The time-course of behavioural phase change in nymphs of the desert locust, Schistocerca gregaria

Abstract. The time-course of behavioural phase change was investigated in nymphs of Schistocerca gregaria , using logistic regression analysis of behaviour recorded in a standard assay. Gregarization occurred very rapidly. Solitary-reared nymphs became markedly gregarious in behaviour within 1-4h of being placed in a crowd. These insects re-solitarized equally quickly if removed from the crowd. Crowd-reared locusts also solitarized within l-4h, but this effect was not complete. Results indicate that, while behavioural gregarization is maximal within a few hours of crowding, solitarization is a two-stage process, changing rapidly at first, then more slowly as a function of the period of previous crowding.     

Effects of hatchling body colour and rearing density on body colouration in last-stadium nymphs of the desert locust, Schistocerca gregaria

Abstract.  The influences of hatchling character and rearing density on body colour at the last-nymphal stadium are investigated for the desert locust, Schistocerca gregaria . Hatchlings are divided into five groups based on the darkness of the body colour and reared either under isolated or crowded conditions. Two types of body colour variation at the last-nymphal stadium are separately analysed (i.e. the background colour and black patterns). Under isolated conditions, the background body colour is either greenish or brownish. Most individuals are greenish and the highest percentage of brownish insects is obtained from hatchlings with the darkest body colour. Under crowded conditions, the background colour is yellow or orange and the percentage of yellowish nymphs tends to decrease when they are darker at hatching. The intensity of black patterns differs depending on the body colour at hatching and subsequent rearing density. Most isolated-reared nymphs exhibit few or no black patterns but nymphs with some black patterns also appear, particularly among those that had been dark at hatching. Under crowded conditions, the black patterns become more intense when they are darker at hatching. Therefore, last-stadium nymphs with typical solitarious or gregarious body colouration appear when they have the phase-specific body colouration at hatching as well. The present results demonstrate that both body colour at hatching and rearing density during nymphal development influence body colouration at the last-nymphal stadium.     

PCR primers for polymorphic microsatellite loci in the desert locust,Schistocerca gregaria (Orthoptera: Acrididae)

Nine pairs of polymerase chain reaction (PCR) primers that amplify polymorphic microsatellite loci in the desert locust, Schistocerca gregaria (Forskal), were developed using a magnetic bead‐based enrichment protocol. A sample of 48 locusts collected during the 1993 and 1995 upsurge periods in Eritrea, East Africa, were genotyped. The number of alleles per locus ranged from six to 20; the average was 12.67. Allelic distributions were significantly different between samples from different localities.     

Morphological and behavioural characteristics of a gynandromorph of the desert locust, Schistocerca gregaria

Abstract.  Morphological and behavioural characteristics are investigated for a gynandromorph of the desert locust, Schistocerca gregaria , appearing under isolated rearing conditions in the laboratory. It has both male and female external reproductive organs bilaterally. The body size and dimensions are similar to a normal male. Morphometric traits (fore wing length/maximum head width ratio and fore wing length/hind femur length ratio) of the gynandromorph are typical for the values of solitarious locusts. When the gynandromorph is placed into an arena holding ten sexually mature gregarious females, it shows a distinct male behaviour: it jumps on a female and tries to mate with her. When kept together with males, males recognize this gynandromorph as a female because some of them try to mount, although no successful copulation is observed. The results suggest that the gynandromorph might have had a female-specific pheromone. Dissection reveals that the gynandromorph has no testis but abnormal ovaries containing vitellogenic oocytes. These observations indicate that the gynandromorph obtained has a mixture of male and female morphological characteristics and behaves like a male but is recognized as a female by conspecific males.     

Tactile stimuli perceived by the antennae cause the isolated females to produce gregarious offspring in the desert locust, Schistocerca gregaria

Maternal determination of progeny body size and coloration in the desert locust, Schistocerca gregaria, depends on the crowding conditions experienced during the short sensitive period that occurs two to six days before the deposition of the egg pod. Solitarious (isolated-reared) females produce relatively small eggs that yield solitarious green hatchlings but, females that are exposed to crowded conditions during the sensitive period, produce larger eggs that yield the dark-colored hatchlings characteristic of gregarious forms. The present study aimed to determine the stimuli influencing the maternal determination of progeny characteristics as well as the site at which such stimuli are perceived. By exposing isolated female adults to various combinations of visual, olfactory and tactile stimuli from a crowd of other adults, we found that no crowding effects could be elicited without tactile stimulation. Coating of various body surfaces with nail polish followed by exposure to crowding stimulation suggested that female adults perceive crowding stimuli with their antennae. This finding was supported by another experiment in which the antennae were either removed or covered with wax before the isolated females were exposed to crowded conditions. Neither serotonin nor an antagonist of its receptor affected the density-dependent maternal determination of progeny characteristics when injected into isolated or crowded female adults.     

Fine structure and distribution of antennal sensilla of the desert locust, Schistocerca gregaria (Orthoptera: Acrididae)

The fine structure and distribution of various types of antennal sensilla in three nymphal stages and in adults of both solitary-reared (solitary) and crowd-reared (gregarious) phases of the desert locust, Schistocerca gregaria, were investigated by scanning and transmission electron microscopy. Four types of sensilla were identified: sensilla basiconica, s. trichodea, s. coeloconica and s. chaetica. S. basiconica contain up to 50 sensory neurons, each of which displays massive dendritic branching. The sensillar wall is penetrated by a large number of pores. In contrast, s. trichodea contain one to three sensory neurons that branch to give five or six dendrites in the sensillar lumen; the sensillum wall is penetrated by relatively few pores. The s. coeloconica are situated in spherical cuticular pits on the antennal surface. The s. coeloconica are of two types: one type contains one to three sensory neurons with double sensillar walls penetrated by slit-like pores, whereas the second type contains four sensory neurons with non-porous double sensillar walls. The s. chaetica have a flexible socket and a thick non-porous sensillum wall and contain four sensory neurons that send unbranched dendrites to a terminal pore. A fifth sensory neuron of the s. chaetica terminates in a tubular body at the base of the hair. S. basiconica and coeloconica are normally distributed over the entire antennal flagellum, with a concentration in the middle segments; s. trichodea have three areas of concentration on the 5th, 10th and 14th flagellar segments. Sensilla chaetica are most abundant on the terminal segment. Locusts raised in solitary conditions have more olfactory sensilla (s. basiconica and s. coeloconica) than crowd-reared locusts. The difference in sensillar numbers is more evident in adults than in nymphs. These results suggest that differences in the odour-mediated behaviour of nymphs and adults, and between the phases of S. gregaria, may be attributable to differences at the sensory input level.     

Yellowing,morphology and behaviour in sexually mature gynandromorphs of the desert locust Schistocerca gregaria

The morphology, colouration and sexual behaviour of two gynandromorph adults of the desert locust Schistocerca gregaria Forskål are described. In each of these specimens, the left half of the external genitalia has male characteristics and the right half has female characteristics. Yellowing of the epidermis occurs to different degrees in normal sexually mature male and female adult S. gregaria under crowded conditions. Two hypotheses are known to explain this phenomenon. One suggests involvement of a sex‐hormone/receptor complex, whereas the other proposes different sensitivities of the epidermis to the same hormonal environment. To examine which hypothesis is more likely, one gynandromorph adult is kept under crowded conditions and the other is maintained under isolated conditions. In the former, the left half of the body turns bright yellow, as in a sexually mature crowded male, and the right half is brownish with little yellowing, as in a sexually mature crowded female of the same age. The bilaterally divided body colouration may support the second hypothesis. In the second gynandromorph, which is isolated after adult emergence, the body colour remains brownish. Upon sexual maturity, this individual is kept together with normal male or female adults for behavioural observations. It attempts to mount a female but is approached and mounted by males.     

Phase-related body-color polyphenism in hatchlings of the desert locust, Schistocerca gregaria: re-examination of the maternal and crowding effects

The mechanism controlling the body color of hatchlings was studied for the desert locust, Schistocerca gregaria. A pheromonal factor secreted by gregarious female adults into the foam plugs of egg pods has been suggested to cause darkening in their progeny. We re-examined the role of this maternal factor by washing or separating eggs at deposition. Eggs produced by crowd-reared female adults were washed with saline or separated individually without being washed immediately after deposition and the body color of the hatchlings from them was compared with that from the eggs unwashed and kept in the egg pod until hatching. Most hatchlings were dark and no significant difference was found in the proportions of dark- and light-colored hatchlings between the treatments and controls. Likewise, eggs separated before the foam plug deposition produced dark-colored hatchlings as in the un-separated controls. These results demonstrated that neither washing nor separation of eggs at deposition affected the hatchling body coloration. The variation in hatchling body color was correlated closely to the body weight at hatching, indicating that hatchling body color had been determined maternally. Green hatchlings reared under crowded conditions remained green until the second stadium at which black patterns were induced. It was concluded that body color at hatching has been determined maternally and crowding during the first nymphal stadium influences nymphal body color but its effect is not manifested until the second stadium. The present study casts doubts on the presence of a recently suggested pheromonal factor on the color of the hatchlings.     

Phase-independent responses to phase-specific aggregation pheromone in adult desert locusts, Schistocerca gregaria (Orthoptera: Acrididae)

Abstract. Volatiles from solitary-reared (solitarious) and crowd-reared (gregarious) adult male desert locusts, Schistocerca gregaria (Forskal) (Orthoptera: Acrididae), were quantitatively and qualitatively different.In particular, solitarious males did not emit phenylacetonitrile, a key component of the aggregation pheromone produced by gregarious adult males.In laboratory bioassays, solitarious and gregarious adults of both sexes responded similarly to the natural aggregation pheromone blend, the major pheromone component phenylacetonitrile, and a synthetic pheromone blend comprising benzaldehyde, guaiacol, phenylacetonitrile and phenol.EAG measurements showed significant differences in the responsiveness of adults of the two phases to the four synthetic components at high doses; however, the general response patterns were similar.These results suggest that the gregarious adult male aggregation pheromone may play a role in the arrestment and subsequent recruitment of solitarious individuals into gregarious or gregarizing groups during the early stages of a locust outbreak.     

Phase-specific responses to different qualities of food in the desert locust, Schistocerca gregaria: developmental, morphological and reproductive characteristics

Solitarious female adults are known to produce smaller hatchlings than those produced by gregarious adults of the desert locust, Schistocerca gregaria. This study investigated developmental, morphological and reproductive responses to different qualities of food in hatchlings of different phases. Mortality was higher, the duration of nymphal development longer and adult body weight lighter with a low-quality food than a high-quality food. Gregarious hatchlings showed better survivorship, grew faster and became larger adults than did solitarious ones. The incidence of locusts exhibiting extra molting, which was typically observed in the solitarious phase, was dramatically increased when a low-quality food was given to the solitarious hatchlings. Low-quality food caused locusts to shift morphometric ratios toward the values typical of gregarious forms; smaller F/C (hind femur length/maximum head width) and larger E/F (elytra length/hind femur length). Solitarious hatchlings grown at either high- or low-quality foods and then given high-quality food after adult emergence revealed that food qualities during the nymphal stage influence their progeny quality and quantity via adult body size that influenced reproductive performance. Female adults showed an overshooting response to a shift from low- to high-quality food by increasing egg production that was specific to body size. This study may suggest that gregarious hatchlings are better adapted to adverse food conditions than solitarious counterparts and extra molting is induced even among gregarious hatchlings under poor food conditions.     

Physiological responses and central nervous projections of antennal olfactory receptor neurons in the adult desert locust, Schistocerca gregaria (Orthoptera: Acrididae)

Olfactory receptor neurons present in two morphological sensillum types on the male Schistocerca gregaria antenna were for the first time investigated physiologically when stimulated with behaviourally relevant odours. Neurons present in trichoid/basiconic sensilla showed clear excitatory responses to compounds present in the male-produced aggregation pheromone and also to a plant produced compound. Sensilla could be categorised physiologically according to the responses of their receptor neurons to the tested stimuli. Also receptor neurons present in sensilla coeloconica responded to aggregation pheromone components, but always in an inhibitory fashion. These neurons could, however, be excited by a plant produced compound and by some acids present in the nymphal odour. The antennal lobe of the male S. gregaria was observed to contain about 1000 very small glomerular structures. Single receptor neurons were stained from the antenna to the antennal lobe using a cobalt lysine technique. These stainings revealed a multi glomerular axonal branching pattern of antennal receptor neurons.Abbreviations AN antennal nerve - AL antennal lobe - RN receptor neuron     

Subspecific taxonomy of the desert locust,Schistocerca gregaria (Orthoptera: Acrididae), based on molecular and morphological characters

We evaluated the validity of the subspecific designation for Schistocerca gregaria gregaria (Forskål) and Schistocerca gregaria flaviventris (Burmeister), isolated in distinct regions along the north–south axis of Africa. Towards this goal, we assessed the variation of multiple morphological and molecular traits within species. We first used elliptic Fourier and landmark‐based relative warps analyses to compare the size and shape of two internal and two external structures of male genitalia. We provide a discriminant function which classified the specimens with 100% accuracy and selected shape elements of the external structures only (cercus and epiproct). We also tested eight molecular markers, and because of either absence of variation or contamination by mitochondrial DNA (mtDNA)‐like sequences, we used a clone‐and‐sequence analysis of the standard cytochrome c oxidase subunit I mitochondrial DNA barcode only. We differentiated 185 true mitochondrial sequences from 66 mitochondrial DNA‐like sequences, most of which were from S. g. gregaria specimens. On the dataset of mitochondrial origin, we identified three characteristic point mutations that diagnosed the two allopatric subspecies with 94% accuracy. Minimum spanning network and parsimony tree analyses identified S. g. flaviventris as a monophyletic lineage distinct from the nominate subspecies. Accordingly, microsatellite data indicate rarely occurring admixture events only, showing that independent evolutionary history is the norm.     

Effects of isolation and crowding on the haemolymph pigment composition of the desert locust, Schistocerca gregaria (Orth., Acrididae)

Abstract:  An experimental study on the effect of isolation and crowding of the desert locust, Schistocerca gregaria (Forskål) on haemolymph pigment composition (as measured by absorbance ratios at 460 and 680 nm) was carried out at the International Centre of Insect Physiology and Ecology, Nairobi. The results showed that haemolymph absorbance ratios of gregarious and solitarious locusts were different (P < 0.05) and that there was no phase-specific sex differentiation with regard to this measure (P < 0.05). The haemolymph pigment composition changes during isolation and grouping showed contrasting trends. It changed rapidly in nymphs, but slowly and erratically in adults both resulting from isolation and grouping at hopper stage. Significant shifts in locust phase status were recorded in both situations in nymphs in F₀ generation, and in the adults by the end of the F₂ and F₃ generations of solitary and gregarious conditions, respectively. Shifting of fledglings did not induce significant changes in their mean absorbance ratios in both situations, i.e. isolation and crowding within F₀ generation. The ratio of length of posterior femur (F) to greatest width of the head capsule (C) changed either at a similar rate or more often slower than the haemolymph pigment ratio and both required more than a generation to transform fully. The study has shown that haemolymph pigment composition could be a suitable measure for monitoring phase changes at nymphal stages of the desert locust.     

Effects of juvenile hormone treatment on phase changes and pheromone production in the desert locust,Schistocerca gregaria (Forskal) (Orthoptera: Acrididae)

The roles of juvenile hormone III (JH III) on phase changes and pheromone production were examined in laboratory-reared gregarious desert locust, Schistocerca gregaria (Forskal). The hormone was applied to 5th instar nymphs and newly emerged adult locusts. Generally, the 5th instar nymphs exhibited a higher sensitivity to hormone treatments than the adults. Hormone applications inhibited pheromone production (as measured by the amounts of phenylacetonitrile released). In addition, JH III had a significant effect on the external colouration and absorbance ratios of the haemolymph pigments. It is concluded that the effects of exogenous JH III on gregarious locusts represent a shift towards the solitarious phase.     

Effect of phase status on responses to AKHI in the desert locust,Schistocerca gregaria gregaria

Hyperlipaemic response to adipokinetic hormone (AKH I) was demonstrated in both solitary and gregarious phases of the desert locust, Schistocerca gregaria gregaria. Time-course studies showed that the gregarious locusts had a faster response to the hormone than their solitary counterparts. At peak response time (90 min), the gregarious locusts were more sensitive to AKH I doses below 2 pmol while the solitary locusts had a higher response above this dose. Upon injection of the hormone, lipoprotein conversion occurred, resulting in the formation of the low density lipoprotein (LDLp). The LDLp formed in the gregarious locusts was much larger than that of the solitary locusts. The fat body lipid reserve (expressed as % fat body dry weight) was significantly (P < 0.01) higher in the gregarious (79.02 ± 2.77%) than in the solitary locusts (65.23 ± 2.55%). Triacylglycerol was the major lipid class representing 83.9 and 73.9% of the total lipids in gregarious and solitary locusts, respectively. The higher fat body lipid reserves and efficient LDLp formation in response to AKH in gregarious locusts compared to solitary locusts suggests a physiological adaptation for prolonged flights. © 1996 Wiley-Liss, Inc.     

Genetic and hormonal control of melanization in reddish–brown and albino mutants in the desert locust Schistocerca gregaria

The genetic and hormonal control of body colouration is investigated using two recessive genetic mutant strains, the reddish–brown (RB) mutant and an albino mutant, as well as a normal (pigmented) strain of the desert locust Schistocerca gregaria. The colour patterns of the RB nymphs are similar to those of a normal strain, although the intensity of the melanization is weaker in the former. Reciprocal crosses between the RB and albino mutants produce only normal phenotypes in the F₁ generation. In the F₂ generation, the normal, RB and albino phenotypes appear in a ratio of 9 : 3 : 4, indicating that two Mendelian units might determine the appearance of dark body colour and the intensity of melanization, respectively. In other words, at least two steps of regulation might be involved in the expression of body colour. Injections of [His⁷]‐corazonin, a neuropeptide inducing dark colour in this locust, fail to induce dark colour in albino nymphs but show a dose‐dependent darkening in RB nymphs in the range, 10 pmol to 1 nmol. Some RB nymphs become indistinguishable from normal individuals after injection of the peptide. Implantation of corpora cardiaca (CC) taken from RB mutants into other RB individuals induces darkening in the latter and CC from RB, albino and normal strains have similar dark colour‐inducing activity when implanted into albino Locusta migratoria. These results suggest the possibility that the RB mutant gene regulates the intensity of melanization, possibly through controlling the pathway of pigment biosynthesis associated with [His⁷]‐corazonin.     

Take-off activity in caged desert locusts,Schistocerca gregaria (Forsk.) (Orthoptera,Acrididae) in relation to meteorological disturbances

Take-off activity of the desert locust,Schistocerca gregaria (Forsk.), phasegregaria, was studied in a temperature-controlled laboratory environment and possible relationships with certain meteorological factors were analysed. A statistically significant relationship between the passage of cold fronts and an increased take-off rate was found. The reaction of the locusts could not be correlated to changes in VLF electromagnetic radiation or to the atmospheric pressure.     

The integument and sensilla of the pregenital abdomen of the Desert locust (Schistocerca gregaria) (Orthoptera: Acrididae)

The integument and sensilla of the abdominal segments II–VII, together with the tracheal and nerve supply, are described in so far as they can be seen using whole mounts. Such a method often enables the neurons of the sensilla to be seen clearly and to be associated definitely with a certain cuticular structure. The sensilla are compared with those already known from the palps, and it is concluded that probably all those on the abdomen also occur on the palps.