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1. Long-term changes in the numbers of Zonocerus at oviposition sites reflect changes in the maturity of the general population. 2. Short-term changes in the numbers of Zonocerus at oviposition sites are associated partly with normal diurnal behaviour in which roosting occurs at night. 3. Immigration of Zonucems, particularly females, into the oviposition site occurs prior to periods of oviposition activity. Emigration follows oviposition. 4. The majority of Zonocerus move upwind into the oviposition site suggesting an odour and/or sound attraction. 5. Periods of intense oviposition activity follow heavy rainfall.

1. About 166,000 African armyworm moths, Spodoptera exempta (Walk.), were marked at an emergence site near Nairobi when they fed at night on trees baited with dyed molasses. 2. Six marked moths were captured in pheromone traps, including one at 90 km after flying for only one night, and another at 147 km. 3. Moth flight trajectories deduced from radar and from marking showed that migration was downwind. 4. During migration, moths become dispersed; hence the high densities that lead to outbreaks must be produced by concentration. 5. Some moths were ready to mate on the same night they completed their long-distance flight.

1. The dry-season population of Zonocerus in southern Nigeria oviposits in dense aggregations under single or clumped woody plants or on the edge of tall continuous stands of vegetation. No egg pods were found oviposited singly. 2. The vegetation under which oviposition occurs is the most common available at the time of the year. There seems to be no selection of particular species of plants so long as they provide good low shade. 3. Oviposition can take place in a wide range of gravelly soils. 4. Topography is not important as long as the oviposition site is on high ground away from damp areas subject to flooding. 5. There was a mean of 618 pods per oviposition site from forty-one sites which were dug up, and there was a significant correlation between the area of a site and the number of pods. 6. There was a marked tendency for oviposition sites to be found close to cassava. At two localities of approximately 20 ha each 90% of the sites were found within 30 m of cassava. 7. It is suggested that permanent shade is the most important factor in the selection of a site and this is probably related to the survival of the eggs during their 6–7 months development.

1. Onset and spread of the severe outbreaks of African armyworm, Spodoptera exempta (Walk.), over Ethiopa, Kenya, Tanzania and Burundi in late 1984 have been analysed in relation to trap and weather records. 2. Although first outbreaks in each country appeared progressively further south, winds on the best estimated dates of moth concentration and egg laying blew from directions precluding moth movements between them. 3. Such independently developing outbreaks are called ‘primary’ to distinguish them from any ‘secondary’ outbreaks that may develop from them. 4. Parent moths giving rise to all seven areas of primary outbreaks were derived from low-density populations that had persisted through the dry season, probably near the coast of East Africa. 5. Primary outbreaks developed on the south-eastern sides of individual rainstorms over the highlands, most likely because windborne moths were concentrated there by convergent winds. 6. Of the secondary outbreaks within Kenya, Tanzania and Burundi, most were derived from only one area of primary outbreaks in Kenya, which were therefore critical for effective control of spread; the paucity of other secondaries was presumably due to a lack of timely rainstorms or other concentrating mechanisms.