To be or not to be a locust? A comparative analysis of behavioral phase change in nymphs of Schistocerca americana and S. gregaria

Phenotypic plasticity in behavior induced by high rearing density is often part of a migratory syndrome in insects called phase polyphenism. Among locust species, swarming and the expression of phase polyphenism are highly correlated. The american grasshopper, Schistocerca americana, rarely swarms even though it is closely related to the swarming Old World desert locust, S. gregaria, as well as two swarming New World locusts. Anecdotal field observations of locust-like behavior in S. americana indicate that it may express behavioral phase polyphenism, but empirical investigations are lacking. In this study, I tested the hypothesis that S. americana expresses locust-like density-dependent changes in behavior during both the first and final nymphal instars. I then compared the expression of behavioral phase change between S. americana and S. gregaria. First instar S. americana exhibited significant geographic variation in behavior with grasshoppers from a North Carolina population expressing more pronounced density-dependent changes relative to grasshoppers from a Texas population. The behavior of final instar S. americana was only slightly affected by rearing density and there was no evidence for a difference between populations. Comparison with S. gregaria revealed that the magnitude of density-dependent behavioral change, particularly among final instar nymphs, was much reduced in S. americana.     

Relationship between the successful infection by entomopathogenic nematodes and the host immune response

Reproduction of entomopathogenic nematodes requires that they escape recognition by a host's immune system or that they have mechanisms to escape encapsulation and melanization. We investigated the immune responses of larvae for the greater wax moth (Galleria mellonella), tobacco hornworm (Manduca sexta), Japanese beetle (Popillia japonica), northern masked chafer (Cyclocephala borealis), oriental beetle (Exomala orientalis) and adult house crickets (Acheta domesticus), challenged with infective juveniles from different species and strains of entomopathogenic nematodes. The in vivo immune responses of hosts were correlated with nematode specificity and survival found by infection assays. In P. japonica, 45% of injected infective juveniles from Steinernema glaseri NC strain survived; whereas the hemocytes from the beetle strongly encapsulated and melanized the Heterorhabditis bacteriophora HP88 strain, S. glaseri FL strain, Steinernema scarabaei and Steinernema feltiae. Overall, H. bacteriophora was intensively melanized in resistant insect species (E. orientalis, P. japonica and C. borealis) and had the least ability to escape the host immune response. Steinernema glaseri NC strain suppressed the immune responses in susceptible hosts (M. sexta, E. orientalis and P. japonica), whereas S. glaseri FL strain was less successful. Using an in vitro assay, we found that hemocytes from G. mellonella, P. japonica, M. sexta and A. domestica recognized both nematode species quickly. However, many S. glaseri in M. sexta and H. bacteriophora in G. mellonella escaped from hemocyte encapsulation by 24h. These data indicate that, while host recognition underlies some of the differences between resistant and susceptible host species, escape from encapsulation following recognition can also allow successful infection. Co-injected surface-coat proteins from S. glaseri did not protect H. bacteriophora in M. sexta but did protect H. bacteriophora in E. orientalis larva; therefore, surface coat proteins do not universally convey host susceptibility. Comparisons of surface coat proteins by native and SDS-PAGE demonstrated different protein compositions between H. bacteriophora and S. glaseri and between the two strains of S. glaseri.     

Factors affecting the ability of the wasp parasite Pseudeucoila bochei to inhibit tumourigenesis in Drosophila melanogaster.

The cellular immune reaction of Drosophila melanogaster larvae which results in encapsulation of internal metazoan parasites is similar to the autoimmune reaction made by certain tumourmutant stocks against abnormally developing host tissues. The wasp parasite Pseudeucoila bochei is capable of actively inhibiting the haemocytic encapsulation reaction of host larvae, and this activity can also inhibit the haemocytic response made by the bw tu mutant flies against abnormal fat body tissue. The ability of the parasite to suppress tumourigenesis is dependent on the age of the host at the time of infection, and on the amount of previous oviposition of the parasite. The earlier during host development parasitisation occurs the more tumour formation is inhibited. partially spent females are less able to inhibit tumours than are newly emerged parasites and unspent parasites of the same age. In some parasitised individuals the tumour response is incomplete and consists of small pigment particles instead of the large melanotic growths characteristic of the strain. The autoimmune response made against “alien” host tissues is specific and does not interfere with the successful development of the parasite.     

Laboratory evidence showing the separate species status of Schistocerca gregaria, americana and cancellata (Acrididae, Crytacanthacridinae)

Abstract. Results are given of experimental crosses of Schistocerca species from the New World and the Old World Desert Locust. It is concluded that S. cancellata (Serville), S. gregaria (Forskal) and S. americana (Drury) are separate species in their own right not subspecies of americana. The African Desert Locust should thus be called S. gregaria gregaria and its S. W. African race S. gregaria flaviventris (Burmeister). The name S. vicaria (Walker) is proposed for the Central American Locust. The relationship of S. paranensis to Sxancellata is discussed.     

Intermediate host specificity in Schistosoma mansoni

Miracidia of Schistosoma mansoni penetrate into many kinds of snails, but development of normal sporocysts takes place only in certain species of Biomphalaria. Different populations of this snail vary greatly in laboratory infection rates with S. mansoni originating from diverse geographic localities. Cross-exposure experiments show that compatibility factors exist in both snails and parasites. Susceptibility of stocks of Biomphalaria to particular strains of S. mansoni is genetically determined and may be modified by selection in the laboratory. In a compatible snail, the sporocyst develops without host tissue reaction; in incompatible snails the early larvae are rapidly surrounded by amebocytes and fibroblasts, and destroyed. This reaction resembles the generalized host cellular response elicited by any foreign body. An individual snail exposed to many miracidia may have both developing and encapsulated sporocysts side by side within its tissues. The weight of current evidence suggests that elicitation or absence of this cellular response resides in the recognition or nonrecognition of the sporocyst as a foreign body. The sporocyst tegument surface, which forms within a few hours after miracidial penetration, may have a molecular conformation identical with that of the snail, or may be able to bind specific host molecules, so that detection and subsequent encapsulation by host cells are averted. Presuming genetic determination of the sporocyst surface structure and of the host cell detection capability, differing infection rates would result from the particular frequencies of relevant genes in the populations concerned.     

Ultrastructural aspects of the host cellular immune response to Taenia crassiceps metacestodes.

When three Taenia crassiceps metacestodes were injected intraperitoneally into C3H mice primed by previous subcutaneous inoculation of metacestodes, larvae which were resistant to early immune damage by the humoral response were encapsulated by host cells and rejected. Initially, normal larvae were encapsulated primarily by eosinophils and macrophages. In the early stages of encapsulation, both cell types showed severe degenerative changes and disruption of cell membranes, but there was no evidence of tegumental damage to the encapsulated larvae. Later, mast cells appeared in the capsules surrounding the larvae. After mast cells became common, all of the cell types present were normal, and damage to the larval Tegument became apparent. Ultimately, interaction of eosinophils, mast cells, macrophages, and lymphocytes resulted in death of the encapsulated larvae. These results suggest that larvae may secrete substances toxic to host cells, and that mast cells are necessary for rejection of larvae.     

Heritability of immune function in the caterpillar Spodoptera littoralis

Phenoloxidase (PO) is believed to be a key mediator of immune function in insects and has been implicated both in non-self recognition and in resistance to a variety of parasites and pathogens, including baculoviruses and parasitoids. Using larvae of the Egyptian cotton leafworm, Spodoptera littoralis, we found that despite its apparent importance, haemolymph PO activity varied markedly between individuals, even amongst insects reared under apparently identical conditions. Sib-analysis methods were used to determine whether individuals varied genetically in their PO activity, and hence in one aspect of immune function. The heritability estimate of haemolymph PO activity was high (h(2) = 0.690 +/- 0.069), and PO activity in the haemolymph was strongly correlated with PO activity in both the cuticle and midgut; the sites of entry for most parasites and pathogens. Haemolymph PO activity was also strongly correlated with the degree to which a synthetic parasite (a small piece of nylon monofilament) was encapsulated and melanized (r = 0.622 +/- 0.142), suggesting that the encapsulation response is also heritable. The mechanism maintaining this genetic variation has yet to be elucidated.     

Levels of encapsulation and melanization in two larval instars of Ostrinia furnacalis Guenée (Lep., Pyralidae) during simulation of parasitization by Macrocentrus cingulum Brischke (Hym., Braconidae)

Abstract:  The hymenopteran Macrocentrus cingulum is a polyembryonic endoparasitoid that uses larvae of the lepidopteran Ostrinia furnacalis as one of its host insects. Previous studies indicated that although this parasitoid does not transmit polydnaviruses when it lays its eggs, a layer of fibrous tissue on the surface of the eggs helps them to avoid being encapsulated by the immune system of the host insect. However, as eggs of M. cingulum that are laid into late instar larvae of O . furnacalis often do not survive, there is a tendency for the adults to lay their eggs in earlier instar larvae. We studied the amounts of encapsulation and melanization around beads of DEAE-Sephadex A-25 injected into the haemoceol of fourth and fifth larval instars of O. furnacalis . The beads were injected to simulate the presence of eggs of the parasitoid M. cingulum . We found that the levels of encapsulation and melanization around the beads increased with the age of the O. furnacalis larvae. Likewise, the total counts of the haemocytes circulating within the haemolymph increased with the age of the O. furnacalis larvae and were correlated negatively with the percentage of larvae parasitized by M. cingulum . It appears that young O. furnacalis possess a weak cellular defence, and as a result are more susceptible to being parasitized. Hence, the correlation between the levels of encapsulation and the age of the host insect appears simply to reflect passive evasion.     

Immune response of the hibiscus mealybug, Maconellicoccus hirsutus Green (Homoptera: Pseudococcidae), to oviposition of the parasitoid Anagyrus kamali Moursi (Hymenoptera: Encyrtidae)

Anagyrus kamali Moursi has been recently introduced into the Caribbean as a biological agent against the hibiscus mealybug, Maconellicoccus hirsutus Green. This host has a cellular defense reaction that involves encapsulation and melanization of the endoparasitoid egg. The impact of this immune response on the parasitoid progeny was assessed, as well as the response of the parasitoid countermeasures to overcome it. Under laboratory conditions, significant differences in the immune response were found for different developmental stages of M. hirsutus. The intensity of the immune response varied between second instar, third instar and adult mealybugs. After 30 h, the level of encapsulation was the highest for eggs oviposited in adults: 58% of eggs were encapsulated, followed by third (32%) and second (4%) instars. Three days after oviposition 23, 44 and 86% of the parasitoid eggs oviposited, respectively, in adult, third and second instars were not encapsulated. The unencapsulated parasitoid eggs could hatch and continue their development. Adult mealybugs required 30 h to encapsulate 50% of the eggs, whereas in second and third instars, 50% level encapsulation was never reached. Superparasitism had a saturating effect on the immune system; reduced levels of encapsulation occurred when more than 10 eggs were oviposited in a single mealybug. Wasp larvae were never encapsulated by M. hirsutus.     

Role of cellular response in elimination of the monogenean Neoheterobothrium hirame in Japanese flounder Paralichthys olivaceus

Adult worms of the blood-feeding monogenean parasite Neoheterobothrium hirame, which cause anemia in the Japanese flounder Paralichthys olivaceus, attach to the host fish by embedding their posterior part deeply into the host tissue. To investigate the possibility that cellular responses of the host fish can eliminate N. hirame, flounder were experimentally infected with N. hirame larvae and reared in either fed or starved conditions. Mature parasites were identified on the buccal cavity wall of the fish 33 d post-infection (Day 33). Monocytes/macrophages and granulocytes increased rapidly in the blood and infected sites after the appearance of mature parasites. These cells adhered to the tegument of the parasites. In addition, a few cells with large electron-dense granules (DGCs) were observed in the inflammatory foci. On Day 47, the tegument of some parasites collapsed partially and were phagocytosed by the infiltrated host cells. Some infiltrated cells adhered directly to the inner tissues of the parasites. On Day 54, in the fed fish group, the loss of the tegument led to damage of the parasites' inner tissue by a large number of infiltrated cells. In this group, the elimination of the parasites was noted from Day 47 to 54. These observations probably suggest that the cellular response of the host fish destructed the parasite's posterior part embedded in the tissue, thereby eliminating the parasites. On the other hand, a high mortality was observed in the starved group. The starved fish developed much more severe anemia than the fed fish, and the elimination of the parasites was not observed in this group. The results of the present study suggest that flounder can eliminate N. hirame if they are fed sufficiently.     

Differential expression of the CrV1 haemocyte inactivation-associated polydnavirus gene in the African maize stem borer Busseola fusca (Fuller) parasitized by two biotypes of the endoparasitoid Cotesia sesamiae (Cameron)

Polydnaviruses are rarely studied for their natural variation in immune suppressive abilities. The polydnavirus harboring braconid Cotesia sesamiae, a widespread endoparasitoid of Busseola fusca and Sesamia calamistis in sub-Saharan Africa exists as two biotypes. In Kenya, the western biotype completes development in B. fusca larvae. However, eggs of the coastal C. sesamiae are encapsulated in this host and ultimately, no parasitoids emerge from parasitized B. fusca larvae. Both biotypes develop successfully in S. calamistis larvae. Encapsulation activity by B. fusca larvae towards eggs of the avirulent C. sesamiae was detectable six hours post-parasitization. The differences in encapsulation of virulent and avirulent strains were associated with differences in nucleotide sequences and expression of a CrV1 polydnavirus (PDV) gene, which is associated with haemocyte inactivation in the Cotesia rubecula/Pieris rapae system. CrV1 expression was faint or absent in fat body and haemolymph samples from B. fusca parasitized by the avirulent C. sesamiae, which exhibited encapsulation of eggs. Expression was high in fat body and haemolymph samples from both B. fusca and S. calamistis larvae parasitized by the virulent C. sesamiae, encapsulation in the former peaking at the same time points as CrV1 expression in the latter. Non synonymous difference in CrV1 gene sequences between virulent and avirulent wasp suggests that variations in B. fusca parasitism by C. sesamiae may be due to qualitative differences in CrV1-haemocyte interactions.     

Genetic basis of encapsulation response in Anopheles gambiae

The phenomenon of encapsulation of invading organisms is widespread in insects. Co-evolution has produced an intricate balance between the immune responses of the host and immune-suppressive (or immune-evading) properties of the parasite. Genome-wide genetic mapping revealed different loci in Anopheline mosquitoes were involved in melanotic encapsulation of different malaria parasites. Certain isolates of human malaria parasites can still suppress or avoid the immune response from refractory mosquitoes. Similar interactions with parasitoids were observed in Drosophila melanogaster. Species-specific encapsulation locus was identified for two parasitoids, respectively, and virulent strain of parasitoid can suppress the immune system of an otherwise resistant fruitfly. It is believed that the encapsulation loci in both mosquitoes and fruitfly may encode gene products that function at the early stages of parasite/parasitoid recognition or immediate signaling events. Future research on membrane receptor molecules and their roles in insect immunity will yield interesting insights into mosquito-parasite interactions.     

Host refractoriness of the tobacco hornworm, Manduca sexta, to the braconid endoparasitoid Cotesia flavipes

Cotesia flavipes (Hymenoptera:Braconidae) is a gregarious endoparasitoid of several pyralid stemborer larvae of economic significance including the sugarcane borer, Diatraea saccharalis. In this study, the ability of this parasitoid to develop in a sphingid host, Manduca sexta, was tested. First, second, third, fourth, and even pharate fifth instar host tobacco hornworm larvae were readily parasitized by the female C. flavipes parasitoids but no wasp larvae hatched from the eggs in this refractory host. Instead, the parasitoid eggs were invariably encapsulated by the host's hemocytes and, ultimately, no parasitoids emerged from tobacco hornworm hosts. The first stages of encapsulation were evident at 2 h post-parasitization of the host M. sexta larvae, when the beginning stages of capsule formation were seen. The developmental fate of the host larvae with encapsulated parasitoids was variable. Most succumbed as abnormally small fifth instars or as post-wandering prepupal animals, while a few developed normally to the pupal stage. Dissection of all the larvae or pupae with encapsulated wasp eggs showed evidence of hemocytic encapsulation and melanization of the C. flavipes eggs. This report describes the association between C. flavipes and M. sexta, which appears to be an excellent model system for studying the physiological processes accompanying wasp egg encapsulation that result in death of the host as well as the parasitoid. Since the parasitoid egg never hatches, the system offers an excellent opportunity to identify and study the effects of parasitoid-injected polydnavirus and venom on host physiology.     

The effects of host age and superparasitism by the parasitoid, Microplitis rufiventris on the cellular and humoral immune response of Spodoptera littoralis larvae

To study the dynamics of stage-dependent immune responses in Spodoptera littoralis (Boisd.) larvae (Lepidoptera: Noctuidae), single and superparasitism experiments were carried out using the parasitoid Microplitis rufiventris Kok. (Braconidae: Hymenoptera). Compared to younger (preferred) host larvae, the older (non-preferred) host larvae displayed a vigorous humoral response that often damaged and destroyed the single wasp egg or larva. Superparasitism and host age altered both the cellular and humoral immune responses. Younger host larvae showed a stronger encapsulation response compared to older host larvae. Moreover encapsulation rates in younger hosts (e.g., second instar) decreased with increasing numbers of parasitoid eggs deposited/larvae. In older larvae, the encapsulation rate was low in fourth, less in fifth and absent in sixth instar hosts. Conversely, the order and magnitude of the cellular immune response in S. littoralis hosts were highest in second instar larvae with the first instar larvae being a little lower. The immune response steadily decreased from the third through to the fifth instar and was least obvious in the sixth instar. In contrast, the general humoral immune response was most pronounced in sixth instar larvae and diminished towards younger stages. The results suggest that both cellular and humoral responses are stage-dependent. Wasp offspring in younger superparasitized host larvae fought for host supremacy with only one wasp surviving, while supernumerary wasp larvae generally survived in older superparasitized larvae, but were unable to complete development. Older instars seem to have a method for immobilizing/killing wasp larvae that is not operating in the younger instars.     

Pharmacology of the octopamine-stimulated adenylate cyclase of the locust and tick CNS

The presence of an octopamine-stimulated adenylate cyclase is shown in homogenates of the cerebral ganglion of the locust Schistocerca americana gregaria and the synganglion of the tick Boophilus microplus. A detailed study of the pharmacology of the activation of the enzyme of the two species is reported and compared with the pharmacology of octopamine receptors in other invertebrate preparations.     

Overview of parasitism associated effects on host haemocytes in larval parasitoids and comparison with effects of the egg-larval parasitoid Chelonus inanitus on its host Spodoptera littoralis

In the first part we review the effects of larval endoparasitoids and their polydnavirus and venom on the immune system of their hosts. In all systems investigated, haemocyte spreading and encapsulation activity was reduced; in some cases effects on total (THC) or differential (DHC) haemocyte count as well as modification of haemocyte morphology and ultrastructure were also documented. In many cases polydnavirus (and venom) were shown to play a major role in abrogation of the host's immune reaction. In the second part we present the first investigation of effects of parasitism and polydnavirus/venom on the immune system of the host for an egg-larval parasitoid, Chelonus inanitus. We observed that in 4th and 5th instar larvae, i.e. 7 to 10 days after parasitization, neither haemocyte spreading and encapsulation activity, nor DHC, nor haemocyte ultrastructure were altered. After parasitization with X-ray irradiated wasps, which inject polydnavirus and venom and infertile eggs, there was no alteration of the above mentioned parameters. Nevertheless, parasitoid larvae implanted into 4th instar larvae which developed from eggs parasitized with X-ray irradiated wasps were not encapsulated, whereas co-injected latex beads were. These results show that parasitism by this egg-larval parasitoid does not generally suppress the host's immune system but that polydnavirus/venom injected at oviposition prevent, by, as yet unknown mechanisms, encapsulation of the parasitoid larva.     

Extraembryonic membrane of the polyembryonic parasitoid Macrocentrus cingulum Brischke (Hym., Braconidae) is essential for evasion of encapsulation

Abstract:  The wasp Macrocentrus cingulum (Hymenoptera) is an endoparasitoid that uses larvae of the Asian corn borer Ostrinia furnacalis as one of its hosts. The wasp is polyembryonic and a single wasp egg gives rise to several dozens of embryos. Earlier studies showed that the fibrous layer on the surface of M. cingulum eggs protects them from the immune system of the moth larvae. However, the way in which the embryos of the parasitoid avoid being encapsulated remained unknown. In this paper, we show that the evasion of encapsulation is mediated through the extraembryonic membrane. We also show that M. cingulum embryos developed normally in the larvae of O. furnacalis but were encapsulated when injected into the larvae of Helicoverpa armigera , which is not a host species for the wasp larvae. When the extraembryonic membrane was removed, either chemically using the enzyme dispase or mechanically using a dissecting needle, the 'unprotected' embryos were also encapsulated both in vivo and in vitro by the haemocytes of the normal host O. furnacalis . It was also shown that the extraembryonic membrane was labelled strongly with fluorescein isothiocyanate (FITC)-conjugated Helix pomatia ( H.p. ) lectin. This suggests that a chemical in, or on, the extraembryonic membrane, that helps the embryos of M. cingulum to avoid encapsulation, is possibly a glycodeterminant produced in the haemocoel of the wasp.     

Racing against host's immunity defenses: a likely strategy for passive evasion of encapsulation in Asobara tabida parasitoids

The hymenopteran Asobara tabida Nees (Braconidae, Alysiinae) develops as a solitary endophagous parasite in larvae of several Drosophila species. Most A. tabida eggs possess a sticky chorion which attaches to the tissue of the host organs within a few hours following oviposition. A. tabida sticky eggs usually avoid encapsulation, though the probability of survival decreases in hosts carrying a larger number of circulating hemocytes. Here, we hypothesized that the elicitation of the encapsulation reaction may result from a race between two phenomena: the host's hemocytic reaction and the embedment of the parasitic egg within the host tissues. In order to test this hypothesis, we measured the speed of capsule formation in D. melanogaster larvae of different ages, knowing that the number of circulating hemocytes increases with the age of the larvae. Using a non-virulent A. tabida strain, the eggs of which do not attach to the host tissue, we found that the speed of capsule formation increased correlatively with the age of the D. melanogaster larva. Therefore, the hypothesis of a physiological race between host's immunity defenses and parasite's avoidance of host's defenses is strongly supported by our results. Also, A. tabida eggs which attach to the host's tissue before the attack by the hemocytes has taken place may be considered as a strategy of passive evasion from encapsulation.     

Effect of the venom of the gregarious parasitoid Apanteles glomeratus on its hemocytic encapsulation by the host, Pieris

When eggs from the lateral oviduct of the gregarious parasitoid Apanteles glomeratus were injected with calyx fluid and venom apparatus material into host larvae, Pieris rapae crucivora, most of the eggs were not encapsulated. Apanteles eggs deposited by the parasitoid from which the venom apparatus was removed were usually encapsulated by the host. These results indicate that the parasitoid venom apparatus material is an important factor in suppressing the encapsulation of 1- or 2-day-old eggs in the host. In order to clearly demonstrate that the venom suppresses egg encapsulation but not the encapsulation of other foreign objects, DEAE-Sephadex A-50 ion-exchange particles stained with 0.001% (w/v) Congo Red solution were injected into hosts together with venom apparatus material. The Sephadex particles were encapsulated by host hemocytes. The results suggest that the venom does not inhibit the encapsulation ability of the host.