LEPTOCORISARICE SEED BUGS (HEMIPTERA: ALYDIDAE) IN ASIA: A REVIEW

Of the nine species of Leptocorisa rice bugs (RBs) in Asia and Oceania, L. oratorius is the most important in tropical climates, whereas L. acuta and L. chinensis prevail in upland areas or temperate climates. RB taxonomy has been clarified only since the 1970s, thus many host and distribution records need verification. RBs were a more serious pest in the th first half of the 20 century when rice fields were interspersed among extensive grasslands that were readily located by adults flying distances up to 24 km. If grasslands were composed of alternative host species, RBs could complete several generations before the rice planting season, particularly in years when the rains came early and fell intermittently to encourage many flushes of grasses. Practices that further favored RB build-up occurred in regions where farmers planted both early and long maturing varieties and in years when the planting season became extended over 2-5 months. In such situations, up to five RB generations could be produced on a single rice crop of long-maturing, traditional varieties, particularly where weed control in rice fields was lax. Dryland rice is more susceptible than wetland rice cultures because of isolated fields that are readily reached by adults and the concentrating effect caused by small fields . RB adults can gain nourishment from feeding on plant sap to sustain them for a number of weeks, but eventually they need to feed on rice endosperm to reproduce. Rice crops are only susceptible for about 30 days from flowering through soft dough as RBs emigrate at the hard dough stage to seek younger crops. Nymphs are particularly vulnerable as they cannot fly to seek new fields, thus their survival depends on their mothers ovipositing at the time of panicle exsertion. As more land became settled throughout Asia, the ratio of grassland to rice land decreased, greatly reducing the frequency of RB outbreaks. Higher-tillering, modern varieties also dilute the impact of RB damage, as for a given RB density , proportionally more spikelets escape RB feeding. In addition, high yielding varieties bear more spikelets per plant thus can compensate better by filling younger spikelets during the milk stage. Any agronomic practice that increases yield will minimize yield loss from RBs. RBs can pass periods where neither rice nor alternative hosts are present by entering quiescence for several months or diapause for longer periods depending on the floral composition of the local ecosystem. Masses of dormant adults congregate in moist, shady wooded or grassy areas which offer off-season shelter. In general, RBs are less of a pest in irrigated multi-crop rice environments particularly in areas where suitable shelter is not available and there is an offseason break between rice crops. RBs have become perennial pests of irrigated rice areas that are within dispersal distance to other regions where rice is grown during the off-season, where shelter is available, or where the period between rice crops is less than a month. RBs are attacked by a range of predators and parasitoids that focus mainly on the egg stage. The dispersal ability of RBs reduces the impact of natural enemies compared to other rice insect pests, thus cultural control methods are needed. Particularly effective is for farmers to synchronize the flowering period between fields and to create a rice-free period lasting more than two months between rice crops. Most insecticide use occurs in Japan to prevent production of pecky rice where rice is priced eight times more than the world price. Action thresholds are as low as 2 0.5 RBs/m in Japan, but in other countries thresholds range from 2 to10 2 RBs/m