Novel cross-stage solitarising effect of gregarious-phase adult desert locust (Schistocerca gregaria (Forskål)) pheromone on hoppers

Previous studies had demonstrated stage differentiation in the cohesion (aggregation) pheromone systems of the desert locust, Schistocerca gregaria. In laboratory arena, the nymphal and adult stages responded aggregatively to their own pheromone, but dispersed evenly within the arena in the presence of the other. In the present study, we explored the effects of longer-term contact of field gregarious hopper bands and laboratory crowd-reared nymphs with the major constituent of the adult pheromone. During the first few days, hoppers in treated bands became relatively hyperactive. Over the next few days, their movements became random and they stopped marching as coherent groups, they started to roost for longer periods on vegetations, and they fragmented into smaller and smaller groupings and individuals. When attacked by birds, they demonstrated subdued levels of collective defensive behaviour compared to normal hoppers, and there were clear signs of increased predation and cannibalism at the roosting sites. In cage experiments, crowd-reared nymphs treated with the pheromone component became hyperactive, showed abnormal diel patterns and reduced feeding on plants but increased cannibalism. Our observations show that the major adult pheromone constituent has a solitarising effect on gregarious hoppers. The mechanism underlying this effect and the potential of the agent in desert locust control are discussed.     

The trans-generational phase accumulation in the desert locust: Morphometric changes and extra molting

To understand the underlying trans-generational phase accumulation, a classical morphometric characteristic, the F/C ratio (F, hind femur length; C, maximum head width), of adult desert locusts (Schistocerca gregaria) was monitored over eight consecutive generations. Adult F/C ratios, which are larger in solitarious locusts than in gregarious ones, were negatively correlated to the darkness of body color at hatching. Two successive generations were required for a complete shift from the gregarious (crowd-reared) to the solitarious (isolated-reared) phase and vice versa in the laboratory. That is (1) female adults needed to be exposed to crowded (or isolated) conditions so that their hatchlings would become large (or small) and dark (or green) in color, and (2) the hatchlings then needed to be exposed to crowded (or isolated) conditions for their entire nymphal stage. Solitarious locusts exhibited extra molting that influenced the F/C ratio in the adult stage, but did not exert significant influences on the trans-generational changes in this trait because the incidence was low. The incidence of extra molting was negatively correlated with nymphal survival rates. The morphometric trans-generational changes may be explained without assuming any accumulating internal factor.     

Maternal effects on phase characteristics in the desert locust, Schistocerca gregaria: a review of current understanding

Desert locusts demonstrate pronounced density-dependent polyphenism: a complex suite of traits shifts over the lifetime of an individual in response to crowding or isolation. These changes also accumulate across generations through a maternal effect. Female desert locusts alter the developmental trajectory of their offspring in response to their own experience of crowding. The mother possesses a memory of both the recency and extent of crowding and shifts the phase state of her hatchlings accordingly. Extensive experimental work has shown that offspring behaviour is controlled by a low molecular weight, polar compound (or compounds) released from the mother's accessory glands. The chemical identity of this agent is not yet known.     

Concentration-dependent parsimonious releaser roles of gregarious male pheromone of the desert locust, Schistocerca gregaria

The responses of (i) groups of crowd-reared mature males of desert locust, Schistocerca gregaria to a choice of two columns of air, one permeated with different concentrations of phenylacetonitrile (PAN), the major component of gregarious-phase male-produced pheromone, and the other untreated, and (ii) individual crowd-reared mature males of the insect to varying concentration gradients of PAN, were studied in two different types of arena. In the choice assay, locusts preferred to be within PAN-permeated air column at low relative doses of the pheromone, but away from PAN at high relative doses. In the second assay, individual locusts were arrested close to PAN source at low PAN concentration gradients, but away from the source at high concentration gradients. The results are consistent with two reported releaser functions of the adult male-released pheromone that are dependent on different sensory thresholds: arrestment and cohesion at lower relative concentrations and male-male homosexual avoidance at higher relative concentrations.     

The role of octopamine in locusts and other arthropods

The biogenic amine octopamine and its biological precursor tyramine are thought to be the invertebrate functional homologues of the vertebrate adrenergic transmitters. Octopamine functions as a neuromodulator, neurotransmitter and neurohormone in insect nervous systems and prompts the whole organism to “dynamic action”. A growing number of studies suggest a prominent role for octopamine in modulating multiple physiological and behavioural processes in invertebrates, as for example the phase transition in Schistocerca gregaria. Both octopamine and tyramine exert their effects by binding to specific receptor proteins that belong to the superfamily of G protein-coupled receptors. Since these receptors do not appear to be present in vertebrates, they may present very suitable and specific insecticide and acaricide targets.     

Effects of [His(7)]-corazonin on the phase state of isolated-reared (solitarious) desert locusts,Schistocerca gregaria

[His⁷]-corazonin is a neuropeptide produced in the pars lateralis of the brain. It is stored in the corpora cardiaca and probably released from there. The only well-documented effect in locusts is increased melanization of the cuticle. We investigated whether this hormone might also be causally related to changes in behavior and morphometrics that, like melanization, occur during crowding-induced gregarization. Solitary fourth-instar nymphs of Schistocerca gregaria (Forsk.) were injected thrice with 1 nmol [His⁷]-corazonin. After molting to the 5th stadium their behavioral phase state was measured in an arena assay and analyzed using multiple logistic regression analysis. The hormone was found not to induce behavioral phase changes. Upon reaching adulthood, morphometrics shifts occurred towards values typical for crowd-reared and regregarized animals. Our results thus indicate that [His⁷]-corazonin is not involved in behavioral gregarization but may participate in morphometrical phase change.     

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.     

Artificial miniaturization causes eggs laid by crowd-reared (gregarious) desert locusts to produce green (solitarious) offspring in the desert locust, Schistocerca gregaria

The mechanism underlying the phase-dependent polyphenism in hatchling body coloration was studied by testing for a possible causal relationship with egg size in the desert locust, Schistocerca gregaria. Crowd-reared (gregarious) females typically produce large, black offspring, whereas females reared in isolation (solitarious) deposit small, green offspring. We first tested for possible genetic differences in the role of egg foam by washing or separating eggs from two strains of locust. No solitarizing effect was found in either of the strains tested, supporting a previous finding, using another laboratory strain, to show that the hatchling body coloration and size are pre-determined in the ovary of the mother and no egg foam factor is involved in the control of the hatchling body coloration. Topical application of fenoxycarb, a juvenile hormone analog (JHA), and implantation of extra corpora allata (CA), taken from Locusta migratoria, caused gregarious female adults of S. gregaria to produce small eggs. Some eggs laid by CA-implanted females produced green hatchlings. All large eggs chosen among those deposited by gregarious females produced black hatchlings. When eggs were either kept on dry filter paper at nearly saturated relative humidity during embryogenesis or pricked with a needle so that some egg yolk was squeezed out, some produced small, green hatchlings. These results suggested that the amount of egg yolk or the availability of yolk material may determine the body coloration of hatchlings.     

Effects of crowding, isolation, and transfer from isolation to crowding on total ecdysteroid content of eggs in Schistocerca gregaria

We examined the ecdysteroid content of eggs from crowd-reared and solitary-reared desert locusts, Schistocerca gregaria, throughout embryogenesis from the day of egg laying until shortly before hatching on day 14. Depending on the time during incubation, ecdysteroid content in eggs from crowd-reared females was 5-10 times higher than in eggs from solitary-reared females. Our investigation revealed two peaks in ecdysteroid content of eggs from crowd-reared females, a small one at day 3 and a major peak at day 10 of incubation. At days 10, 12 and 14 during incubation of eggs from crowd-reared females, we found a positive correlation between egg mass and ecdysteroid content. There was no difference between eggs from the bottom and the top of individual egg pods, but variation in ecdysteroid content between egg pods from different females was considerable in all treatment groups. A brief crowding of solitary-reared females at the time of egg laying, a treatment that initiates maternally mediated gregarization of the developing offspring, had no effect on the consistently low ecdysteroid content in the eggs. This result rules out the possibility that the crowding experience of females is transmitted to the offspring by variation in the total amount of ecdysteroids in their eggs.     

A review of maternal and embryonic control of phase-dependent progeny characteristics in the desert locust

Hatchlings of the desert locust, Schistocerca gregaria, display phase polyphenism in body coloration and size. This phenomenon has been found to be maternally controlled and two different mechanisms have been proposed to explain the underlying process. One mechanism claims that a water-soluble pheromonal factor secreted by gregarious female adults into the foam plugs of egg pods induces darkening in their progeny. The other mechanism states that hatchling body coloration is pre-determined in the ovaries and that no foam factor is involved in this phenomenon. This mechanism was supported by the observation that hatchling melanization was not prevented by the early washing or separation of eggs, which should have removed the pheromonal factor from the eggs and produced green hatchlings according to the other mechanism. This paper reviews the latest findings related to this phenomenon with special reference to reproductive cycles and genetic differences. The close relationships between egg size and the degree of melanization in hatchlings may provide strong evidence against the possible involvement of the pheromonal factor, because egg size is determined in the ovarioles. Furthermore, the absence of “solitarizing” effects of early washing and separation on hatchling melanization was also confirmed in different genetic strains. A hypothesis proposed by others that such effects occur only in eggs from the first reproductive cycle was tested and rejected. Based on these and other results, a model to explain the mechanisms underlying the maternal control of progeny characteristics and embryonic control of melanization in the hatchling was proposed.     

Neuroanatomy and neurophysiology of the locust hypocerebral ganglion

The insect stomatogastric ganglia control foregut movements. Most previous work on the system has concentrated on the frontal ganglion (FG), including research into the role of the FG in feeding as well as molting-related behavior, mostly in locusts, but also in other insect species. The stomatogastric system exerts its physiological actions by way of careful interaction and coordination between its different neural centers and pattern-generating circuits. One such hitherto unstudied neural center is the hypocerebral ganglion (HG), which is connected to the FG via the recurrent nerve. It sends two pairs of nerves along the esophagus and to the posterior region of the crop, terminating in the paired ingluvial ganglia. Very little is known about the neuroanatomy and neurophysiology of the insect HG. Here we investigate, for the first time, the neuronal composition of the locust HG, as well as its motor output. We identify rhythmic patterns endogenous to the isolated HG, demonstrating the presence of a central pattern-generating network. Our findings suggest interactions between the HG and FG rhythm-generating circuits leading to complex physiological actions of both ganglia. This work will serve as a basis for future investigation into the physiology of the HG and its role in insect behavior.     

Maternal effects on progeny body size and color in the desert locust, Schistocerca gregaria: examination of a current view

Hatchling body color and size of the desert locust, Schistocerca gregaria, are determined by the population density of the mothers during their reproductive period. Smaller green hatchlings are produced by adults at low population density (solitarious conditions) and larger dark hatchlings at high population density (gregarious conditions). One claim states that a pheromonal factor secreted by gregarious mothers into foam plugs of egg pods induces darkening in hatchlings. Previous research suggests that the foam factor can be removed by separating eggs individually within 1h of deposition, causing presumptive gregarious eggs to hatch without darkening. The present study re-examined this claim and possible factors that have been proposed which could account for the difference between our results and those reported earlier. Early separation was performed on eggs with a low mortality rate. The results showed that the egg separation did not increase the incidence of green hatchlings. Once chorionated in the ovary, eggs remained unchanged in size until the second day after oviposition in either isolated or crowded locusts. This and other results suggest that the phase-dependent differences in body size and color of hatchlings are established in the ovary and that modifications by the accessory gland factor either in the oviduct or after deposition are unlikely.     

The inhibitory effect of the fungal toxin, destruxin A, on behavioural fever in the desert locust

During an infection locusts behaviourally fever by seeking out higher environmental temperatures. This behaviour places the pathogen at sub-optimal growth temperatures while improving the efficiency of the immune system, thereby prolonging the lifespan of the host. It is therefore in the interest of the pathogen to either adapt to fever-like temperatures or to evolve mechanisms to interfere with, or inhibit fever. We investigated the behavioural fever response of desert locusts to two fungal pathogens. A prolonged fever was observed in locusts infected with Metarhizium acridum. However, fever was comparatively short-lived during infection with Metarhizium robertsii. In both cases restriction of thermoregulation reduced lifespan. Destruxin A (dtx A) produced by M. robertsii, but not M. acridum has previously been associated with the inhibition of the insect immune system. Injection of dtx A during infection with the fever-causing M. acridum inhibited fever and was particularly effective when administered early on in infection. Furthermore, locusts injected with dtx A were more susceptible to M. acridum infection. Therefore engineering M. acridum isolates currently used for locust biocontrol, to express dtx A may improve efficiency of control by interfering with fever.     

Final steps in juvenile hormone biosynthesis in the desert locust, Schistocerca gregaria

Two genes coding for enzymes previously reported to be involved in the final steps of juvenile hormone (JH) biosynthesis in different insect species, were characterised in the desert locust, Schistocerca gregaria. Juvenile hormone acid O-methyltransferase (JHAMT) was previously described to catalyse the conversion of farnesoic acid (FA) and JH acid to their methyl esters, methyl farnesoate (MF) and JH respectively. A second gene, CYP15A1 was reported to encode a cytochrome P450 enzyme responsible for the epoxidation of MF to JH. Additionally, a third gene, FAMeT (originally reported to encode a farnesoic acid methyltransferase) was included in this study. Using q-RT-PCR, all three genes (JHAMT, CYP15A1 and FAMeT) were found to be primarily expressed in the CA of the desert locust, the main biosynthetic tissue of JH. An RNA interference approach was used to verify the orthologous function of these genes in S. gregaria. Knockdown of the three genes in adult animals followed by the radiochemical assay (RCA) for JH biosynthesis and release showed that SgJHAMT and SgCYP15A1 are responsible for synthesis of MF and JH respectively. Our experiments did not show any involvement of SgFAMeT in JH biosynthesis in the desert locust. Effective and selective inhibitors of SgJHAMT and SgCYP15A1 would likely represent selective biorational locust control agents.     

The circadian clock genes affect reproductive capacity in the desert locust Schistocerca gregaria

The circadian clocks govern many metabolic and behavioral processes in an organism. In insects, these clocks and their molecular machinery have been found to influence reproduction in many different ways. Reproductive behavior including courtship, copulation and egg deposition, is under strong influence of the daily rhythm. At the molecular level, the individual clock components also have their role in normal progress of oogenesis and spermatogenesis. In this study on the desert locust Schistocerca gregaria, three circadian clock genes were identified and their expression profiles were determined. High expression was predominantly found in reproductive tissues. Similar daily expression profiles were found for period (per) and timeless (tim), while the clock (clk) mRNA level is higher 12 h before the first per and tim peak. A knockdown of either per or tim resulted in a significant decrease in the progeny produced by dsRNA treated females confirming the role of clock genes in reproduction and providing evidence that both PER and TIM are needed in the ovaries for egg development. Since the knockdown of clk is lethal for the desert locust, its function remains yet to be elucidated.     

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.     

Quantitative studies on anti-allatin and lethal effects of a synthetic precocene in different larval instars of the desert locust

The lethal and anti-allatin effects of various doses of 7-ethoxy-precocene II (PIII), topically applied to crowdedSchistocerca gregaria nymphs, 0–27 h after the previous moult, were investigated for the five nymphal instars. The LD (lethal dose) probit lines were parallel for the different instars. The ED (effective anti-allatin dose) probit lines were also parallel. But for each instar the LD and ED probit lines seemed to be convergent toward the higher doses. The LD₅₀ and the ED₅₀ (g PIII/g fresh weight) were not widely separated; the LD₅₀/ED₅₀ ratio was instar dependent and varied between 1.26 (Vth instar) and 5.35 (IIIrd instar). Thus, separation between lethal effect, anti-allatin effect and no effect was rather poor; indeed in many instances all these types of results were obtained in a group of hoppers treated with the same dose in the same instar. A low precocene susceptibility was observed for the Ist instar and for each subsequent instar the susceptibility further decreased; the LD₅₀ and the ED₅₀ were respectively 886 g/g and 328 g/g for the Ist instar, but increased to 14 307 g/g and 11 367 g/g for the Vth instar. The anti-allatin effect of the precocene induced various morphogenetic responses: (1) permanent adultiform in the instar next to that treated; (2) death in the moult from the next to the second consecutive instar; (3) permanent adultiform in the second consecutive instar. The occurrence and/or distribution of these responses were instar dependent and for the first three instars also limitedly dose dependent. The anti-allatin effect of the precocene applied to Vth (last) instar nymphs became overt by sterility and lack of yellow colour in the adult. An overt anti-allatin effect was always coupled with a response dependent increase of the duration of the instar treated and/or of the next instar.

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Etudes quantitatives des effets létaux ou antiallates d'un précocène de synthèse sur différents stades larvaires du criquet pélerin

Résumé L'étude a porté sur les effets létaux ou antiallates de différentes doses de 7-éthoxy-précocène II (PIII), déposées de 0 à 27 heures après la mue sur la surface dorsale de l'abdomen de larves groupées de chacun des 5 stades larvaires deSchistocerca gregaria. Les courbes des doses létales (LD) ont été parallèles pour les différents stades. Les courbes pour les doses antiallates (ED) étaient elles aussi parallèles. Cependent pour chaque stade, ces deux courbes semblaient converger et même se croiser vers les doses les plus fortes. Les LD₅₀ et ED₅₀ (en g de PIII par gramme de poids frais) n'étaient pas nettement séparées; le rapport LD₅₀/ED₅₀ dépendait du stade et variait entre 1,26 (5è stade) et 5,35 (3è stade). Ainsi la distinction entre effet létal, effet antiallate et absence d'effet était plutôt faible; à la vérité, dans de nombreux cas, tous ces effets ont été obtenus dans un groupe de criquets traités avec la même dose au même stade. Une sensibilité assez faible au précocène a été observée, même au ler stade, et pendant chaque stade suivant la sensibilité ultérieure diminuait; LD₅₀ et ED₅₀ ont été respectivement 886 g/g et 328 g/g pour le ler stade, mais ont augmenté à 14 307 g/g et 11 367 g/g pour le 5è stade. Les effets antiallates ont induit différentes conséquences morphologiques: (1) des adultiformes permanents lors du stade suivant immédiatement le stade traité; (2) mort pendant la mue du ler ou 2nd stade suivant le traitement; (3) des adultiformes permanents dans le 2nd stade après le traitement. L'apparition et/ou la distribution de ces réponses dépendaient du stade, et pour les 3 premiers stades étaient partiellement dépendantes de la dose. L'effet antiallate du précocène au 5è stade (dernier) devenait évident avec la stérilité et la perte de la coloration jaune de l'adulte. Un effet antiallate manifeste était toujours associé à un accroissement d dépendant de la dose de la durée du stade traité et/ou du stade suivant.

     

Effects of glucobrassicin, epiprogoitrin and related breakdown products on locusts feeding: Schouwia purpurea and desert locust relationships

The glucosinolates of a Saharan crucifer Schouwia purpurea (Forskål) (Brassicaceae) were determined by liquid chromatography. Two of these glucosinolates and sinigrin were tested for their deterrent effect on Schistocerca gregaria (Forskål) (Orthoptera: Acrididae). Glucobrassicin, three indolyls and epigoitrin were synthesized for this purpose. Epiprogoitrin was extracted from Crambe seeds. Choice tests on artificial substrate compared feeding responses to glucosinolates and to related breakdown products released when the plant is eaten. Breakdown products were more efficient in deterring the generalist locust than were glucosinolates. Two patterns of dose responses were recorded: glucosidic compounds deterred or stimulated feeding, depending on the concentration tested; aglycones did not stimulate feeding at any concentration. Allyl isothiocyanate, a volatile compound, was a 100-fold higher deterrent than its substrate (sinigrin).