Trans-generational transfer of gregarious-phase traits in the desert locust Schistocerca gregaria (Forskål, 1775) is mediated by primer gregarizing pheromonal signals produced by ovipositing females that experience crowding. We monitored time-course proteomic events in eggs from solitary-reared locusts that had been exposed for 1, 3, 5, 7, 10 and 12 days to different levels of the sand-associated gregarizing signal originating from 0, 3, 5 or 10 ovipositions by crowd-reared females. Evidence for the phase transition was sought by comparing the protein patterns of embryos thus exposed with those from crowd-reared (gregarious) controls; this comparison was continued until the stage of the first instars. Expressed proteins were analysed by two-dimensional protein gel electrophoresis, and patterns from the different treatments within stages were compared by profile matching and χ2 analyses. Eggs derived from crowd- and solitary-reared females showed essentially similar protein patterns at early stages of embryogenesis; however, mature stages (particularly, days 10 and 12) and hatchlings demonstrated significantly different patterns. Protein patterns of eggs from solitary-reared females that were incubated in sand contaminated with the pheromonal signal and of the hatchlings that emerged were similar to those derived from gregarious females and dependent on the level of the pheromone to which the embryos had been exposed. The results confirm the gregarizing effect of the signal and constitute a useful basis for unravelling the mechanism of the signalling cascades associated with gene expressions triggered by the pheromone.

The effect of vegetation cover (millet) on the efficacy and conidial persistence of Metarhizium acridum (Driver & Milner) J.F. Bisch., Rehner & Humber was evaluated in semi-field conditions using breeding cages (2 × 2 × 1 m). A mixed population of third- and fourth-instar desert locust larvae, Schistocerca gregaria Forsskål, was used as a target. The insects were exposed in two different vegetation cover types classified as low (about 10%) and high (about 90%). Metarhizium acridum was used at a dose of 2.5 × 1012 conidia/ha in two different application volumes: 1 and 2 l/ha. Untreated insects kept in contact with treated vegetation were monitored to evaluate the persistence of conidia. The results showed that vegetation cover did not significantly (F= 1.320; P= 0.334) affect the efficacy of M. acridum. Under the high vegetation cover, the increase in the applied volume rate to 2 l/ha significantly improved the speed of mortality. Conidia persisted 6 days after treatment with a remarkable effect on untreated larvae exposed to the treated vegetation. In addition, the results of this study showed the efficacy of M. acridum in the low vegetation cover. With an important mass of vegetation, M. acridum conidia could persist even under high temperature conditions.

The storability of five new different formulations of Metarhizium acridum (two oil, one paste, one semi-solid and one powder) was investigated under ambient and low temperatures. Fourth-instar nymphs of the desert locust Schistocerca gregaria (Forskål) were used to evaluate their virulence. Under ambient conditions (23–38°C), all the formulations maintained acceptable viability (66.1 ± 2.30 to 71.8 ± 0.88% germination) after 6 months of storage. After 10 months, only conidia from the powder formulation remained viable. In contrast, samples of the formulations kept under low temperatures ( − 1.2 to 15°C) did not show significant loss of viability after 30 months of storage despite repeated power cuts, lasting sometimes several days. During this period, M. acridum conidia remained virulent to nymphs with 100% mortality 5 to 7 days after the treatment. These results show that all the formulations have greater stability when stored under low and even fluctuant temperature changes. Under ambient conditions, the formulations could remain effective for at least 6 months of storage.

Resilin is a soft and elastic protein, which is found in many exoskeletal structures of arthropods. Proteins with similar chemical properties have been described for other invertebrates including monogenean fish parasites. However, for the latter taxon no clear microscopic evidence for a resilin-like protein has been shown so far. Here, we present the results of detailed microscopic analyses of the clamp sclerites (attachment devices) of the monogenean Diplozoon paradoxum. Toluidine blue, which is known to stain resilin, selectively stained the material in the clamp sclerites. In addition, when exposed to UV light, this material exhibited an intense blue autofluorescence. The emission spectrum of this autofluorescence has its maximum at 424 nm and is nearly identical to emission spectra of blue autofluorescences observed in 2 well-studied structures containing high proportions of resilin in the exoskeleton of the locust Schistocerca gregaria. The results strongly indicate that the sclerite material of D. paradoxum contains a resilin-like protein. The presence of such a protein likely enhances the attachment efficiency of the clamp sclerites and increases their lifetime.

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.

The levels of lipophorin, the principal insect haemolymph lipoprotein, were estimated during the development of solitary and gregarious phases of Schistocerca gregaria using single radial immunodiffusion. In the gregarious phase, lipophorin titres ranged from 6.69 ± 0.57 mg/ml in the 3rd nymphal instar to 14.42 ± 2.04 mg/ml in mature adults. The titres in the solitary phase were 3.33 ± 0.59 mg/ml in the 3rd nymphal instar and 8.44 ± 0.67 mg/ml in mature adults. Peak lipophorin titres occurred in mature adults (14.42 ± 2.04 mg/ml) and 5th nymphal instar (8.75 ± 0.25 mg/ml) for gregarious and solitary locusts, respectively. Gregarious locusts had significantly (P< 0.05) higher lipophorin titres than their solitary counterparts during the 3rd and 4th nymphal instars as well as in the adult stage (P< 0.01). Estimation of the haemolymph protein levels over the same period showed a general increase from the 3rd nymphal instar to mature adult stage in both phases. It is proposed that the higher lipophorin titre in gregarious locusts is a physiological adaptation that ensures high lipid reserves that are necessary to cope with the high metabolic requirements of this phase.

The influence of post-treatment temperature on the insecticidal activity of Azadirachta indica (A. Juss) seed extract against the desert locust, Schistocerca gregaria (Forskål) is reported. In tests on adults, the toxicity of crude A. indica seed extract increased by about 10-fold when the post-treatment temperature was raised from 22°C to 40°C. This temperature-dependent toxicity was observed in insects treated either topically or via injection. This phenomenon could partly explain the wide variability in efficacy of neem extracts reported by different investigators.

Different blends of five chromatographically prominent components of the pheromone emission of gregarious-phase mature desert locust Schistocerca gregaria (Forskål) (Orthoptera: Acrididae) males were bioassayed for their maturation-accelerating effects on immature counterparts. The blend of all five components, namely anisóle, veratrole, benzaldehyde, phenylacetonitrile and 4-vinylveratrole was as effective as the emission from live mature males in accelerating the onset of mating in immature males. Subtraction of anisóle had no significant effect but removal of any one of the other four components significantly reduced the acceleration potency of the resulting blends. The maximum reduction occurred in the absence of phenylacetonitrile which appears to be critical to the activity of the blend. The magnitude of reduction in maturation acceleration that resulted from the subtraction of the other three components suggests that, although present in lesser relative amounts, these compounds contribute significantly to the activity of the full blend. Some differences were found between the onset of integumental yellowing and mating which indicate that blends of slightly different compositions are involved in promoting these two physiological processes.

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.

The blue biliverdin pigment in the haemolymph of Schistocerca gregaria Forskål (Orthoptera: Acrididae) was shown to be associated with all the stages and ages of the solitary-reared (solitarious) insects, irrespective of their origin or diet. The pigment was absent or present in very small quantities in the crowd-reared (gregarious) insects. The ratio of absorbance at 460 nm and 680 nm, the former representing the λmax for the carotenoid pigment and the latter for the biliverdin, provides a clear-cut index for partitioning the phases. The mean value of the index for the gregarious group fell in the range 3.69 to 4.78. That of the solitarious group ranged from 0.64 to 1.67. The absorbance ratio at 460/680 nm appears to represent a simple and convenient means of differentiating the two phases of the desert locust, Schistocerca gregaria.

The already known benzoquinone, embelin (mixed with rapanone), and two new benzoquinones, myrsinone and myrsinaquinone, were isolated from the Myrsinaceae plant, Rapanea melanphloes (L), Mez. All the three compounds showed antifeedant activity when bioassayed on nymphs of the desert locust, Schistocerca gregaria (Forsk). They also inhibited growth of second-instar larvae of mosquito Aedes aegypti at low doses, and had acute lethal effects at higher doses.

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.

Laboratory studies were carried out in Egypt to examine the effects of treatment of the nymphs of Schistocerca gregaria during the later period of third instar with different dosages of gamma radiation ranging from 1000 to 3000 rad. The data indicate that treatment with a dose of 3000 rad induced complete mortality for all treated nymphs. However, irradiation with a dose of 2000 rad produced fourth instar nymphs which enter a state of suspended development for about 14 days. Exposure to a dose of 1000 rad induced three types of response associated with the endocrine activities. Some of the treated nymphs developed to the fifth instar with soft and thin cuticle without tanning or darkening, these nymphs did not survive more than a few hours after ecdysis. Other treated nymphs became fifth instar with anomalous wings and normal cuticle which survived about 18 days as permanent fifth instar nymphs before dying, while the rest of the treated nymphs moulted to permanent fourth instar which survived more than 1 month and eventually died. Corpora allata (CA) volumes in the permanent nymphs were estimated by planimeter method, revealing a pronounced reduction in their size in comparison with the untreated controls. It reflects a sharp inhibition in their synthetic activity of juvenile hormone (JH).

We deduce that the permanent nymphs may be produced as a result of a direct inhibition on the cerebral neurosecretory cells of the brain, the source of stimulating factors for JH and ecdysone production. Gamma rays prevented the production of the tanning and darkening factor (Bursicon Hormone) in these cells.