Nephotettix nigropictus (rice green leafhopper)

Damage done by N. nigropictus occurs by removal of plant sap and the insertion of stylet feeding sheaths secreted during feeding to ensure that the mouthparts block the flow of plant sap within the plant. This results in wilting.

Cultural Control

A number of cultural practices have been identified to either reduce population build-up or mitigate against diseases vectored by N. nigropictus and other green leafhoppers (Litsinger, 1994). Although N. nigropictus prefers alternative grass hosts to rice, the irrigation water or rains that favour planting a rice crop also favour its alternative hosts. Therefore reducing the number of rice croppings to two per year and synchronizing establishment across farms within each irrigation system will reduce N. nigropictus as well as other insect vectors of rice virus or phytoplasma diseases. There is an apparent trade-off, however, for large irrigation systems (>100,000 ha) between reducing the recolonization of disease vectors and natural enemies (W. Settle, Indonesian National IPM Programme, Indonesia, personal communication, 1995). At this scale, breaking up the area into blocks to allow nearby reservoirs of natural enemies to recolonize may be considered, as the rice-free period also severely limits their numbers as well. Loevinsohn et al. (1988) suggested dividing the irrigation system into blocks and transplanting each block (2-3 km diameter) sequentially from one end to the other at 10-day intervals.

Tungro and other diseases vectored by N. nigropictus are more severe when the crop is infected at an early stage. Transplanting and placing the seedbed away from disease sources is a way of avoiding early infection. Transplanting older seedlings (>3 weeks old) also reduces the susceptible vegetative period in the field (Chancellor and Cook, 1995). Avoiding planting during months when historical records show peak green leafhopper activity is another escape mechanism.

Early planting within a given planting period, particularly in the dry season, is suggested as a means of reducing the risk of insect-vectored disease (T. Chancellor, IRRI, Philippines, personal communication, 1995).

Nitrogen use increases green leafhopper numbers, but it also promotes tillering and the plant vigour necessary to boost the crop's ability to compensate for pest damage (Litsinger, 1993). Timely fertilizer use also has been found to reduce the symptoms of tungro. On balance, nitrogen should be applied at an optimal but not excessive level.

Good weed management not only removes the preferred grassy hosts for N. nigropictus but promotes crop vigour. Trimming the vegetation on rice bunds is a good weed control practice and also removes plant hosts of N. nigropictus.

Crop rotation with a non-rice crop during the dry season, where tillage is used, removes weed hosts as well as volunteer rice which may act as a disease reservoir. Upland rice intercropped with soyabean reduced incidence of N. nigropictus on rice compared to rice alone (Gyawali, 1988).

Mechanical and Physical Control

Traditional practices have included light traps to capture green leafhoppers and to dislodge insects onto kerosene-coated water from foliage by dragging a rope across the field or using large brooms (Litsinger, 1994). These methods, however, are not very effective. Light traps appear to capture many insects, but in fact only a very small percentage is actually collected. Kerosene is expensive and is phytotoxic to the crop if not removed within a few days.

Biological Control

Although a number of fungal pathogens are known to infect N. nigropictus and other green leafhoppers, their use as microbial insecticides has had mixed results, usually because of a lack of a microclimate-favouring leaf wetness when applied. A wide array of parasitoids and predators can normally contain green leafhopper numbers if insecticide usage does not disrupt their balance. Way and Heong (1994) advocate the use of restriction insecticides to favour natural enemies as a primary control strategy. Often weather factors upset parasitoid and predator numbers in much the same way as insecticides. For greater assurance of continuous pressure against population build-up, supplemental methods such as host-plant resistance and cultural control practices should be utilized.

Host-Plant Resistance

A study by Razzaque et al. (1987) reported that rice varieties having six different genes for resistance to N. virescens were also resistant against N. nigropictus based on plant damage ratings, feeding activity measured by honeydew production, nymphal survivorship, and fecundity. These results were confirmed by Viswanathan and Kalode (1984) and Dutt and Biswas (1979) who indicated the mechanism of resistance was antibiosis. As most modern rices released as new varieties in Asia have resistance against N. virescens, most of these same varieties therefore are also resistant against N. nigropictus. Among the rice germplasm tested, more sources of resistance (>1200 accessions) have been found against green leafhoppers than any other pest group. Low population build-up is evident when N. nigropictus was reared on resistant cultivars which exhibit antibiosis, causing direct mortality to nymphs and adults feeding on the plants. On susceptible rices, feeding predominantly occurs from the phloem, but on resistant varieties the feeding site commonly switches to the xylem, as phloem ingestion is inhibited by antibiosis effects (Sogawa, 1973; Razzaque et al., 1987). Ingestion time is markedly reduced on resistant cultivars. The efficiency of disease transmission is directly related to the duration of phloem feeding (Heinrichs and Rapusas, 1990). RTBV particles prefer to locate in phloem rather than xylem tissue. More probing occurs in resistant varieties, as the green leafhoppers desperately seek more acceptable feeding sites. Increased probing, however, increases disease transmission. A clumped pattern of disease development illustrates that leafhoppers make short rather than long distance movements between plants (Bottenberg and Litsinger, 1989).

Green leafhoppers rarely become sufficiently abundant to cause direct yield loss through sap removal, as is common in planthoppers.