Locust Phase Research Articles

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.

Haemolymph pigment composition as a chemometric indicator of phase in the desert locust, Schistocerca gregaria

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.

Juvenile hormone and locust phase

Juvenile hormone (JH) is directly and indirectly involved in the determination of many phase characteristics of locusts. Temporal differences in JH titer and the biosynthetic capacity of excised corpora allata of solitary nymphs are correlated to some extent with the effects on them of treatment with JH or JH analogs. Juvenile hormone modulates cuticular melanization and the rate of reproductive maturation—specifically regulating vitellogenesis at the transcriptional level, and nonspecifically stimulating the translational capacity of the locust fat body. Juvenile hormone does not appear to be involved in behavioral phase transition of locusts. Long-term treatment of crowded nymphs with the JH analog methoprene does not lessen their gregarious behavior, and does not reduce hemolymph lipids or carbohydrates. Reducing endogenous JH levels of solitary nymphs by chemical allatectomy with precocene III does not induce gregarious behavior. However, methoprene does affect nymphal coloration. The relevance of these results to locust control is discussed. Arch. Insect Biochem. Physiol. 35:375–391, 1997. © 1997 Wiley-Liss, Inc.

Proprioceptive feedback in locust kicking and jumping during maturation.

Campaniform sensilla monitor the forces generated by the leg muscles during the co-contraction phase of locust (Schistocerca gregaria) kicking and jumping and re-excite the fast extensor (FETi) and flexor tibiae motor neurones, which innervate the leg muscles. Sensory signals from a campaniform sensillum on the proximal tibia were compared in newly moulted locusts, which do not kick and jump, and mature locusts which readily kick and jump. The activity pattern of FETi during co-contraction was mimicked by stimulating the extensor tibiae muscle. Less force was generated and the spike frequency of the sensory neurone from the sensillum was significantly lower in newly moulted compared to mature locusts. Depolarisation of both FETi and flexor motor neurones as a result of sensory feedback was consequently less in newly moulted than in mature locusts. The difference in the depolarisation was greater than the decrease in the afferent spike frequency suggesting that the central connections of the afferents are modulated. The depolarisation could generate spikes in FETi and maintain flexor spikes in mature but not in newly moulted locusts. This indicates that feedback from the anterior campaniform sensillum comprises a significant component of the drive to both FETi and flexor activity during co-contraction in mature animals and that the changes in this feedback contribute to the developmental change in behaviour.

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.

Lipophorin and apolipophorin-III in solitary and gregarious phases of Schistocerca gregaria

Lipophorin was isolated from the hemolymph by KBr density gradient ultracentrifugation. Apolipophorin-III (apoLp-III) was isolated from low-density lipophorin (LDLp). High-density lipophorin (HDLp) ( M r ∼-620,000) consisted of two apoproteins, apoLp-I (M r ∼-224,000) and apoLp-II ( M r ∼-81,000). Isolated apoLp-III ( M r ∼-20,000) had two isoforms in non-denaturing gels. Injection of adipokinetic hormone-I resulted in the formation of LDLp in both locust phases. Lipid content of HDLp was estimated at 48.4% and 51.8% for solitary and gregarious locusts, respectively. In LDLp, lipids constituted 59.8% and 57.1% for gregarious and solitary locusts, respectively. The results suggest that the gregarious locusts were more efficient in the conversion of HDLp to LDLp.

A neuropeptide controlling the dark pigmentation in color polymorphism of the migratory locust, Locusta migratoria

The neuroendocrine control of the dark body color in hoppers of Locusta migratoria was studied using an albino strain as a bioassay system. Methanol or saline extracts of corpora cardiaca originating from 5th instar hoppers of a normal (non-albino) strain were dried, dissolved in 10% aqueous ethanol or mixed with peanut oil, and injected into albino hoppers. No dark color was induced by samples of the methanol extracts dissolved in 10% aqueous ethanol. However, samples of the extracts mixed with peanut oil induced dose-dependent darkening in the albino hoppers within a few days after injection; the higher the dose, the darker the color induced. Methanol extracts dissolved in saline and heated at 95°C for 10 min retained the dark-color inducing effect after being redried, mixed with peanut oil and injected, but those incubated with a proteinase for 2 h lost it completely. Therefore, this dark-color inducing factor seems to be a heat-stable neuropeptide. Organ implantation into albino hoppers revealed that the corpora cardiaca and brain of normal solitary hoppers contained this neurohormone as did those of normal gregarious hoppers. Injection of a high dose (2 pairs of corpora cardiaca equivalent) of extract mixed with peanut oil, or daily repeated injections on the first 4 days of the 4th instar, induced very dark color after the following ecdysis, but black patterns characteristic of gregarious hoppers were not induced. However, some albino hoppers developed such patterns after receiving 3 daily repeated injections of 1 pair of corpora cardiaca equivalent during the second half of the 4th instar. The role of this neurohormone in locust polychromatism and in locust phase polymorphism is discussed.

Sex differentiation studies relating to releaser aggregation pheromones of the desert locust, Schistocerca gregaria

AbstractBehavioural responses of nymphs and adults in the gregarious phase of the desert locust, Schistocerca gregaria (Forskal) (Orthoptera: Acrididae) were investigated in a single‐chamber bioassay system to a choice of two columns of air, one permeated with airborne volatiles emanating from either sex of nymphs or adults and the other untreated. There was no sexual differentiation in the production of or response to nymphal volatiles. Young adults of either sex did not produce a stimulus with significant activity. Of the older adults, only the males produced the aggregation stimulus to which both sexes were equally responsive. Charcoal‐trapped volatiles from the two sexes of nymphs and adults evoked similar aggregation responses. Antennae of the older adults showed significantly higher EAG responses than those of fifth instar nymphs to all four volatile collections, of which volatiles from older adult males were the most stimulatory and evoked the highest EAG amplitudes.

Analysis of phase-related changes in behaviour of desert locust nymphs

An assay is developed in which the behavioural responses of an individual locust to a group of conspecifics are used to derive an index representing the ‘behavioural phase status’ of that insect. The index is generated from logistic regression models based on 11 behavioural parameters. Data are presented for locusts reared under crowded conditions for many generations and in isolation for one, two and three generations. Crowd-reared locusts differ in behaviour from insects reared in isolation in several ways. They tend to be attracted by a group of locusts, whereas insects reared in isolation are repelled. Solitaryreared locusts also show a characteristic set of behavioural responses which are all consistent with a cryptic lifestyle. Such responses become more evident with increasing number of generations of rearing in isolation. These data represent the most integrated behavioural analysis of locust phase differences yet undertaken. The procedure will be used in future work to quantify the dynamics of gregarization in congregated solitarious locusts. This will provide a much-needed basis for studying underlying controlling mechanisms, the nature of which is at present unclear.

Evaluation of Juvenoid Insect Growth Regulators on Schistocerca Americana

Three aromatic juvenoids were evaluated for morphogenetic activity on Schistocerca americana . Fenoxycarb, the most active insect growth regulator examined, was found to induce a significant mortality in developing fifth stage nymphs and caused severe morphogenetic damage during ecdysis to the adult. Juvenoid treatments were also observed to increase the duration of the last nymphal stage and to induce chromotropic colour changes similar to that observed in the solitary phase of the desert locust, Schistocerca gregaria Forskal.

Phase polymorphism in Schistocerca gregaria: Reproductive parameters

Solitarious phase Schistocerca gregaria were selected according to the criterion of the occurrence of an extra stadium during larval development. The proportion of locusts undergoing an extra ecdysis increased within one generation of rearing under isolated conditions but it failed to reach 100% throughout an observation period encompassing six generations. A higher proportion of female locusts exhibited an extra stadium indicating sexual dimorphism in this phase character. The number of eggs per pod produced by solitarious females was larger than their gregarious counterparts. This was shown to be due to an increase in the number of ovarioles and a relative decrease in the proportion of non-functional oöcytes in solitary females, confirming earlier work. The inverse relationship between ovariole numbers and oöcyte size characterizing locust phase polymorphism was examined. Vitellin content of ovaries containing mature oöcytes was similar in gregarious and solitarious females. However, vitellin content per oöcyte was less in the latter suggesting that the increase in ovariole number may occur at the expense of oöcyte size and vitellin content.

Locust phase colouration

Locust phase colouration

PHASE AND MOULTING POLYMORPHISM IN LOCUSTS

According to the Phase Theory formulated by Uvarov (1921, 1928), locusts exhibit a graduated type of densitydependent dimorphism. So marked is this dimorphism that the phases of most common locusts were once accorded specific rank. Phase polymorphism is characterized by two limiting forms linked by a whole range of intermediates; the form found in bands or swarms is known as phase gregaria, and the sedentary isolated form as phase solitaria: intermediate or transiens forms are designated by the terms congregans and dissocians, according to the direction of the phase-change. The ideas presented in this essay stem from different kinds of morphometric analysis of the locusts. First, the occurrence of an extra moult in some individuals is found to be part of a twin homeostatic growth-regulating process during ontogeny. The polymorphism that ensues is stochastically related to the density of the parental population. This density-dependent quality, though inherited, is not genically determined. Careful study of the wing-pads of the developing nymphs distinguishes insects whose innately determined phase status is not in equilibrium with the density of the larval population of which they are part. Congregans and dissocians nymphs form two separate categories of this transiens phase, illustrating at once the dynamic conception of phase polymorphism and the existence of hysteresis in cycles of densitydependent changes of shape in locusts. New methods for the quantitative study of form in locusts show that the generic differences between Red and Desert Locusts are independent of their phase polymorphism and of their normal growth. Finally, a necessarily speculative discussion of the role of polymorphism in the life-cycle of locusts is attempted.

Biochemical Differences between the solitary and gregarious Phases of Locusts and Noctuids

The theory of phases, Uvarov (1921), had the object of explaining the interrelationship between different forms ofLocusta migratoriaandLocustana pardalina. This theory has been accepted by most entomologists working on locust problems. Authors such as Faure (1923 and 1932), Johnston and Maxwell-Darling (1931) and later a series of others including Hussein and Ramchandra Rao, proved and confirmed the truth of Uvarov's theory.

On the Ecology of Acrididae near Lake Chad

1. The shore of Lake Chad was suspected of being a reservation of both the Migratory Locust and the Red Lcoust. In 1931 a preliminary survey of the shore was made, as a result of which it was tentatively concluded that about one-half or less of the shore of British Chad may offer conditions suitable for the production of swarms of the Migratory Locust from the solitary phase. The Red Locust was present only in the southern half of the shore. A village named Kalkala, at the south-western corner of the Lake, was selected as the most suitable locality in which to carry out ecological research.2. The ecology of the two species of locusts and of about 60 other species of ACRIDIDAE was studied at Kalkala during the first six months of 1933. A description is given of agricultural and climatic conditions in the Kalkala area and some notes on the hydrography of Lake Chad are included. The vegetation, soils, fauna and microclimates of the habitats frequented by ACRIDIDAE are described.3. Collections of Acrididae were made each month in all the principal habitats. It was found that the Acridinae occurred principally in the wettest habitats and that this subfamily exhibited a small degree of ecological plasticity. The Oedipodinae consisted chiefly of geophilous species which were most abundant on bare soil in farm-land. The Pyrgomorphinae were most abundant in two non-gramineous habitats, but were not uncommon in Cynodon dactylon; one species was associated with Calotropis procera. The Catantopinae showed a greater degree of ecological plasticity than did any of the other subfamilies; the majority of the species encountered were most abundant in the drier habitats.4. Bionomical notes are given for the majority of the species of Acrididae met with in 1931 and 1933 in the Chad area. Evidence was obtained that many species of Catantopinae have a prolonged imaginal diapause. Certain species of Acridinae and Catantopinae are characterised by an embryonic diapause during the hottest months of the year, and one species of Pyrgomorphinae has a nymphal diapause during the early part of the dry season.5. The Red Locust frequents tall grass habitats from November to the end of April, the adults then gradually move to short grass and farm-land and begin to breed in late June. The imaginal diapause lasts from early October to late July. Biometrical examinations of adults and other data indicate that the Red Locust is in phase transiens (congregans). There is no evidence that swarms have left the Chad area. It seems probable that conditions in this area are unsuitable for the production of phase gregaria.6. The solitary phase of the Migratory Locust was present in the survey area throughout the six months; it was never numerous and decreased steadily in numbers from January to June in spite of the fact that it was breeding from late February to June. The decrease in numbers is thought to have been due, chiefly, to the attacks of the Carmine Bee-eater and to bush fires.Phase transiens (dissocians) was found until mid-March; there is no doubt that most of the locusts present were derived from the survivors of hopper bands which had been baited or trenched in the autumn of 1932. Swarms arrived in late May and early June and repopulated the area. The principal habitats of the Migratory Locust were Cynodon dactylon, Brachiaria ramosa and Chloris gayana. The scarcity of this species in the southern half of Chad indicated that this area is unlikely to be a reservation; but research over a number of years is necessary to confirm this opinion.

Observations on Phases of the Red-Winged Locust in Northern Rhodesia

Summary1. The egg, vermiform larva, and the nymphal stages of ph. gregaria are described. It is shown that six stages occur, or seven instars including the vermiform larva. These stages can be distinguished by (a) the number of antennal joints, (b) the degree of backward prolongation of the pronotum, (c) the differentiation of the terminal abdominal structures. Measurements of the pronotum and femur served to distinguish the stages in material from the same swarm, but the absolute measurements cannot be used to distinguish material from different sources.2. The colour pattern of ph. dissocians is analysed, and colour changes are indicated as divergences from the gregaria pattern. Forms of ph. dissocians observed in the field are grouped into several types the relationships of which are illustrated, and the environmental factors which produce them are discussed.3. Hoppers believed to be of ph. congregans could not be distinguished from certain forms of ph. dissocians.4. Cage experiments in which hoppers were reared under varying degrees of crowding are described. It is strongly indicated that activity which is induced by crowding influences the development of gregaria colouring. The green colouring of dissocians hoppers appears to be associated with high humidity conditions and the presence of green food. Other colours of dissocians hoppers are conditioned by those of the environment.5. An aberrant pallid type of hopper is described and its origin is ascribed to the effects of parasitism by Nematodes and possibly also by Dipterous larvae.6. Certain inferences regarding the processes in the formation of the colour pattern are inferred from a consideration of the effects of parasitism and of dissocians producing factors.7. The colourings of young adults of ph. gregaria a few days after the ultimate moult and of types taken from large swarms in August are described. It is shown that colour changes occur gradually in the field, the conspicuous pattern of light and dark coloration of the young adult being obscured and finally hidden by the development of a bright red pigmentation.8. The colouring of young adults of ph. dissocians is described. These show a pattern of light and dark coloration similar to that of the young adult of ph. gregaria, from which it is indistinguishable, immediately after the ultimate moult. In ph. transiens no marked colour change takes place throughout the life of the individual except the development of a purple coloration on the hind wings.9. Factors influencing adult colouring are discussed. Adults crowded in cages and derived from hoppers reared under crowded conditions did not undergo the colour changes observed in the field. Material taken from large swarms in the field and showing some development of swarm colouring were maintained under crowded and uncrowded conditions. The red pigment of the body changed to a dark brown in both cases. Under uncrowded conditions no further change took place except a conspicuous development of purple coloration on the hind wings in all individuals. Under crowded conditions the brown pigment spread, its final distribution resembling that of the red pigment in later swarm types from the field, but the red colouring of the hind wings was developed only to a very slight extent in a few individuals. In the case of individuals isolated completely the swarm colouring was lost entirely.10. It is inferred that swarm colouring in the adult is associated with crowding and possibly with activity and that development of the red and purple coloration on the hind wings in gregaria and transiens is affected by different factors and possibly differs in composition.11. Biometrical data obtained from a variety of types is tabulated and discussed. Material varied from types taken from very large and dense swarms to what might be described as “solitaria beginning to congregate.”12. It is shown that certain measurable characters, especially the relative size of the sexes, the degree of development of the femur in relation to the wing, and the relative length and degree of constriction of the pronotum differ greatly in gregaria and transiens types and may be used to distinguish them.13. In the case of material from very large and dense swarms and from very small loose swarms no differences in the characters on which the types could readily be distinguished were observed. It may be that these small groups were derived from larger swarms.14. Interesting aberrations occur in the case of a swarm believed to have been one of the earliest to migrate any considerable distance after the commencement of swarming. The quantity of material was insufficient to permit of very reliable comparison but it would appear that in these forms the pronotum of the female was generally not shortened or constricted to the same extent as in extreme gregaria, although the construction of the pronotum of the male approximated fairly closely to that of more extreme gregaria

The Phases of African Locusts

The Phases of African Locusts

The Phases of Locusts in South Africa

Adding together all the hoppers listed above, we find that 637 individuals were reared in these experiments from 34 different pairs of adults; 394 were reared in complete isolation, and 243 under crowded conditions, but each crowd consisted of the progeny of one pair only. The majority of those isolated were placed in isolation on the day of hatching, but some first lived in a crowd for a week or two before being used to replace isolated individuals that had died.Comparing the results in Tables XXI and XXII, we noted that the isolated hoppers acquired thesolitariacharacters, while those crowded tended to develop thegregariacharacters, regardless of whether the parents had beengregariaorsolitaria. Only two isolated hoppers from the pair L.p.352 showedgregariacharacters, and one of these had lived in a crowd and had acquiredgregariacharacters before it was placed in isolation in its second instar. Table XXIII shows that crosses betweengregariaandsolitariagave the same results.In the second generation (Table XXIV) the progeny of the P1gregariapair 352 reacted to isolation and crowding in the same way as all other hoppers, and the same was true of the F2back-cross togregaria(L.p.569).Examining the hypothesis of Potgieter in the light of these experiments, we find that the data presented in Tables XXI to XXIV are in direct contradiction with the three phases enumerated above. Instead of giving a majority ofgregaria, the matings ofgregariabygregariaproduced bothgregariaandsolitariaaccording to the conditions under which the progeny were reared. According to the second thesis, the crossing ofsolitariaandgregariashould always result in a majority ofgregaria, but Table XXIII shows that 90 isolated hoppers all becamesolitaria. Thirdly, the mating ofsolitariawithsolitariais supposed never to producegregariaprogeny; Table XXII shows that the progeny of these crosses developedgregariacharacters when crowded andsolitariawhen isolated.

The Locust Problem

THE theory of the phases of locusts, advanced by B. P. Uvarov in 1921, is now well known to entomologists and has proved a fertile stimulus to further investigation of this important problem. It recognised the existence among these insects of two definite or extreme forms—one gregarious and the other solitary—which are connected by a continuous series of less defined transitional forms. Messrs. B. P. Uvarov and B. N. Zolotarevsky1 have recently discussed certain aspects of the problem, in the light of new observations made by S. A. Predtechensky in Russia, and by the junior author in Madagascar. Although their remarks apply more especially to the well-known species Locusta migratoria, these authors believe that a standard phase nomenclature, applicable to all species, would be both possible and advantageous. According to their interpretation a locust can exist in three unstable biological phases, namely, a solitary one, phasis solitaria: a gregarious one, phasis gregaria, and a transitional phase between these two which they term phasis transiens. These phases differ from each other in morphological and colour characteristics, on one hand, and in biological features (mainly behaviour) on the other. Whether it will prove possible to distinguish such phases solely by the con- venient method of examining their morphological characters, can only be determined by studying the whole series of phases of a given locust in a specific locality.

Phases of Locusts and their Interrelations

The theory of phases of locusts put forward as a working hypothesis by the senior author (Uvarov, 1921) has led, as might have been expected, to further researches in that direction, but the results obtained have been in some cases apparently contradictory. A summary of the work done on these phases has been published recently in a book (Uvarov, 1928), and we do not propose to repeat it here, the purpose of the present joint paper being to discuss very briefly certain aspects of the problem in the light of new observations made by Predtechensky (1928) in Russia and by the junior author in Madagascar.