Solitarious Desert Locusts Research Articles

Microbiome-related aspects of locust density-dependent phase transition.

Locust plagues are a notorious, ancient phenomenon. These swarming pests tend to aggregate and perform long migrations, decimating cultivated fields along their path. When population density is low, however, the locusts will express a cryptic, solitary, non-aggregating phenotype that is not considered a pest. Although the transition from the solitary to the gregarious phase has been well studied, associated shifts in the locust's microbiome have yet to be addressed. Here, using 16S rRNA amplicon sequencing, we compared the bacterial composition of solitary desert locusts before and after a phase transition. Our findings revealed that the microbiome is altered during the phase transition, and that a major aspect of this change is the acquisition of Weissella (Firmicutes). Our findings led us to hypothesize that the locust microbiome plays a role in inducing aggregation behaviour, contributing to the formation and maintenance of a swarm. Employing a mathematical model, we demonstrate the potential evolutionary advantage of inducing aggregation under different conditions; specifically, when the aggregation-inducing microbe exhibits a relatively high horizontal transmission rate. This is the first report of a previously unknown and important aspect of locust phase transition, demonstrating that the phase shift includes a shift in the gut and integument bacterial composition.

The potential habitat of desert locusts is contracting: predictions under climate change scenarios.

Desert locusts are notorious for their widespread distribution and strong destructive power. Their influence extends from the vast arid and semiarid regions of western Africa to northwestern India. Large-scale locust outbreaks can have devastating consequences for food security, and their social impact may be long-lasting. Climate change has increased the uncertainty of desert locust outbreaks, and predicting suitable habitats for this species under climate change scenarios will help humans deal with the potential threat of locust outbreaks. By comprehensively considering climate, soil, and terrain variables, the maximum entropy (MaxEnt) model was used to predict the potential habitats of solitary desert locusts in the 2050s and 2070s under the four shared socioeconomic pathways (SSP126, SSP245, SSP370, and SSP585) in the CMIP6 model. The modeling results show that the average area under the curve (AUC) and true skill statistic (TSS) reached 0.908 ± 0.002 and 0.701, respectively, indicating that the MaxEnt model performed extremely well and provided outstanding prediction results. The prediction results indicate that climate change will have an impact on the distribution of the potential habitat of solitary desert locusts. With the increase in radiative forcing overtime, the suitable areas for desert locusts will continue to contract, especially in the 2070s under the SSP585 scenario, and the moderately and highly suitable areas will decrease by 0.88 × 106 km2 and 1.55 × 106 km2, respectively. Although the potentially suitable area for desert locusts is contracting, the future threat posed by the desert locust to agricultural production and food security cannot be underestimated, given the combination of maintained breeding areas, frequent extreme weather events, pressure from population growth, and volatile sociopolitical environments. In conclusion, methods such as monitoring and early warning, financial support, regional cooperation, and scientific prevention and control of desert locust plagues should be further implemented.

Soil moisture from remote sensing to forecast desert locust presence

Abstract Preventive control of desert locusts is based on monitoring recession areas to detect outbreaks. Remote sensing has been increasingly used in the preventive control strategy. Soil moisture is a major ecological driver of desert locust populations but is still missing in the current imagery toolkit for preventive management. By means of statistical analyses, combining field observations of locust presence/absence and soil moisture estimates at 1 km resolution from a disaggregation algorithm, we assess the potential of soil moisture to help preventive management of desert locust. We observe that a soil moisture dynamics increase of above 0.09 cm3/cm3 for 20 days followed by a decrease of soil moisture may increase the chance to observe locusts 70 days later. We estimate the gains in early warning timing compared to using imagery from vegetation to be 3 weeks. We demonstrate that forecasting errors may be reduced by the combination of several types of indicators such as soil moisture and vegetation index in a common statistical model forecasting locust presence. Policy implications. Soil moisture estimates at 1 km resolution should be used to plan desert locust surveys in preventive management. When soil moisture increases in a dry area of potential habitat for the desert locust, field surveys should be conducted two months later to evaluate the need of further preventive actions. Remote sensing estimates of soil moisture could also be used for other applications of integrated pest management.

Sexual Behavior of the Desert Locust During Intra- and Inter-Phase Interactions

Mating and reproduction behaviors and strategies are fundamental aspects of an organism’s evolutionary and ecological success. In locusts, intra- as well as inter-phase reproductive interactions among gregarious and solitarious locust populations have a major impact on the locust population dynamics. However, practically all previous work on locust sexual behavior has been limited to the gregarious phase. Here we provide a first detailed description of pre-copulatory behavior of solitarious desert locusts. We compare our findings with those of previous reports of pre-copulatory behavior of gregarious locusts, focusing on the behavioral elements that serve in inter-sex signaling and communication. We also studied inter-phase (mixed pairs) reproductive interactions. Solitarious males were found to invest more in pre-copulatory courtship and signaling compared to their gregarious counterparts; and solitarious females played a more dominant role in the inter-sex communication than gregarious females. The solitarious females were also less likely to demonstrate rejection-related behavioral patterns than gregarious females. As a consequence of the particular characteristic behavior of each phase, the most successful among intra- and inter-phase pairs were gregarious males with solitary females. Least successful were solitary males paired with gregarious females, indicating a strong asymmetry in inter-phase reproductive interactions.

Discontinuous gas-exchange cycle characteristics are differentially affected by hydration state and energy metabolism in gregarious and solitarious desert locusts

The termination of discontinuous gas exchange cycles (DGCs) in severely dehydrated insects casts doubt on the generality of the hygric hypothesis, which posits that DGCs evolved as a water conservation mechanism. We followed DGC characteristics in the two density-dependent phases of the desert locust Schistocerca gregaria throughout exposure to an experimental treatment of combined dehydration and starvation stress, and subsequent rehydration. We hypothesized that, under stressful conditions, the more stress-resistant gregarious locusts would maintain DGCs longer than solitary locusts. However, we found no phase-specific variations in body water content, water loss rates (total and respiratory) or timing of stress-induced abolishment of DGCs. Likewise, locusts of both phases re-employed DGCs after ingesting comparable volumes of water when rehydrated. Despite comparable water management performances, the effect of exposure to stressful experimental conditions on DGC characteristics varied significantly between gregarious and solitary locusts. Interburst duration, which is affected by the ability to buffer CO2, was significantly reduced in dehydrated solitary locusts compared with gregarious locusts. Moreover, despite similar rehydration levels, only gregarious locusts recovered their initial CO2 accumulation capacity, indicating that cycle characteristics are affected by factors other than haemolymph volume. Haemolymph protein measurements and calculated respiratory exchange ratios suggest that catabolism of haemolymph proteins may contribute to a reduced haemolymph buffering capacity, and thus a compromised ability for CO2 accumulation, in solitary locusts. Nevertheless, DGC was lost at similar hydration states in the two phases, suggesting that DGCs are terminated as a result of inadequate oxygen supply to the tissues.

Demographic processes shaping genetic variation of the solitarious phase of the desert locust

Between plagues, the solitarious desert locust (Schistocerca gregaria) is generally thought to exist as small populations, which are particularly prone to extinction events in arid regions of Africa and Asia. Given the high genetic structuring observed in one geographical area (the Eritrean coast) by former authors, a metapopulation dynamics model involving repeated extinction and colonization events was favoured. In this study, we assessed the validity of a demographic scenario involving temporary populations of the solitarious phase of the desert locust by analysing large-scale population genetic data. We scored 24 microsatellites in 23 solitarious population samples collected over most of the species range during remission. We found very little genetic structuring and little evidence of declining genetic diversity. A Bayesian clustering method distinguished four genetically differentiated units. Three groups were largely consistent with three population samples which had undergone recent bottleneck events. Nevertheless, the last genetically homogeneous unit included all individuals from the remaining 18 population samples and did not show evidence of demographic disequilibrium. An approximate Bayesian computation treatment indicated a large population size for this main genetic group, moderately reduced between plague and remission but still containing tens of thousands of individuals. Our results diverge from the hypothesis of a classical metapopulation dynamics model. They instead support the scenario in which large populations persist in the solitarious phase of the desert locust.

Microarray-Based Transcriptomic Analysis of Differences between Long-Term Gregarious and Solitarious Desert Locusts

Desert locusts (Schistocerca gregaria) show an extreme form of phenotypic plasticity and can transform between a cryptic solitarious phase and a swarming gregarious phase. The two phases differ extensively in behavior, morphology and physiology but very little is known about the molecular basis of these differences. We used our recently generated Expressed Sequence Tag (EST) database derived from S. gregaria central nervous system (CNS) to design oligonucleotide microarrays and compare the expression of thousands of genes in the CNS of long-term gregarious and solitarious adult desert locusts. This identified 214 differentially expressed genes, of which 40% have been annotated to date. These include genes encoding proteins that are associated with CNS development and modeling, sensory perception, stress response and resistance, and fundamental cellular processes. Our microarray analysis has identified genes whose altered expression may enable locusts of either phase to deal with the different challenges they face. Genes for heat shock proteins and proteins which confer protection from infection were upregulated in gregarious locusts, which may allow them to respond to acute physiological challenges. By contrast the longer-lived solitarious locusts appear to be more strongly protected from the slowly accumulating effects of ageing by an upregulation of genes related to anti-oxidant systems, detoxification and anabolic renewal. Gregarious locusts also had a greater abundance of transcripts for proteins involved in sensory processing and in nervous system development and plasticity. Gregarious locusts live in a more complex sensory environment than solitarious locusts and may require a greater turnover of proteins involved in sensory transduction, and possibly greater neuronal plasticity.

Diel Behavioral Activity Patterns in Adult Solitarious Desert Locust,Schistocerca gregaria(Forskål)

The responses of adult solitarious desert locust to odors from a host plant were evaluated in a two-choice wind tunnel. Solitarious desert locusts collected from the field (Red Sea Coast) were more attracted to volatiles from pottedHeliotropium ovalifoliumin scotophase than in photophase. The attraction towards the host plant odors rather than to clean air, in both photophase and scotophase, concurs with previous observations on oviposition preferences near these plants. Diel behavioral activity patterns of adult solitarious desert locustsSchistocerca gregaria(Forskål) that were collected from the field in Port Sudan were investigated by monitoring, scanning, resting, taking off, and walking/running in a wind tunnel. Solitarious locusts that had been propagated in the laboratory for 20 generations were also observed for comparison. In both groups of locusts, insects were significantly more active after sunset and this activity attained peak level at 1-2 hours after dusk. Of the two groups, solitarious locusts collected from the field were significantly more active. In the scotophase, the former traversed distances that were about seven times those covered by laboratory-reared locusts. Overall, the results show that the repertoire of behavioral activities of solitarious locusts is maintained in laboratory-reared insects, albeit at a lower level. The implications of these observations in the behavioral ecology of the desert locust are discussed.

Neuroparsin transcripts as molecular markers in the process of desert locust ( Schistocerca gregaria) phase transition

Desert locust swarms occasionally cause severe ecological and economic damage, particularly in countries of northwest Africa. However, the physiological mechanisms underlying locust phase transition, the switch of the solitarious to the gregarious phase, remain elusive. Therefore, identification of molecular changes linked to this phenomenon represents a primary requirement to start unraveling this enigma. The present paper provides novel information on phase-related molecular markers for locust phase transition. We present a detailed quantitative real-time RT-PCR analysis of two distinct neuroparsin precursor transcripts ( Scg-NPP3 and Scg-NPP4) in the brain and in abdominal tissues of gregarious and solitarious desert locusts ( Schistocerca gregaria). Our data reveal different temporal changes of these transcripts in the fat body during the adult stage of both phases. We, hereby, present novel scientific evidence for a phase-dependent regulation of these particular peptide hormone encoding transcripts and assign them as possible molecular markers in the process of locust phase transition.

Plant communities can predict the distribution of solitarious desert locust Schistocerca gregaria

Summary The desert locust is a migratory pest whose population development in remote areas must be monitored to prevent outbreaks, upsurges and plagues. Monitoring would be very much facilitated if the area of search could be restricted to sites of likely population increase. The spatial distribution of solitarious desert locusts on the Red Sea coastal plain of Sudan was determined over 3 years from November to March. Additional observations were made on habitat factors, such as plant community, soil texture, soil moisture and land use. Locust densities varied according to the amount and distribution of rainfall and longevity of the annual green vegetation, with virtually no locusts being observed in the driest season. Samples on a grid of 120 sites within a 120‐km stretch of coastal plain showed that locusts were prevalent only in the millet–Heliotropium plant community, which is found at sites with a fine sandy soil texture and comparatively high and long‐lasting soil moisture in wadi deltas. These sites constitute less than 5% of the area of this part of the plain. An accessory study showed association between desert locust distribution and millet cropping in an area where no Heliotropium was found. Other samples confirmed the association between solitarious desert locust and millet agriculture. Synthesis and applications. The results indicate that surveys for early detection and control of desert locust on the Red Sea coast of Sudan can focus on millet cropping areas. The results suggest that the efficiency of monitoring migratory pest outbreaks in remote areas could be enhanced by using associations between plant communities and herbivorous insects to predict risk areas and target survey efforts.

Quantitative real‐time RT‐PCR analysis in desert locusts reveals phase dependent differences in neuroparsin transcript levels

In different parts of the world, locust swarms cause severe ecological and economic damage. However, the physiological mechanisms underlying this gregarization process remain elusive. In this study, we present a detailed quantitative analysis of two neuroparsin precursor (Scg-NPP1 and Scg-NPP2) transcripts in the brain, fat body, gut, gonads and accessory glands of male and female, gregarious and solitarious desert locusts (Schistocerca gregaria). These transcripts are generally more abundant in solitarious than in gregarious animals. In contrast to their gregarious congeners, solitarious locusts contain detectable Scg-NPP1 and Scg-NPP2 transcript levels in the fat body. Moreover, our data reveal temporal changes of neuroparsin mRNA levels in the brains and fat bodies of adult isolated-reared locusts. This paper provides the first scientific evidence for phase-dependent transcriptional regulation of neuropeptide hormone encoding genes.

Developmental changes in the structure and function of the central olfactory system in gregarious and solitary desert locusts

Desert locusts are guided by olfactory cues in different behavioural contexts. In order to understand the basis for the variable olfactory guided behaviour displayed by different developmental stages and by solitary and gregarious locusts, we investigated their central olfactory system with neuroanatomical and neurophysiological methods. The primary olfactory centre of the brain, the antennal lobe (AL), increases in size during development due to an increased number and size of glomeruli. These glomeruli are innervated by a constant number of projection neurons that display increased dendritic arborizations during the development of the locust. The anatomical parameters do not differ between gregarious and solitary locusts. In parallel with the observed neuroanatomical changes, neurophysiological changes in response spectra and response specificity of AL neurons were found. During development, the percentage of neurons responding specifically to aggregation pheromone components decreases, whereas an increase in both pheromone-generalists and plant-pheromone generalist neurons is observed. The percentage of neurons responding to green leaf volatiles, however, remains constant. A decrease in the number of nymph blend-specific neurons was also observed. Our data show that anatomical and physiological properties of the AL and its neurons to a large extent reflect the changes in olfactory guided behaviour during development and between phases. The majority of our results are also in accordance with findings that the number of olfactory receptor neurons increases during development, resulting in increasing convergence on AL neurons.

Role of Olfactory and Visual Cues in the Attraction/Repulsion Responses to Conspecifics by Gregarious and Solitarious Desert Locusts

Desert locusts [Schistocerca gregaria Forskal (Orthoptera, Acrididae)] change phase in response to population density: solitarious insects avoid one another, but when crowded they change to the gregarious phase and aggregate. The attraction/repulsion responses of gregarious and solitarious locusts maintain phase differences in locust populations. Despite considerable research, the cues for aggregation are poorly understood; moreover, the repulsion response of solitarious locusts has not previously been investigated. This study analyzes the role of visual and olfactory stimuli in triggering these different responses to conspecifics. Isolation-reared insects were repelled by both olfactory and visual stimuli from other locusts. Crowd-reared insects were attracted by the combination of olfactory and visual cues. In addition, olfactory stimuli affected other behaviors in both phases, and behavioral differences between isolation- and crowd-reared locusts were clear even in the absence of conspecifics. The sensory and neurological mechanisms underlying these responses are not well understood and will form the basis for neurobiological investigations of locust phase.

What defense doesSchouwia purpurea (Cruciferae) have against the desert locust?

A field study of the food eaten by solitary desert locusts was carried out in a winter breeding area in Mauritania. The food eaten, determined by the plant epidermis found in fecal pellets, was compared to the plant's availability in the habitat.Schouwia purpurea, well represented in the diets, was dominant at the study site. Adults had a preference forTribulus terrester. Growth and feeding on these two plants were compared. The high water content ofS. purpurea leaves limited the dry matter eaten and slowed down growth. Glucosinolates were separated and quantified by gas chromatography. There are 132μmol/g dry matter in green leaves. In multiple choice tests, with paper disks, glucosinolate extracts were phagostimulant at a low concentration (21μmol/g dry matter) and repulsive at a higher one (214μmol/ g dry matter). Biting behavior onS. purpurea was recorded and analyzed on video. The importance ofSchouwia purpurea in desert locust habitats and its defenses is discussed.

Theoretical rates of increase of gregarious and solitarious populations of the Desert Locust.

Leslie matrices are used to compare theoretical populations of gregarious and solitarious Desert Locusts, Schistocerca gregaria (Forsk.). Despite their lower fecundity, the synchrony and faster maturation of gregarious populations permit them to have much faster rates of increase than solitarious populations. When realistic mortality estimates are assumed the differences can be very pronounced even when the rates of mortality are the same for both phases; this suggests that the longer period during which solitarious locusts are susceptible to predation is critical. The conclusions are briefly discussed with respect to the genesis and maintenance of locust plagues and the evolutionary significance of gregarisation.

The Outbreak Centres of Schistocerca gregaria, Forsk., on the Red Sea Coast of the Sudan

1. An ecological description of the Red Sea coast of the Anglo-Egyptian Sudan is given, with special reference to the occurrence of the solitary phase of Schistocerca gregaria in its different parts and zones.2. The greatest numbers of solitary Desert locusts were found amongst raingrown dukhn (Pennisetum typhoideum, Rich.) cultivated in the Suaeda belt of the coast, near the sea.3. The factors responsible for the higher population of solitary locusts among the cultivation are probably the soil consisting of soft loose sand, and the more or less evergreen condition of the weeds on cultivated plots. It appears possible, however, that in some localities with naturally loose sandy soil all areas suitable for vegetation are cultivated, and in this case the cultivation in itself is not of primary importance.4. Incipient outbreaks were observed in the type of cultivated areas described above in 1933–34, and again in 1934–35 winter seasons. Hoppers of the phase congregans were found mostly on a weed, Heliotropium undulatum, which is preferred by young hoppers to other food-plants, and seemed to act as a concentrating factor for the hoppers. No hoppers of the ph. congregans were found on clean cultivation where this weed was absent.5. The chief factor determining the commencement of an outbreak seems to be heavy winter rains (over 240 mm. during the season), starting early (end of October) and continuing late (end of February), which allows for the development of two successive winter generations, as follows:—(a) Early maturation of old adults (November); (b) plentiful oviposition of old adults (November-December); (c) hoppers beginning to emerge (end of November); (d) hoppers beginning to become adult (early January); (e) majority of hoppers adult (end January); (f) these young adults remaining among dukhn in a fairly crowded condition (February); (g) young adults maturing (end February); (h) young adults ovipositing (March); (i) hoppers hatching (end of March-April); (j) young hoppers crowded on Heliotropium, assuming congregans coloration and later forming marching bands (April); (k) emigration of young flying swarms (May).6. The exact cause of the rapid maturation of young adults of the first winter generation has not been discovered, although a short period of very high humidity seems to have some effect. There is no doubt that late rains are necessary for most adults to mature and oviposit in the field.7. The Red Sea coast of the Sudan is distinguished from the interior of that country by its higher air humidity, by winter rains, and by the fact that the rainy season can extend for a longer time than the summer rainy period inland. The northern parts of the coast, north of the latitude 20°N., are less likely to produce outbreaks only because heavy and prolonged rains are unlikely to occur there.8. General conditions in Eritrea, south of the Sudanese border, are such that some outbreak areas should be expected to exist there.9. It appears probable that the Arabian coast of the Red Sea also contains some outbreak areas. As regards inland parts of Arabia, their climate, the distribution and the life-cycle of the solitary locusts there, are too little known to estimate the possibility of the existence of outbreak areas, until a special investigation is made.10. The results of the investigations on the Red Sea coast put under suspicion of harbouring outbreak areas of the Desert locust all those parts of the sea-coasts of Africa and Asia which may obtain in some years winter rainfall of not less than 240 mm. extending for a period sufficiently long to allow for the development of two successive winter generations of the locust.