Azadirachtin, the principal active ingredient of neem seed and aqueous neem seed extracts (ANSE) reduced food intake and utilisation efficiency in the penultimate instar level of the African armyworm (Spodoptera exempta) when applied either topically or mixed with the diet of the insect. These observations reflect interference of neem with centres in the insect that regulate feeding and metabolism. The decreased food utilisation efficiency in treated insects indicated higher metabolic costs as they had to direct some energy into dealing with neem deposits in their bodies rather than biomass production.

A range of materials were collected from a number of ecological and environmental zones within Kenya as being likely sources of entomopathogenic bacteria. These were soils, insect frass and dead insect material. Using an isolation protocol involving use of a selective medium, over 150 Bacillus thuringiensis (B. t.) strains were recovered from the sample material. Their identity as B. t. strains was confirmed by their growth characteristics, morphology and presence of a parasporal delta-endotoxin crystal.

All strains were screened for activity against two Lepidopteran pest species, the spotted stem borer, Chilo partellus and the African army worm, Spodoptera exempta. Isolates causing over 80% mortality in the screening assays were retained for further evaluation. The selected isolates were subjected to bioassay against C. partellus and S. littoralis. Following bioassay, the most toxic isolates were selected for screenhouse trials against C. partellus and field trials against a natural outbreak of S. exempta.

In the screenhouse trials all of the B. t. treated plants showed reduced levels of damage and yields were 5 to 7 times higher than the yield obtained from the untreated, infested control plots. Three isolates, A-3, A-C-2 (isolated from insect material) and M-44–2 (isolated from soil) seemed to offer superior levels of protection. Two new B. t. isolates, K-26–21 and MF-4B-2, both isolated from soils, showed high levels of toxicity to S. littoralis and S. exempta in the laboratory. When isolate K-26–21 was applied to maize seedlings in the field, almost total control of the larval population was achieved within 48 hr at each of the concentration levels tested.

Armyworm, Spodoptera exempta (Wlk.) feed on cereals and grassland in eastern and southern Africa. Data on the yield losses and economic effects of an attack on cereals is sparse. They can be an indirect cause of cattle deaths. The need to examine the situation at the farm level has been recognized. A national survey was conducted in 1987, and is currently being analysed. The work entails identifying representative areas and farmers, completing a questionnaire with each farmer, noting the frequency of outbreaks experienced, the farmers' response, the level of government support, the effectiveness of control measures and the costs incurred. Proposals are outlined for future work within the region to determine the reduction in cereal yields due to this insect and to set up action thresholds for control activities. A major difficulty with the survey approach is the disaggregation of various loss-causing factors. At a sub-sample of survey sites, where outbreaks are present, the damage to the crop and the subsequent yield of the damaged crop or the planting of a replacement crop would be recorded to build up data from which action thresholds might be determined.

Field and laboratory evidence now suggests that the African armyworm, Spodoptera exempta, is specifically adapted for survival at low population densities, providing a new basis for understanding the significance of its phase polyphenism and migratory strategy. This conclusion allows deductions to be made as to the expected extent of displacement of flying moths from low and, when they occur as a result of impeded or inadequate dispersal of parent moths, high density populations. The proposed migratory strategy which depends on the genetic determination of flight potential also provides a framework for interpreting observed patterns of armyworm outbreaks through the season and in relation to rainfall. Traits associated with a low-density, “armyworm” strategy are shared by other Noctuids which may have similar life histories.

The African armyworm is a migrant moth which, in its larval form, can cause severe plagues which destroy cereal crops, sugarcane and pastures in many parts of Africa. This paper reviews the relevant aspects of a multidisciplinary study into the dispersal behaviour and re-concentration of moths, migratory flight, source areas, population upsurges and the seasonal movement of moths. It introduces the new concepts of primary and critical outbreaks.

Pheromones have been successfully used on a large scale for control of pink bollworm, Pectinophora gossypiella, by mating disruption in Egypt and Pakistan using various slow release formulations. Similar formulations are being developed for control of other cotton bollworms, Earias insulana and E. vittella. Small-scale trials are in progress to evaluate pheromone formulations for control of sugar-cane borers, Chilo sacchariphagus and C. auricilius in Mauritius and India, and the tropical warehouse moth, Ephestia camelia, as a pest of stored cocoa in Brazil. Traps baited with the pheromone of the African armyworm, Spodoptera exempta, have been extensively used in Eastern and Southern Africa as the basis of a forecasting system of pest incidence.

Traps baited with the synthetic aggregation pheromone of the larger grain borer, Prostephanus truncatus, have been developed and are in use in E and W Africa for detection and monitoring of this pest of stored grain.

Limonin, deoxylimonin, citrolin, obacunone, harrisonin and acetoxyharrisonin were assayed as potential antifeedanfe against the African crop pests, Spodoptera exempta, Eldana saccharina and Manica testulalis. Even at the highest applied concentration, limonin, deoxylimonin, citrolin and acetoxyharrison were not active against S. exempta. At high concentrations, obacunone and harrisonin were mildly active in S. exempta assays. Harrisonin and obacunone displayed potent activity against E. saccharina and M. testulalis.

Laboratory reared populations of Spodoptera exempta (Wlk.) had higher net reproductive rate and capacity for increase on maize, Zea mais L. and star grass, Cynodon dactylon (L.) Pers. than on Kikuyu grass, Pennisetum clandestinum Chiov., Guinea grass, Panicum maximum Jacq. and Setaria plicatilis (Hochst.) Hack. These parameters were greatly influenced by the larval mortality, but in all cases they were greater than one demonstrating that S. exempta populations can multiply on all the host plants tested. Since no populations with stable age distributions are known for S. exempta the net reproductive rate is more appropriate than the capacity for increase in estimating its population growth.

The courtship behaviour of male Spodoptera exempta (Wlk.) has been quantified and used as a method of bioassay for the female sex pheromone. Virgin females become attractive to sexually mature males from 72 hr after emergence and henceforth their attractiveness increases gradually until they have mated. Mated females start losing their attractiveness to males gradually after mating and 72–96 hr after mating they cannot produce any courtship response in males. Amongst virgin females, crude pheromone-gland extract and synthetic pheromone compounds, it is the older live females and the crude extract at 0.1 female equivalent that produce highest responses in males. A certain degree of cross-attraction between three coexisting species has been noted, both by using live insects as well as synthetic pheromone compounds of these species under laboratory bioassays. Possible interspecific pheromone-related implications are discussed.

The African armyworm, the larva of the night flying moth Spodoptera exempta (Walk.), feeds on young stages of cereal crops, sugar-cane and all types of grasses in some 30 countries of Africa, from Mauritania and Senegal in the north-west to Ethiopia and Somalia in the north-east, and southwards to Namibia and South Africa. Between 1960 and 1982, outbreaks of armyworms were recorded regularly in East Africa during the ‘outbreak season’ between November/December and June/July, often from southern Tanzania towards the north or north-west at monthly intervals. Analysis of occurrence of outbreaks in Tanzania, Kenya and Uganda in the 1971/72–1973/74 seasons, and rainfall records in Tanzania for 1969–1975, showed that the occurrence of armyworms in Tanzania coincided with the onset and northward progression of the rains up to Kenya and Uganda. Additional results from monthly maps, tables, graphs and histograms of armyworm occurrences in administrative units (Districts and Provinces) in Tanzania, Kenya and Uganda for 20 years (1960–1979), showed that the onset, progress and duration of armyworm attacks in all districts and provinces in each country, tend to follow certain geographical patterns which can be monitored through the use of insect traps and Agricultural Extension Services. In this way, coordinated surveillance of priority areas could enable farmers, pastoralists, Plant Protection and Pest Control Units of the Ministries of Agriculture to identify likely areas and periods of armyworm attacks, so that they could organize strategic or emergency control of armyworms with insecticides when the larvae are still young.

At 25°C and 70% r.h. larvae of Spodoptera exempta develop faster and pass through five instars on star grass, Cynodon dactylon (L.); Pers. maize, Zea mais L.; and Kikuyu grass, Pennisetum clandestinum Chiov. whereas on guinea grass, Panicum maximum Jacq.; and Setaria plicatilis (Hochst.) Hack, the development is not only slower, but the larvae pass through six and seven instars, respectively. Samples of larvae reared on the former grass species suffer high mortality only at the first and the last instar stages so that the distribution of per cent mortality is U-shaped. On the latter host plants, the mortality during the first instar stage is equally high and continues through the subsequent instars with no obvious peak towards the end of the larval life. The samples on C. dactylon and Z. mais had much higher growth indices than those on P. maximum and S. plicatilis. C. dactylon is a preferred natural food plant of S. exempta in the field, and these investigations demonstrate its value and importance.

The relationship between seasonal numbers of outbreaks of African armyworm (Spodoptera exempta) in Kenya and Tanzania, and early season rainfall in source areas has been examined using records for 21 seasons. Significant inverse relationships were found between numbers of Kenya outbreaks and early season rains in the Kisumu area, and between numbers of Tanzania outbreaks and early season rains in the Morogoro and Mtwara areas. Total Kenya and Tanzania outbreak numbers were significantly inversely related to early season rainfall for 8 areas combined. These results suggest that early season rainfall totals, either from specific likely source areas or countrywide, could be used in January to give a seasonal forecast of likely numbers of armyworm outbreaks during the rest of the season. Seasons with more than average numbers of outbreaks often follow poor early season rains and visa versa. Further work is needed to establish biological causes of this apparent relationship.

Field observations were carried out on a natural outbreak of a nuclear polyhedrosis virus of the armyworm Spodoptera exempta. Records were kept in four study sites with varying larval density, vegetation cover, and climatic conditions. Biotic factors influencing disease outbreaks were also noted. Initial mortality was recorded 11 days after larval outbreak, and the highest mortality occurred 10 days later. The epizootic was highest in sites of high larval density, wide fluctuations of daily temperatures (range 16.2°C), and high relative humidity. Rainfall was recorded in all disease outbreak sites. An acridid Acrotylus patruelis fed on larval cadavers in the field. Polyhedral inclusion bodies recovered from the gut of A. patruelis caused disease when fed to third instar S. exempta larvae in the laboratory. Predation on larvae by the pied crow Corvus albus in some observation sites reduced larval numbers, and may have prevented NPV from rising to epizootic levels in such areas. In sites of high polyhedrosis incidence, the disease led to larval population collapse in 3 weeks following disease outbreak.

The dwarf variety of coconut, Cocos nucifera L. (Palmae), was found to be eaten by the larvae of the African armyworm, Spodoptera exempta (Wlk.). The comparison of the armyworm infestation on the leaves of the coconut seedlings and the adjacent grass species revealed lower larval density and growth rate on coconut seedlings. This observation suggests that a small population of the African armyworm can survive and develop on the leaves of the dwarf variety of coconut.

Occurrences of outbreaks of the African armyworm, Spodoptera exempta, between January and December for 19 years (1961–1979), and records on nightly moth catches from light trap stations located between southern Kenya and east-central Tanzania (1–8°S and 35–41°E) for 16 years (1963–1978), have been mapped, tabulated, and analysed on a standard weekly time-table, in order to compare events occurring between the ‘off-season’ months of July to November and the beginning of the ‘outbreak seasons’ from December or January. Circumstantial evidence from the above, coupled with analysis of climatological statistics for rainfall, temperature and windflow over the area and period, support the hypothesis that populations of S. exempta can survive between southern Kenya and eastcentral Tanzania during the off-season months, and contribute to the initial development of ‘outbreak seasons’ in Tanzania or Kenya. Methods for monitoring and analysing such developments for improving the efficiency of the Armyworm Forecasting Service in East Africa are suggested.

During sporadic outbreaks of S. exempta, population densities sometimes reach extremely high numbers. Monitoring at one such outbreak revealed that a considerable synchrony exists in the developmental processes of such populations, so that at times up to 97% of the population is represented by only one stage of development. Heavy pupal mortality with symptoms of a cytoplasmic virus infection was noted. Flight behaviour of emerging adults revealed observable differences in the flight activity of the moths. Although a great majority of the emerging moths flew away from the area, some stayed back. Eggs layed by the latter produced a second brood but, due to a more or less total depletion of the food source, this subsequent brood did not survive in any appreciable numbers and finally died out.