoalkaloid content was associated with greater resistance among potato cultivars. A larger study of 100 species of Solanum found that leafhopper resistance was associated with both the glycoalkaloid tomatine and glandular trichomes, and that artificial selection led to increased susceptibility to the leafhopper (Flanders et al., 1992).E. fabae exhibits differing aggregation and oviposition responses to various varieties or crosses of Solanum tuberosum and other Solanum species. These differences are due in part to alkaloids. The alkaloid, leptine-I, extracted from leaves of Solanum chacoense, markedly reduced both the rate of initial imbibition by the leafhoppers and their survival time. Tomatine, solanine, solandine and demissidine reduced initial imbibition, but did not influence survival time, and tomatidine affected neither imbibition nor survival. Thus, the alkaloids, in relation to their qualitative and quantitative distributions in the plants and their antifeeding properties, probably play decisive roles in the natural interactions of leafhoppers with Solanum species. The glycoalkaloid contents of foliage were measured in several populations derived from potato crosses that had been improved for resistance to the leafhopper by recurrent selection. These were analyzed for tuber glycoalkaloid content for many years (Slessman and Bushnell 1937; Sanford et al., 1972, 1992, 1995; Schalk et al., 1975; Tingey and Plaisted, 1976; Raman, 1979; Sanford and Ladd, 1979, 1983, 1985, 1986, 1987; Sanford, 1982; Flanders et al., 1997).The trichomes of soyabean influenced the ability of the leafhopper to get to the leaf surface in order to feed on it. Reductions in populations size on pubescent plants are due to interference with feeding. The potential of using semiochemicals in crop protection has been evaluated by several authors and is reviewed in Canada (Pelletier and King, 1987).In other cropsIn sweet potato, there are pubescent clones (with glandular trichomes) which are a resistance mechanism to E. fabae under insectarium coordinations (Ramos, 1993).Schaafsma et al. (1990) reported the resistance of common bean (Phaseolus vulgaris) lines to the leafhopper. Plant lectins affect the survival time of adult female E. fabae (Habibi et al., 1993).