Host-feeding and egg maturation byPachycrepoideus vindemiae

Pachycrepoideus vindemiae Rondani (Hymenoptera: Pteromalidae) is a facultative hyperparasitoid ofDrosophila parasitoids in Europe. FemaleP. vindemiae host-feed from the same hosts into which they lay eggs and this enables them to mature additional eggs.P. vindemiae females were allowed to host-feed from puparia containingDrosophila melanogaster Meigen (Diptera: Drosophilidae) pupae or pupae ofAsobara tabida Nees (Hymenoptera: Braconidae). Wasps which had host-fed carried significantly more eggs; the species of host which was fed from had no significant effect on the number of mature eggs in the ovaries. Host-feeding caused no significant reduction in the size of the emerging offspring.P. vindemiae were allowed to forage over patches containing different frequencies of the two host species. No significant oviposition preference was found but there were marked host-feeding preferences which were affected by the age of the host pupae. It is suggested that these preferences were due to the physical nature of the hosts which were fed from.     

Intraguild interactions between two egg parasitoids exploring host patches

Intraguild interactions between two egg parasitoids, Trissolcus basalis (Hymenoptera: Scelionidae) and Ooencyrtus telenomicida (Hymenoptera: Encyrtidae), exploring egg masses of the Southern Green Stink Bug (SGSB) Nezara viridula (Heteroptera: Pentatomidae), were investigated in laboratory conditions by single, simultaneous and sequential host attack experiments. Mortality of N. viridula eggs was higher in simultaneous and sequential releases compared to single species releases. In simultaneous host exploitations, T. basalis females displayed an aggressive behavior against O. telenomicida females. The outcome of multiparasitism showed that interspecific larval competition was dominated by O. telenomicida regardless of the sequence in which oviposition occurred and which parasitoid was or was not simultaneously released in the patch. Finally, O. telenomicida can successfully develop in hosts already parasitized by T. basalis up to seven days earlier, acting as a facultative hyperparasitoid, so that intraguild predation (IGP) also occurred. The ecological factors that allow species coexistence and the role played by competition in biological control programs are discussed.     

Host Age Preference Behavior in <Emphasis Type="Italic">Aphidius ervi</Emphasis> Haliday (Hymenoptera: Aphidiidae)

Many parasitoid species have preference for certain stages of hosts to parasitize but the underlying behavioral mechanisms of such preference are still poorly understood, making it difficult to evaluate host-parasitoid interactions and their effects on the success of biological control programs. Here, we report our work on a parasitoid Aphidius ervi Haliday on the pea aphid Acyrthosiphon pisum (Harris). We show that with the increase of host age, female parasitoids are more likely to encounter and to attack their hosts but the hosts develop increasingly greater defensive capabilities. Encounter almost always triggers attack attempt; however, increasing attack attempts do not proportionally lead to ovipositor probings and increasing ovipositor probings do not proportionally translate into ovipositions. These asymmetric responses may be interpreted as that A. ervi females prefer to parasitize older aphids for higher fitness return but those aphids can better defend themselves, and as a consequence, A. ervi females may achieve the highest gain by attacking aphids of intermediate ages. We suggest that A. ervi females forage in a manner consistent with the optimal foraging theory, trading off host handling time with fitness returns.     

The developmental strategy of an idiobiont ectoparasitoid, Dendrocerus carpenteri : influence of variations in host quality on offspring growth and fitness

Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae) is a solitary hyperparasitoid, which attacks prepupal and pupal stages of hymenopteran parasitoids inside mummified aphids. The larva feeds externally on the host, which is envenomed by the female at oviposition. To evaluate the influence of variations in host quality on the growth, development and fitness of D. carpenteri, we varied the size and developmental stage of the primary parasitoid host (Aphidius ervi Haliday), which was reared on different instars of pea aphid [Acyrthosiphon pisum (Harris)] and English grain aphid [Sitobion avenae (F.)]. Within each kind of host, females eclosed from the relatively larger mummies, while males eclosed from the smaller mummies. Host size and hyperparasitoid size were correlated, and females were larger than males. In hyperparasitoids developing on prepupal and pupal hosts, development time from oviposition to adult eclosion was proportional to size; females required more time for development than males. The mean relative growth rate was the same in males and females and increased with host quality, as predicted by the growth model of Mackauer and Sequeira (1993) for idiobiont parasitoids. Larvae developing on late-pupal stages and pharate adults of A. ervi were unable to consume sclerotized host tissues; they were smaller and needed more time for development. The average number of mature eggs at eclosion was six, except in females developing on suboptimal hosts, which contained only one egg or none. Egg volume was correlated with female size, possibly reflecting differences in larval ontogeny. We provide equations describing the relationship between host quality as indexed by hind-tibia length of the mummified aphid and adult body size in terms of dry mass, development time and mean relative growth rate of D. carpenteri. We discuss the usefulness of host size as a proxy of host quality for idiobiont parasitoids, and provide examples of exceptions. Received: 14 December 1997 / Accepted: 23 July 1998     

Role of Caterpillar Silk Thread in Location of Host Pupae by the Parasitoid <Emphasis Type="Italic">Diadromus pulchellus</Emphasis>

Diadromus pulchellus is a solitary ichneumonid parasitoid. Its only known host is the pupa of Acrolepiopsis assectella, a specialist herbivore of Allium species. D. pulchellus females parasitize A. assectella pupae within 48 h after the caterpillars spin their cocoon and begin to pupate. Having observed that the cocoon produced by the leek moth caterpillar stimulates parasitoid egg-laying and that caterpillar leaves a silk thread, we studied the hypothesis that silk thread might be involved in host-finding by the parasitoid. Behavioral tests showed that when D. pulchellus females encounter a host silk thread, they change directions, follow the thread, and quickly locate the host. These findings show that pupal parasitoids can use signals produced by their hosts at the developmental instar preceding the one that they parasitize.     

Fitness of Encarsia sophia (Hymenoptera: Aphelinidae) parasitizing Trialeurodes vaporariorum and Bemisia tabaci (Hemiptera: Aleyrodidae)

Abstract Fitness and efficacy of Encarsia sophia (Girault & Dodd) (Hymenoptera: Aphelinidae) as a biological control agent was compared on two species of whitefly (Hemiptera: Aleyrodidae) hosts, the relatively smaller sweetpotato whitefly, Bemisia tabaci (Gennadius) biotype ‘B’, and the larger greenhouse whitefly, Trialeurodes vaporariorum (Westwood). Significant differences were observed on green bean (Phaseolus vulgaris L.) in the laboratory at 27 ± 2°C, 55%± 5% RH, and a photoperiod of 14: 10 h (L: D). Adult parasitoids emerging from T. vaporariorum were larger than those emerging from B. tabaci, and almost all biological parameters of E. sophia parasitizing the larger host species were superior except for the developmental times of the parasitoids that were similar when parasitizing the two host species. Furthermore, parasitoids emerging from T. vaporariorum parasitized more of these hosts than did parasitoids emerging from B. tabaci. We conclude that E. sophia reared from larger hosts had better fitness than from smaller hosts. Those from either host also preferred the larger host for oviposition but were just as effective on smaller hosts. Therefore, larger hosts tended to produce better parasitoids than smaller hosts.     

Mating and host density affect host feeding and parasitism in two species of whitefly parasitoids

Abstract The parasitoids in the genera of Encarsia and Eretmocerus (Hymenoptera: Aphelinidae) are important biological control agents of whiteflies, and some of them not only parasitize hosts but also kill them with strong host‐feeding capacity. Two whitefly parasitoid species, Encarsia sophia and Eretmocerus melanoscutus were examined to determine if mating and host density affected their host feeding and parasitism. The whitefly host, Bemisia tabaci, was presented to these two wasp species in densities of 10, 20, 30, 40, 50 and 60 third‐instar nymphs per clip cage. Mated whitefly parasitoid females fed on more hosts than unmated females under a range of host densities (under all six host densities for En. sophia; under the densities of 40 nymphs or more for Er. melanoscutus). Meanwhile, mated females parasitized more whitefly nymphs than unmated females under all host densities for both species. With increase of host density, mated or unmated Er. melanoscutus females killed more hosts by host feeding and parasitism. Mated En. sophia females killed more hosts by host feeding with increase of host density, whereas unmated females did not parasitze whitefly nymphs at all. Our results suggest that only mated female parasitoids with host‐feeding behavior should be released in crop systems to increase their bio‐control efficiency.     

The influence of plant species on attraction and host acceptance inCotesia glomerata (Hymenoptera: Braconidae)

Females of the larval parasitoidCotesia glomerata (L.) use plant-associated cues to locate their lepidopteran host,Pieris rapae L. In this study we investigated the influence of four host plant species,Brassica oleracea var.acephala (‘Vates’ kale),Tropaeolum majus (nasturtium),Lunaria annua (honesty), andCleome spinosa (spider flower), on two components of the host selection process inC. glomerata, namely, attraction and host acceptance. Choice tests in a flight tunnel showed that parasitoids were attracted to some host plant species more than to others in the absence of host larvae.B. oleracea was the most attractive plant species, followed byL. annua, T. majus, andC. spinosa. In previous studies it was shown thatB. oleracea carries highly suitable hosts forC. glomerata and that, in the field, parasitization rates on this plant were the highest. When host larvae were reared on the four host plant species and then transferred to a common substrate (B. oleracea var.capitata, cabbage), plant species that had served as diet for the hosts did not have a significant effect on acceptance for parasitization. Thus, parasitoids were attracted to host plant species differentially, but they did not discriminate among host larvae based on the dietary history of their hosts. ForC. glomerata, it appears that phytochemistry mediates host selection more by influencing parasitoid attraction than it does by affecting host acceptance.     

A re-examination of host relations in the Aphelinidae (Hymenoptera: Ghalcidoidea)

Host relations among the Aphelinidae (Hymenoptera: Chalcidoidea) are highly intriguing from an evolutionary view. Females are usually primary endoparasitoids of whiteflies or scale insects, whereas the development of the male is different and has been used for classification. In heteronomous hyperparasitoids (adelphoparasitoids or autoparasitoids) the male develops as a hyperparasitoid of conspecific females or other endoparasitoid species. We review the consequences of this mode of development which is unique because decisions concerning host selection are inextricably linked with progeny sex ratio. Autoparasitoid field sex ratios can fluctuate dramatically concurrent with changes in the relative availability of male and female hosts. A recent adaptive explanation for these sex ratios involves understanding the reproductive constraints acting on heteronomous parasitoids. Host relations in these parasitoids can show a degree of plasticity. We argue that in many instances autoparasitism may be facultative in nature and should not be used for classification. Heterotrophic parasitism, wherein the male develops as a primary parasitoid of lepidopterous eggs, has been poorly understood in the past due to uncertainties in reports of the biology and taxonomy of heterotrophic parasitoids. The host relations of this group are clarified.     

A chalcid wasp acts chiefly as a hyperparasitoid by mostly using small uncommon hosts

Although ovipositing insects may predominantly use resources that lead to high offspring quality, exceptions to this rule have considerably aided understanding of oviposition decisions. We report the frequency of host species use by a solitary facultative hyperparasitoid, Brachymeria subrugosa Blanchard (Hymenoptera: Chalcididae). In our samples, the wasp attacks the large pupae of the moth Gonioterma indecora Zeller (Lepidoptera: Elachistidae), as well as the considerably smaller, and rarer, pupae of two of its other parasitoids. Consistent with conditional sex allocation models, the wasp produced mainly female offspring on the largest (moth) host, an unbiased sex ratio on the middle‐sized (parasitoid) host, and only males on the smallest (parasitoid) host. Adult offspring size was correlated with the size of the host attacked. These features strongly suggest that the two smaller, primary parasitoid, hosts produce lower‐quality offspring. Despite being more common, the proportion of hosts from which parasitoids emerged was lowest (14%) on the largest host species, and highest on the rarer middle‐sized (34%) and smallest (30%) hosts. This suggests that costs or constraints on attacking high‐quality primary hosts may be a selective force favouring the evolution of hyperparasitism.     

Sex ratio regulation in the waspEupelmus vuilleti (crwf.), an ectoparasitoid on bean weevil larvae (Hymenoptera: Pteromalidae)

In Africa, the seeds and/or pods ofVigna unguiculata andVigna radiata (Papilionacea) are attacked in fields and storage structures by bruchid beetlesCallosobruchus maculatus andBruchidius atrolineatus, on which parasitoid communities can develop. One of these parasitoids is the solitary ectoparasitoidEupelmus vuilleti (Eupelmidae). The storage conditions ofV. unguiculata andV. radiata favor the overlapping of all host stages during several months. These conditions suggest that female parasitoids would vary the sex ratio of their offspring according to the different sizes or developmental stages of hosts. The sex ratio ofE. vuilleti was strongly related to the developmental stage of the hostC. maculatus. Under our experimental conditions, where superparasitism is rare, the proportion of daughters varied between 5 and 25% on the third larval stage but reached 70 to 90% on the pupae. The increase in the proportion of daughters was also observed in the absence of superparasitism. In this case, there was an absolute coincidence between the sex ratio of eggs laid and that of emerged adults. Manipulation of the sex of the egg by the females seems to take place at the time of the egg's fertilization. The relation between host weight and egg sex showed that the male eggs are preferentially laid on lighter host larvae and the female eggs on heavier ones.     

An exotic parasitoid provides an invasional lifeline for native parasitoids

The introduction of an exotic species may alter food webs within the ecosystem and significantly affect the biodiversity of indigenous species at different trophic levels. It has been postulated that recent introduction of the brown marmorated stinkbug (Halyomorpha halys (Stål)) represents an evolutionary trap for native parasitoids, as they accept H. halys egg masses as a host but produce no viable progeny. Interspecific interactions between European egg parasitoid, Trissolcus cultratus (Mayr), and an Asian parasitoid, Trissolcus japonicus (Ashmead), were assessed by providing egg masses to T. cultratus at various time intervals following the initial parasitization by T. japonicus. The suitability of the host for the parasitoid development was re‐assessed by providing T. cultratus with fresh and frozen egg masses of various ages. The likelihood of T. cultratus being able to attack previously parasitized egg masses was determined by assessing the duration of egg mass guarding behavior by T. japonicus following parasitization. The results of experiments examining the interspecific interactions between a native European egg parasitoid, T. cultratus, and an Asian parasitoid, T. japonicus (a candidate for the biological control of H. halys), showed that the native species can act as facultative hyperparasitoid of the exotic one. Although this is only possible during certain stages of T. japonicus development, the presence of the introduced parasitoid may reduce the impact of the evolutionary trap for indigenous parasitoid species. There is a possibility that the occurrence of facultative hyperparasitism between scelionid parasitoids associated with stinkbugs is common. This resulting intraguild predation could promote conservation and stabilization of natural communities by impacting the diversity and population dynamics of native stinkbugs and their parasitoids (e.g., by allowing native parasitoids to avoid wasting reproductive effort on unsuitable hosts), or reduce success of biological control programs (e.g., by reducing the population size of the exotic parasitoids).     

Laboratory studies of host finding,acceptance and suitability of the dung-breeding fly,Haematobia thirouxi potans [Dipt.: Muscidae], byAleochara sp. [Col.: Staphylinidae]

Aleochara sp. from southern Africa is a potential candidate for introduction to Australia for control of the Australian buffalo fly,Haematobia irritans exigua De Meijere. Aspects of its host searching and acceptance behaviour were studied in South Africa usingHaematobia thirouxi potans (Bezzi), to assess, in part, its potential value as a biological control agent and to provide a basis for the development of a mass rearing technique. First instars ofAleochara sp. were found to parasitize most effectively those hosts that had buried themselves in the substrate, probably by orienting to a chemical cue and using the tunnels made by the post-feeding larvae. Lower rates of parasitization occurred in pupae placed on the surface of the substrate or buried by the experimenters. They would accept, parasitize and develop on all ages ofH. thirouxi potans pupae, but showed reduced survival in the very oldest ones. Although there was no difference in relative acceptability among hosts of different ages, a higher proportion of younger hosts were parasitized when these had buried themselves, probably due to a tendency of the parasitoids to follow fresher tunnels into the substrate. Survival was high (>80%) at relative humidities from 10–91% but was less than 20% at an R.H. of 100%. In rearing trials, the most economical ratio of parasitoid eggs: hosts was 1∶1.5, with 70% of the parasitoids developing successfully to adults.      

Multiparasitism of stink bug eggs: competitive interactions between Ooencyrtus pityocampae and Trissolcus agriope

Females of Trissolcus agriope (Platygastridae) avoid host (Brachynema signatum) eggs parasitized by conspecifics or Ooencyrtus pityocampae (Encyrtidae), but females of the latter species will superparasitize and multiparasitize, although they mostly attack unparasitized eggs. Females of T. agriope were more efficient in the laboratory and parasitized more hosts in 24 and 48 h. In multiparasitized hosts, O. pityocampae was a superior larval competitor and could complete also development as a facultative hyperparasitoid. When females of both species foraged together, T. agriope parasitized significantly more than O. pityocampae, about 50 % in both cases, although O. pityocampae almost doubled its parasitism rate as the exposure period was lengthened from 24 to 48 h. When O. pityocampae followed T. agriope in sequential foraging bouts, the former species successfully parasitized more hosts than the latter. The advisability of co-releases of both species to improve biological control of first generation B. signatum in Iranian pistachio orchards is discussed.     

Host preference of the facultative hyperparasitoidTetrastichus howardi (Hym.: Eulophidae)

Tetrastichus howardi (Olliff) preferred the lepidopteran hosts,Chilo partellus (Swinhoe) (Pyralidae) andHelicoverpa armigera (Hübner) (Noctuidae) to their parasitoids,Xanthopimpla stemmator (Thunberg) (Hymenoptera: Ichneumonidae) andPalexorista laxa (Curran) (Diptera: Tachinidae). IfT. howardi had previously experienced parasitising a certain host, its preference for that host increased, but not significantly. When reared on a certain host, the preference for that host increased.T. howardi showed no preference to any particular age of its hosts.T. howardi was able to discriminate between parasitised and unparasitised hosts, initially preferring parasitised hosts, but two days later preferring unparasitised hosts.     

Searching strategies and attack rates of parasitoids of the ash bark beetle (Leperisinus varius) and its relevance to biological control

Abstract.

• 1 Point estimates of parasitoid behaviour indicated that braconid parasitoids of the bark beetle, Leperisinus varius, spend 70% of their host searching time stationary on the bark surface, in sharp contrast to the chalcidoid parasitoids which spend 60–70% of their host searching time moving across the bark surface.
• 2 Aggressive encounters between searching parasitoids are common and the data suggest that they do not occur at random.
• 3 The chalcidoid parasitoids, Cheiropachus quadrum and Eurytoma morio, are more frequently involved in aggressive encounters than expected and can displace ovipositing individuals of other species.
• 4 Attack rates for the parasitoids were estimated to vary from two hosts per day for the braconid Coeloides filiformis to four hosts per day for E.morio, and net reproductive rates vaned from 6.3 for C.filiformis to 0.8 for Coeloides melanotus.
• 5 These data suggest that while all parasitoids of the scolytid can act as primary parasitoids, at least some of the chalcidoid parasitoids are facultative cleptoparasitoids and should be avoided in classical biological control introductions.

     

Host switching in cowbird brood parasites: how often does it occur?

Avian obligate brood parasites lay their eggs in nests of host species, which provide all parental care. Brood parasites may be host specialists, if they use one or a few host species, or host generalists, if they parasitize many hosts. Within the latter, strains of host‐specific females might coexist. Although females preferentially parasitize one host, they may occasionally successfully parasitize the nest of another species. These host switching events allow the colonization of new hosts and the expansion of brood parasites into new areas. In this study, we analyse host switching in two parasitic cowbirds, the specialist screaming cowbird (Molothrus rufoaxillaris) and the generalist shiny cowbird (M. bonariensis), and compare the frequency of host switches between these species with different parasitism strategies. Contrary to expected, host switches did not occur more frequently in the generalist than in the specialist brood parasite. We also found that migration between hosts was asymmetrical in most cases and host switches towards one host were more recurrent than backwards, thus differing among hosts within the same species. This might depend on a combination of factors including the rate at which females lay eggs in nests of alternative hosts, fledging success of the chicks in this new host and their subsequent success in parasitizing it.     

Host preference of Xanthopimpla stemmator (Hymenoptera: Ichneumonidae) and its reproductive performance on selected African lepidopteran stem borers

The Asian pupal parasitoid, Xanthopimpla stemmator Thunberg, was imported into East Africa as a classical biological agent of lepidopteran cereal stem borers. Preference of X. stemmator females for four common borers of maize and sorghum; the crambid Chilo partellus (Swinhoe), the pyralid, Eldana saccharina Walker, and the noctuids, Busseola fusca Fuller and Sesamia calamistis Hampson, was investigated. Pre-adult experience of X. stemmator females did not influence choice of host. In dual choice tests, more B. fusca were attacked than E. saccharina, while E. saccharina were attacked more than Ch. partellus. Life table studies on three of the hosts revealed that the intrinsic rate of increase was highest when X. stemmator was reared on S. calamistis. Net reproductive rates, mean generation times and doubling times were not different between hosts. Results suggest that X. stemmator can be successfully reared on the three stem borer species and released in areas where any combination of the three hosts occurs.     

Effect of host deprivation on egg quality,egg load,and oviposition in a solitary parasitoid,Chetogena edwardsii (Diptera: Tachinidae)

Solitary parasitoids are limited to laying one egg per host because larvae compete within hosts. If host encounter rate is low, females should not increase the number of eggs/host in response. The tachinid fly, Chetogena edwardsii,was used to evaluate the effect of host deprivation on egg accumulation, oviposition behavior, and egg quality in a solitary parasitoid. Females deprived of hosts for 2– 7 days accumulate about 1 day's supply of eggs. Egg output of deprived females once hosts are restored does not differ from that of control females. Deprived females retain one egg in the uterus where it undergoes embryogenesis. Maggots emerging from retained eggs are more likely to survive in hosts molting in 40 h or less after receipt of an egg than are maggots emerging from eggs fertilized shortly before oviposition. Egg retention is a consequence of host deprivation that permits females to broaden the range of hosts they can exploit to include soon-to-molt hosts and possibly multiply parasitized hosts.     

Host selection by the autoparasitoid Encarsia pergandiella on primary (Bemisia tabaci) and secondary (Eretmocerus mundus) hosts

In autoparasitoids, females are generally primary endoparasitoids of Hemiptera, while males are hyperparasitoids developing in or on conspecific females or other primary parasitoids. Female‐host acceptance can be influenced by extrinsic and/or intrinsic factors. In this paper, we are concerned with intrinsic factors such as nutritional status, mating status, etc. We observed the behavior of Encarsia pergandiella Howard (Hymenoptera: Aphelinidae) females when parasitizing primary (3rd instar larvae of Bemisia tabaci Gennadius [Homoptera: Aleyrodidae]) and secondary hosts (3rd instar larvae and pupae of Eretmocerus mundus Mercet [Hymenoptera: Aphelinidae]) for a period of 1 h. Females had different reproductive (virgin or mated younger) and physiological (fed elder or mated elder) status. Virgin females killed a large number of secondary hosts while investing a long time per host. However, they did not feed upon them. Mated females killed a lower number of secondary hosts and host feeding was observed in both consuming primary and secondary hosts. It was common to observe host examining females of all physiological statues tested repeatedly stinging the same hosts when parasitizing, killing or rejecting them. Fed elder females parasitized more B. tabaci larvae than E. mundus larvae or pupae, while investing less time on the primary host than on the secondary host. They also parasitized more B. tabaci larvae than mated elder females, while investing less time per host. The access of females to honey allowed them to lay more eggs.