Intrinsic competition between larval parasitoids with different degrees of host specificity

Abstract.  1. Interspecific competition among parasitoids may play a key role in the community dynamics of tritrophic plant–herbivore–parasitoid systems and has important implications for management of herbivorous insect pests.
2. A model system was used to explore the outcome of interspecific competition between parasitoids that differ in host specificity. The system included the lepidopteran pest Heliothis virescens , the generalist parasitoid Cotesia marginiventris , and two specialist parasitoids, Microplitis croceipes and Cardiochiles nigriceps .
3. The generalist, C. marginiventris , dominated intrinsic competition when given an 8-h developmental head start over C. nigriceps or when its oviposition was simultaneous with that of M. croceipes . Microplitis croceipes and especially C. nigriceps larvae prevailed when they were allowed to oviposit prior to C. marginiventris .
4. Rates of host mortality prior to parasitoid emergence varied with parasitoid species composition and with the order of oviposition.
5. Implications for integrated pest management and the adaptive significance of competition as related to host specialisation are discussed.     

Developmental pathology of Heliothis virescens larvae parasitized by Microplitis croceipes: Parasite-mediated host developmental arrest

The developmental pathology of Heliothis virescens larvae parasitized by the braconid wasp Microplitis croceipes was examined. Parasitized host larvae begin the same precise sequence of developmental events in preparation for pupation as observed in unparasitized larvae. This sequence is initiated even though the host larval weight is below the normal developmental threshold for larval-pupal transformation. After parasite emergence, the host remains in a suspended advanced developmental state but never pupates. The developmental parameters altered by parasitization are normally under the host's endocrine control. Neck ligation of control larvae was used to identify the critical periods in parasitized and unparasitized fourth- and fifth-instar larvae. Control ligated fourth-instar larvae apparently released PTTH between 21:00 AZT of the second day of the instar and 1:00 AZT of the third day. Parasitized fourth-instar larvae were smaller and apparently released PTTH between 18:00 and 23:00 AZT of the third day. Control ligated fifth-instar larvae apparently released PTTH between day 1 and day 2 of the cell formation phase. Ligated fifth-instar parasitized larvae never molted to the pupal stage. Parasite larvae were adversely affected by host neck ligation with their pupal plus cocoon weight being proportional to the age of the host at the time of ligation.     

Intrinsic competition and its effects on the survival and development of three species of endoparasitoid wasps

In natural systems, pre‐adult stages of some insect herbivores are known to be attacked by several species of parasitoids. Under certain conditions, hosts may be simultaneously parasitized by more than one parasitoid species (= multiparasitism), even though only one parasitoid species can successfully develop in an individual host. Here, we compared development, survival, and intrinsic competitive interactions among three species of solitary larval endoparasitoids, Campoletis sonorensis (Cameron) (Hymenoptera: Ichneumonidae), Microplitis demolitor Wilkinson, and Microplitis croceipes (Cresson) (Hymenoptera: Braconidae), in singly parasitized and multiparasitized hosts. The three species differed in certain traits, such as in host usage strategies and adult body size. Campoletis sonorensis and M. demolitor survived equally well to eclosion in two host species that differed profoundly in size, Pseudoplusia includens (Walker) and the larger Heliothis virescens (Fabricius) (both Lepidoptera: Noctuidae). Egg‐to‐adult development time in C. sonorensis and M. demolitor also differed in the two hosts. Moreover, adult body mass in C. sonorensis (and not M. demolitor) was greater when developing in H. virescens larvae. We then monitored the outcome of competitive interactions in host larvae that were parasitized by one parasitoid species and subsequently multiparasitized by another species at various time intervals (0, 6, 24, and 48 h) after the initial parasitism. These experiments revealed that M. croceipes was generally a superior competitor to the other two species, whereas M. demolitor was the poorest competitor, with C. sonorensis being intermediate in this capacity. However, competition sometimes incurred fitness costs in M. croceipes and C. sonorensis, with longer development time and/or smaller adult mass observed in surviving wasps emerging from multiparasitized hosts. Our results suggest that rapid growth and large size relative to competitors of a similar age may be beneficial in aggressive intrinsic competition.     

Interspecific discrimination and larval competition amongMicroplitis croceipes,Microplitis demolitor,Cotesia kazak (Hym.: Braconidae), andHyposoter didymator (Hym.: Ichneumonidae), parasitoids ofHeliothis virescens (Lep.: Noctuidae)

Interspecific host discrimination by adults, and larval competition among the endoparasitoidsMicroplitis croceipes (Cresson),Microplitis demolitor Wilkinson,Cotesia kazak (Telenga) andHyposoter didymator (Thunberg) were investigated usingHeliothis virescens (F.) as the host. In ovipositional choice tests, the mean number of encounters and ovipositions for unparasitized hosts was not significantly different from the mean number of encounters and ovipositions for parasitized hosts for each treatment combination (P>0.05). Thus, none of the parasitoid species discriminated between host larvae recently parasitized once by a female of another species und unparasitized hosts. However, in all but two cases, females did discriminate between unparasitized hosts and hosts in which an early first instar of the first-attacking species was developing.Cotesia kazak andH. didymator did not discriminate between unparasitized hosts and hosts parasitized by an early first instar ofM. demolitor. Larval competition among these parasitoid species was studied for three time intervals between the first and second species parasitization: 1) second species attack immediately (5–15 sec) after the first; 2) second species attack 24 h after the first; and 3) second species attack 48 h after the first. Time until egg eclosion was shortest forM. demolitor, thenC. kazak, thenM. croceipes, and longest forH. didymator. When the second parasitoid species attacked a host immediately after the first species, the species in which egg eclosion occurred first was the victor more frequently, except whenM. demolitor competed withC. kazak andH. didymator. With a 24 h delay between the first and second species to attack, the older first instar from the first parasitization usually outcompeted the younger first instar from the second attack. A first instar from the second species to attack generally outcompeted the second instar of the first species when the second parasitization had been delayed 48 h. Competiors were eliminated mainly by physical attack, butC. kazak andM. croceipes apparently also killedH. didymator eggs by physiological processes.     

Determinants of sperm precedence in a noctuid moth Heliothis virescens: a role for male age

1. Females of the noctuid moth Heliothis virescens F. mate more than once. Thus, sperm from two or more males normally compete for fertilisations within the female reproductive tract. The eggs are typically fertilised by sperm from only one male, either the female's last mate or an earlier mate. Twice‐mated females store only one ejaculate's worth of fertilising sperm (eupyrene) but nearly two ejaculates' worth of a nonfertilising sperm morph (apyrene), which is thought to play a role in sperm competition. 2. The mechanism of sperm use in H. virescens was investigated by examining factors that vary with paternity, which was assigned based on allozyme variation. The factors included male and female body masses and ages, male genital characters, the size of the sperm package, and the number of sperm stored by the female. 3. One male typically gained sperm precedence; this was nearly twice as likely to be the second male as it was to be the first. Two factors were found to vary significantly with paternity: female mass and male age. The second male to mate was more likely to gain sperm precedence if the female was larger and if the male was older than the female's first mate. 4. The significance of male age and female mass to several hypothetical models of the mechanism of sperm use is discussed.     

Intrinsic, inter-specific competition between egg, egg–larval, and larval parasitoids of plusiine loopers

Abstract.  1. Intrinsic, inter-specific competition between parasitoid wasp species is a key factor in ecological community dynamics and is particularly important for application in biological control. Here three parasitoid wasp species with overlapping host ranges and differing life history strategies were chosen to examine parasitoid–parasitoid interactions: the egg parasitoid Trichogramma pretiosum, the egg–larval, polyembryonic parasitoid wasp Copidosoma floridanum, and the gregarious larval parasitoid Glyptapanteles pallipes , with the plusiine loopers Acanthoplusia agnata and Trichoplusia ni as hosts.
2.  Copidosoma floridanum has been shown to be an intrinsically superior competitor against larval parasitoids because of their production and increased investment in a soldier larval caste during development, but little is known of their interactions with egg parasitoid species. Trichogramma pretiosum completely dominated intrinsic competition with C. floridanum regardless of oviposition order or sex of the C. floridanum egg.
3. Competition between C. floridanum and G. pallipes , however, depended on the host stage at which parasitism occurred, the sex of the C. floridanum egg, and parasitoid development time. Copidosoma floridanum outcompeted G. pallipes overall, despite the fact that G. pallipes injects a polyDNA virus into the host.
4. The sex of the C. floridanum egg was a significant factor in its ability to shift caste ratios to produce more soldiers in response to G. pallipes competition.
5. Only developing female C. floridanum responded to competition with G. pallipes by increasing the ratio of soldier to reproductive larvae, and this happened only when multiparasitism occurred in the host's 1st and 2nd instar.     

The role of herbivore‐ and plant‐related experiences in intraspecific host preference of a relatively specialized parasitoid

Parasitoids use odor cues from infested plants and herbivore hosts to locate their hosts. Specialist parasitoids of generalist herbivores are predicted to rely more on herbivorederived cues than plant-derived cues. Microplitis croceipes (Cresson)(Hymenoptera: Braconidae) is a relatively specialized larval endoparasitoid of Heliothis virescens (F.)(Lepidoptera: Noctuidae), which is a generalist herbivore on several crops including cotton and soybean. Using M. croceipes/H. virescens as a model system, we tested the following predictions about specialist parasitoids of generalist herbivores:(i) naive parasitoids will show innate responses to herbivore-emitted kairomones, regardless of host plant identity and (ii) herbivore-related experience will have a greater influence on intraspecific oviposition preference than plant-related experience. Inexperienced (naive) female M. croceipes did not discriminate between cotton-fed and soybean-fed H. virescens in oviposition choice tests, supporting our first prediction. Oviposition experience alone with either host group influenced subsequent oviposition preference while experience with infested plants alone did not elicit preference in M. croceipes, supporting our second prediction. Furthermore, associative learning of oviposition with host-damaged plants facilitated host location. I terestingly, naive parasitoids attacked more soybeathan cotton-fed host larvae in two-choice tests when a background of host-infested cotton odor was supplied, and vice versa. This suggests that plant volatiles may have created an olfactory contrast effect. We discussed ecological significance of the results and concluded that both plant- and herbivore-related experiences play important role in parasitoid host foraging.     

Determinants of Sperm Transfer by Males of the Noctuid Moth Heliothis virescens

Ejaculate composition can be an important determinant of male reproductive success in the face of sperm competition, which varies with the mating history of the female. Here we examine the effect of various male and female mating histories and morphological traits on ejaculate sperm numbers in the polyandrous moth Heliothis virescens. We show that when mating with nonvirgin females, males passed larger sperm packages (spermatophores) but did not alter either the sperm count or the ratio of nucleated-to-nonnucleated sperm. Males also passed fewer sperm in their second ejaculates. Finally, older males passed more sperm than did younger males. Earlier research found that females store more sperm from older males and that older males are more likely to gain sperm precedence over younger rivals. These earlier results, taken together with the present results, indicate that the advantage enjoyed by older males is due to an increased sperm count.     

Modes of action of fraxinellone against the tobacco budworm, Heliothis virescens

Fraxinellone significantly reduced the relative growth rate, food consumption rate as well as the efficiency of conversion of ingested food into biomass of Heliothis virescens when incorporated into artificial diets at concentrations of 4.31 × 10⁻⁵ mol/L and above. After being fed with fraxinellone-treated diets for 24 h, the larval midguts of H. virescens possess significantly lower activities of α-amylase and non-specific proteases and higher activities of cytochrome P450s. In vitro , the compound did not inhibit the activities of α-amylase and non-specific proteases extracted from the larval midguts. Clear evidence of post-ingestive toxicity of fraxinellone to midgut cells was observed under an electron microscope. The modes of action of the compound against insects were discussed.     

Heterospecific larval competition and host discrimination in two species of aphid parasitoids: Aphidius ervi and Aphidius smithi

Females of the solitary aphid parasitoids Aphidius ervi Haliday and A. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae) discriminated between unparasitized pea aphids and those parasitized by the other species. Oviposition restraint varied with the attack sequence and the length of the interval between successive attacks. The tendency to reject a previously parasitized host increased with interval length; A. smithi females rarely oviposited in aphids that had been parasitized 30 h earlier by A. ervi. Early first-instar larvae of A. ervi physically attacked and killed older A. smithi larvae, and older A. ervi larvae killed younger A. smithi, possibly by physiological suppression. Neither species appeared to have a competitive advantage when their eggs hatched at the same time. The evolution of heterospecific host discrimination in A. ervi and A. smithi is discussed. It is suggested that avoidance of multiparasitism is adaptive for both parasitoid species: for A. smithi because it is the inferior larval competitor, and for A. ervi because immatures develop more slowly in multiparasitized than in initially unparasitized hosts.

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Compétition interspécifique et discrimination des hôtes chez deux parasitoïdes de pucerons: Aphidius ervi et A. smithi

Résumé Les femelles des parasitoïdes de pucerons: Aphidius ervi Haliday et A. smithi Sharma & Subba Rao (Hyméno. Aphidiidae) distinguent les pucerons du pois sains des parasités par d'autres espèces. La rétention de la ponte dépend de la séquence de l'attaque et du temps écoulé entre des attaques successives. La tendance au rejet d'un puceron précédemment parasité augmente avec l'importance du délai; A. smithi a rarement pondu dans des pucerons qui avaient été parasités 30 h avant par A. ervi. Les jeunes larves de premier stade de A. ervi ont attaqué physiquement et tué les larves plus âgées de A. smithi, et les larves plus âgées de A. ervi ont tué des larves plus jeunes de A. smithi par élimination physiologique. Aucune espèce ne semble avoir un avantage quand les oeufs ont éclos en même temps. L'évolution de la discrimination interspécifique de l'hôte chez A. ervi et A. smithi est discutée. On estime que la tendance à éviter le multiparasitisme est adaptative chez les 2 espèces: pour A. smithi parce qu'il est dominé dans la compétition larvaire et pour A. ervi parce que les larves se développent plus lentement dans un hôte multiparasité que dans un hôte initialement sain.

     

Genome analysis of Heliothis virescens ascovirus 3h isolated from China

No ascovirus isolated from China has been sequenced so far. Therefore, in this study, we aimed to sequence the genome of Heliothis virescens ascovirus 3h (HvAV-3h) using the 454 pyrosequencing technology. The genome was found to be 190,519-bp long with a G+C content of 45.5%. We also found that it encodes 185 hypothetical open reading frames (ORFs) along with at least 50 amino acids, including 181 ORFs found in other ascoviruses and 4 unique ORFs. Gene-parity plots and phylogenetic analysis revealed a close relationship between HvAV-3h and three other HvAV-3a strains and a distant relationship with Spodoptera frugiperda ascovirus 1a (SfAV-1a), Trichoplusia ni ascovirus 6a (TnAV-6a), and Diadromus pulchellus ascovirus 4a (DpAV-4a). Among the 185 potential genes encoded by the genome, 44 core genes were found in all the sequenced ascoviruses. In addition, 25 genes were found to be conserved in all ascoviruses except DpAV-4a. In the HvAV-3h genome, 24 baculovirus repeat ORFs (bros) were present, and the typical homologous repeat regions (hrs) were absent. This study supplies information important for understanding the conservation and functions of ascovirus genes as well as the variety of ascoviral genomes.

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Mating in Heliothis virescens: Transfer of juvenile hormone during copulation by male to female and stimulation of biosynthesis of endogenous juvenile hormone

Studies were undertaken to determine whether adult males of Heliothis virescens transfer juvenile hormone (JH) to females during copulation, and an in vitro radiochemical assay was used to determine whether mating causes an allatotropic effect, i.e., stimulation of JH biosynthesis by corpora allata (CA). In vitro, CA from 3-day-old mated females synthesized and released approximately 2.5 times total JH as that of CA from comparably aged virgin females. Of the homologues, JH II exhibited significant increase in mated females; JH I also increased but not significantly. JH III remained similar to that of virgin females. This is the first demonstration of an allatotropic effect of mating in moths. In contrast to the female, CA of virgin males did not produce any JH, but accessory sex glands (ASG) in 3-day-old males synthesized small amounts of JH. Immediately after adult emergence, male ASG contained approximately 1.5 ng JH I and II, which increased by 12 h after emergence and remained at this high level up to 54 h after emergence. JH III was barely detected in ASG. JH in ASG of mated male immediately after uncoupling was depleted almost completely, and 24 h later recovered to levels comparable to that of 54-h-old virgin male. Virgin female bursa copulatrix did not contain any JH, but mated female bursa, immediately after uncoupling, had JH at levels comparable to that observed in virgin male ASG. By 6 h after uncoupling, JH levels decreased dramatically in mated female bursa. These data suggest the transfer of JH to females by the male. Arch. Insect Biochem. Physiol. 38:100–107, 1998. © 1998 Wiley-Liss, Inc.     

Differences in the midgut proteolytic activity of two Heliothis virescens strains,one susceptible and one resistant to Bacillus thuringiensis toxins

The development of resistance to Bacillus thuringiensis toxic proteins is a growing concern because it could threaten both conventional and gene transfer use of this environmentally safe biological insecticide. The most common mechanism of resistance involves changes in binding affinity of toxin receptors in the insect midgut membrane. This has not been the case in Heliothis virescens. We have investigated changes in midgut proteolytic activity as a possibility to explain the resistance observed in this insect species. We have developed an improvement of known methods to demonstrate proteolytic activity in crude extracts. Using this method we have found differences in the proteolytic activity profile of midgut extracts of a susceptible and a resistant H. virescens strain. We also have studied the in vitro processing of CrylA(b) toxin and protoxin by midgut contents of both strains. SDS-PAGE of the in vitro degradation products showed differences between the strains. The resistant strain degrades protoxin more slowly and processes the active toxin more quickly than the susceptible strain. © 1996 Wiley-Liss, Inc.     

Rhythmicity of sex pheromone content in female Heliothis virescens: impact of mating

Abstract Pheromone production in virgin females of Heliothis virescens (F.) (Lepidoptera: Noctuidae) peaked between the fourth and seventh hours of scotophase on the second, third, fourth and fifth days following eclosion. The highest peak (186 ng) occurred on day 3 after eclosion. Z-11-Hexadecenal comprised the highest proportion of seven components in the pheromone glands. Disproportionately higher amounts of hexadecanal and Z-11-hexadecenol occurred during photo-phase and other periods when low quantities of total pheromone were recorded.
Mating suppressed pheromone production which remained low until 48 h after mating. Coupling females with males mated three times previously or with 6-day-old males was less effective in causing a drop in pheromone content which peaked again 24 h after mating. This suggests the transfer of a male factor, a pheromonostatic factor, that suppresses pheromone production in mated females and that the factor from older and previously mated males is less effective.     

Flight of Heliothis virescens males in the field in response to sex pheromone

Abstract. The behaviour of Heliothis virescens males flying upwind in the field in a sex pheromone plume was videorecorded and analysed. Males flew faster and straighter, with less counterturning, and heading more directly into the wind when they were 9-11m away from the odour source than when they were 1–3 m away. Regardless of their distance from the source or the windspeed, they maintained an average groundspeed of c. 200 cm s^_1, except when they arrived within 1 m of the source, when their groundspeed slowed significantly. Two or more males flying in the plume at the same instant often exhibited either extremely straight and directly upwind tracks or else zigzagging tracks with significant counterturning (as did males flying through the field of view of the cameras at slighdy different times). The males' position, either in the centre of the plume's axis or along one side, might explain these differences in track straightness, which previous studies with H.virescens have shown to be caused by higher frequencies of contact with plume filaments. When a significant shift in wind direction occurred, males tended to make an initial movement in the direction of the shift, perhaps due to latencies of response in both the olfactory and visual systems associated with flying into clean air. The males' behaviour in the field overall was similar to that observed in the wind tunnel, except that their airspeeds and groundspeeds were significantly higher than those observed in the laboratory. The fact that they flew faster in the field can be explained both by the significandy higher windspeeds that males need to compensate for in the field to attain a preferred velocity of image motion, as well as by a higher height of flight over the ground in die field causing a slower apparent motion of images at a given groundspeed compared with the laboratory.     

Source and characterization of an egg recognition kairomone of Telenomus heliothidis, a parasitoid of Heliothis virescens

ABSTRACT. The ability of Telenomus heliothidis Ashmead (Hymenoptera; Scelionidae) to recognize its host, the eggs of Heliothis virescens (F.) (Lepidoptera; Noctuidae), was determined in laboratory studies. The material present in the accessory gland of female H. virescens adults acted as an egg recognition kairomone for T. heliothidis. Host eggs which lacked the kairomone were not attacked, while glass beads the size of H. virescens eggs and coated with the accessory gland material were. The parasitoid failed to respond to the kairomone unless it was associated with a target the size and shape of normal hosts. The kairomone is heat stable, but was deactivated by proteolytic enzymes. The accessory gland contained 200 mg/ml protein which suggests that the kairomone is proteinaceous.     

Expression and hemocyte-targeting of a Campoletis sonorensis polydnavirus cysteine-rich gene in Heliothis virescens larvae

The polydnavirus associated with the parasitic wasp Campoletis sonorensis is injected into the lepidopteran insect, Heliothis virescens, during parasitization, after which viral gene products suppress the cellular immune system of the hosts. Four related cysteine-rich polydnavirus genes have been identified in parasitized H. virescens larvae and grouped into a family. In this study, we investigated the expression and hemocyte targeting of the cysteine-rich Vhv1.4 protein. Full- length and truncated Vhv1.4 proteins were produced in a bacterial expression system, and the purified proteins were used to raise polyclonal antisera. In immunoblots the Vhv1.4 protein was detected in parasitized insects as early as 6 h and throughout the entire course of parasitism. The Vhv1.4 protein appeared predominantly in the plasma fraction of hemolymph from parasitized larvae, suggesting that this protein is secreted. The Vhv1.4 protein expressed from a recombinant baculovirus was secreted in two lepidopteran cell lines and in larvae injected with the recombinant virus. Digestion with endoglycosidases suggests that the Vhv1.4 protein is glycosylated at multiple N-glycosylation sites. Immunofluorescence assays showed that the Vhv1.4 protein binds to the hemocytes, most notably the granulocytes, in H. virescens larvae. After binding, the Vhv1.4 protein was internalized, probably by endocytosis. Specific binding of the Vhv1.4 to granulocytes implies an important function in the suppression of host cellular encapsulation response. Arch. Insect Biochem. Physiol. 36:251–271, 1997. © 1997 Wiley-Liss, Inc.