Solitarious desert locusts, Schistocerca gregaria (Forskål) (Orthoptera: Acrididae), inhabit the central, arid, and semi-arid parts of the species' invasion area in Africa, the Middle East, and South-West Asia. Their annual migration circuit takes them downwind to breed sequentially where winter, spring, and summer rains fall. In many years, sparse and erratic seasonal rains support phase change and local outbreaks at only a few sites. Less frequently, seasonal rains are widespread, frequent, heavy, and long lasting, and many contemporaneous outbreaks occur. When such seasonal rains fall sequentially, populations develop into an upsurge and eventually into a plague unless checked by drought, migration to hostile habitats, or effective control. Increases in the proportion of gregarious populations as the plague develops alter the effectiveness of control. As an upsurge starts, only a minority of locusts is aggregated into treatable targets and spraying them leaves sufficient unsprayed individuals to continue the upsurge. Spraying all individuals scattered within an entire infested zone is arguably both financially and environmentally unacceptable. More of the population gregarizes and forms sprayable targets after each successive season of good rains and successful breeding. Eventually, unless the rains fail, the entire upsurge population becomes aggregated at high densities so that the infested area diminishes and a plague begins. These populations must continue to increase numerically and spread geographically to achieve peak plague levels, a stage last reached in the 1950s. Effective control, aided by poor rains, accompanied each subsequent late upsurge and early plague stage and all declined rapidly. The control strategy aims to reduce populations to prevent plagues and damage to crops and grazing. Differing opinions on the optimum stage to interrupt pre-plague breeding sequences are reviewed.

1. The brown planthopper, Nilaparvata lugens (Stil), is a major pest of rice in Asia. It is known to make wind-assisted migratory flights each year to colonize the summer rice growing areas of China, Japan and Korea.2. Modelling windborne displacements between rice growing areas in Asia requires migratory behaviour and flight duration t o be established for this insect.3. Field and laboratory observations suggest that N.Zugens take-off at dusk and that some continue flying for u p to 24-26 h if the temperature is 2 17°C. 4. Trajectories for 10 ni above ground level and 1.5 km above mean sea level are used to identify possible sources and, hence, to estimate the flight times of Nlugens caught in nets on ships on the East China Sea in 1973 and 1981.5. Estimated flight times between the sources and the ships ranged from about 9 t o 30 h.6. Results suggest that long-distance migration can occur in surface winds, when they are strong, but that long-distance migration is more likely at 1.5 km.7. When simulating windborne displacements of Nlugens, it can be assumed that in areas and a t heights where the temperature is > 17"C, some migrants will fly downwind for up t o 30 h after a dusk take-off. Others will fly for shorter periods, giving the population as a whole the opportunity to colonize all the rice crops flown over.

No abstract

Reports of the presence and absence of biting by Simulium damnosumTheo. in the Volta River Basin in 1962Basin in , 1966Basin in and 1975 were used to identify occasions when sites were invaded by parous and nulliparous females. Circumstantial evidence suggests that this insect is a windborne migrant, and the weather before and during some of these invasions was examined. Although most invasions studied took place south of the Inter-tropical Convergence Zone, for the first time evidence is presented suggesting that migration also takes place to the north of this zone. Immigrants were captured at the invaded sites only when light winds or calms were present. This cannot, however, be used as proof that S. damnosum migrates and lands only where winds are light or it is calm because host-seeking is inhibited by high winds and the time of arrival, as opposed to capture on a host, is unknown. Until the factors initiating emigration, as well as the height, duration and number of flights in each gonotrophic cycle and the time of immigration are known, the present findings cannot be tested rigorously nor can wind records be used to trace the source of immigrants.