Locusts modify developmental and reproductive traits over successive generations depending on the population density. A trade-off between developmental rate and body size and between progeny size and number is often observed in organisms. In this study, we present evidence that this rule is evaded by desert locusts, Schistocerca gregaria Forskål, which often undergo outbreaks. Under isolated conditions, large hatchlings, typical of the gregarious forms, grow faster but emerge as larger adults than do small hatchlings typical of the solitarious forms, except for some individuals of the latter group that undergo extra molting. Under crowded conditions, large and small hatchlings grow at a similar rate, but the former become larger adults than the latter. Small hatchlings show a trade-off between development time and body size at maturation, but this constraint is avoided by large hatchlings. Phase-specific, as well as body size–dependent, differences are also detected in reproductive performance. As adult body size increases, females of a solitarious line produce more but slightly smaller eggs, whereas those of a gregarious line produce more and larger eggs. Total egg mass per pod is larger in gregarious forms than in solitarious forms. A trade-off between egg size and number is shown by a solitarious line but not by a gregarious line that produces relatively large eggs with similar numbers of eggs per pod. These results suggest that phase transformation involves not just a shift of resource allocation but also an enhanced capability expressed in response to crowding.

Little is known about phase polymorphism in the red locust, Nomadacris septemfasciata (Serville) in Madagascar, despite its economic importance. Only solitary and transiens forms have been reported from the island earlier, but not the gregarious form. Important recent outbreaks have allowed for the studying of the phase polymorphism under experimental and field conditions. Morphometric differences (pronotum size and shape, and F/C and E/F ratios) were noted among hoppers and adults caged for several generations and at various densities, in a field laboratory. These criteria were later used to study adults collected in different areas between 1996 and 2002, from populations with various densities (<1000 to >25,000 imagos/ha). Morphometric measurements revealed that gregarious red locust populations have existed naturally in Madagascar and that these were erroneously classified as transiens forms. The upper threshold density for the solitary phase was estimated at 5000 adults/ha; beyond this limit, locusts develop either into transiens or into highly gregarious forms. The southern part of the island, known as a red locust outbreak area, has harboured gregarious phases despite previous reports that prevailing ecological conditions are not suitable for its complete phase transformation. Deforestation in the northern part of the island has probably led to new favourable biotopes for serious outbreaks and gregarious populations development.

A 1994 field survey of Sahelian grasshoppers and locust populations in natural vegetation, millet and rice fields in The Gambia determined the period between mid-August and early September to be the optimal time for applying oil-formulated conidia of the entomopathogenic fungus Metarhizium anisopliae var acridum to control the acridids. Locust and grasshopper cadavers were also examined for fungus infection. Metarhizium anisopliae was absent from cadavers of natural grasshopper populations, while the entomopathogenic fungus Entomophaga sp. was recovered on Hieroglyphus daganensis cadavers and the nematode Hexamermis sp. from Kraussaria angulifera, The predatory cattle egret fed on swarming adult desert locusts.

The historical account of the rise and fall of Nosema locustae as a biological control agent of grasshoppers has substantial implications for the successful development and implementation of biological control of migratory locusts in Africa. The potential of N. locustae was ultimately not sufficient to overcome a set of impediments, including: efficacy (the rate and extent of mortality were relatively low), target specificity (not all pest species were susceptible), formulation (wheat bran bait excluded feeding by some pests), cost (the price of the formulated product was excessive), storage (the organism lacked long-term stability), complex application (the pathogen and its carrier had to be applied during a narrow set of environmental and logistical parameters) and production (the use of an in vivo production system made high-volume production difficult). Some of the most significant limitations that led to the failure of N. locustae have been overcome by work on other pathogens, including the problems of formulation, storage, efficacy and production. However, at least four relevant lessons can be derived from the N. locustae story and applied to ongoing work with biological control of migratory locusts. First, the erratic population dynamics of acridids necessitates that the production, storage and distribution of an augmentative biological control agent will be driven by a boom-and-bust cycle. Second, the immense spatial scale and low unit value of the resources (rangelands) being protected create a unique set of logistical challenges. Third, the control of acridids involves the management of a native insect pest embedded within complex, native ecosystems, which suggests that our interventions should be undertaken with a great deal of caution, monitoring and, ultimately, humility. Fourth, although pathogens can be used as ‘bio-insecticides’, biological control requires education of end-users regarding a more sophisticated approach to pest management.

Recent studies on the chemical ecology of the desert locust, Schistocerca gregaria, have unravelled a rich and complex system of pheromonal and other semiochemical-mediated communication in this insect. The key pheromones associated with such behavioural processes as social cohesion and communal egg-laying have been characterised. The semiochemical bases of such physiological processes as maturation of solitarious insects before the arrival of seasonal rains, development of gregarious traits in crowded solitarious insects, development of maturation synchrony in gregarious insects, and maternal transfer of gregarious characters to the offspring are better understood. Desert locust semiochemicals are finding two principal uses: as tools for studying the processes associated with gregarisation and, therefore, for elucidating the eco-physiological mechanisms underlying the development of locust outbreaks; and as agents for interfering or reversing the process of gregarisation. The status of this research and its implication for the development of novel ecologically friendly tactics for the management of different locust species are highlighted.

A programme of research into phase change in the desert locust, Schistocerca gregaria, is described. The ability to change phase between solitarious and gregarious forms in response to population density is a key feature of locusts and is central to their occasional yet catastrophic impact on humans. Phase polymorphism is an extreme form of phenotypic plasticity. The most labile phase characteristic is behaviour. It is argued that a fully integrated study of behavioural phase change provides a powerful tool for understanding both the mechanisms of phase change and locust population dynamics, both of which offer possibilities for improved management and control of desert locust plagues. An assay for measuring behavioural phase-state in individual locusts was derived, based on logistic regression analysis. Experiments are described that used the assay to quantify the time-course of behavioural change, both within the life of individual locusts and across generations. The locust-related stimuli that provoke behavioural gregarization were investigated. Complex interactions were found between tactile, visual and olfactory stimuli, with the former exerting the strongest effect. Behavioural analysis also directed a study of the mechanisms whereby adult females exert an epigenetic influence over the phase-state of their developing offspring. Female locusts use their experience of the extent and recency of being crowded to predict the probability that their offspring will emerge into a high-density population, and alter the development of their embryos accordingly through a gregarizing agent added to the foam that surrounds the eggs at laying. There is also a less pronounced paternal influence on hatchling phase-state. An understanding of the time-course of behavioural phase change led to a study of the effect of the fine-scale distribution of resources in the environment on interactions between individual locusts, and hence on phase change. This, in turn, stimulated an exploration of the implications of individual behavioural phase change for population dynamics. Cellular automata models were derived that explore the relationships between population density, density of food resources and the distribution of resources in the environment. The results of the simulation showed how the extent of gregarization within a population increases with rising population size relative to food abundance and increasing concentration of food resources. Of particular interest was the emergence of critical zones across particular combinations of resource abundance, resource distribution and population size, where a solitarious population would rapidly gregarize. The model provided the basis for further laboratory and field experiments, which are described.

Haemolymph from the locusts Schistocerca gregaria and Locusta migratoria contains humoral factors such as lectin and phenoloxidase system. In vitro studies demonstrated that phenoloxidase system can be activated by LPS, laminarin, Malamoeba locustae cysts, trypsin and methanol. Haemolymph showed agglutination activity for mammalian erythrocytes and protozoa (M. locustae, Leishmania major and Trypanosoma brucei). A purified lectin was isolated from haemolymph by affinity chromatography. Analysis by SDS-PAGE gave a single band at 80 kD while on native PAGE, it gave a single band at 650 kD. Inhibitors of the two humoral factors were investigated. Lectin activity was inhibited by nitrophenyl a α D-galactopyranoside in both cases and by lectin antiserum. Inhibitors of the phenoloxidase system were found in plasma. Prospects for use of such inhibitors are discussed. They could be incorporated in biopesticide formulations in such a way that they could depress locust immune reaction and stimulate biocide activity.

Entomopathogens have been isolated and developed for the control of various lepidopteran, coleopteran, dipteran, hymenopteran, homopteran, and even acarine pests. Among the orthopteran pests, the acridid defoliators are important in Africa, and the periodic outbreaks of the desert locust, Schistocerca gregaria; red locust, Nomadacris septemfasciata;migratory locust, Locusta migratoria migratorioides and brown locust, Locustana pardalina; as well as numerous grasshoppers, cause extensive crop losses. Laboratory and field tests carried out on control of locusts and grasshoppers using microorganisms have shown that there are key candidates which could be used for the management of these acridid pests.

This paper describes briefly four different locust species that constitute potential widespread danger to agriculture and pasture in the eastern, central and southern Africa region. Particular reference is made to the red locust, Nomadacris septemfasciata Serville which mostly feeds on monocotyledonous plants. Reference is also made to the African migratory locust, Locusta migratoria migratorioides R&F; the brown locust, Locustana pardalina Walker; and the desert locust, Schistocerca gregaria Forskal. There were frequent reports of damage to cereal crops by the red locust during the most recent plague of the 1930's. However, since that time only a few yield loss data are available owing, primarily, to the fact that locust infestations have mostly been irregular and confined within their natural breeding areas, away from crop land. The paper provides general background information that should enable evaluation of locust incidence in the region and assessment of consequent cereal crop production losses.