WASP‐ASSOCIATED FACTORS ACT IN INTERSPECIES COMPETITION DURING MULTIPARASITISM

Coexistence or displacement of parasitoids in hosts during intrinsic competitive interactions between different parasitoid species (multiparasitism) may depend on their life history traits and behavior. Intense competition for possession of hosts may lead to the elimination of the inferior competitor through physical attack and/or physiological suppression. However, the mechanisms of physiological suppression during multiparasitism remain unclear. Previous work has shown that first instar larvae of the solitary endoparasitoid Meteorus pulchricornis possess well‐developed mandibles that are used to kill competitors. Two gregarious endoparasitoids, Cotesia kariyai and C. rufricus, share host resources especially when the time gap of oviposition is short. Here, we investigated the physiological influence of wasp‐regulatory factors of the three endoparasitoids, M. pulchricornis, C. kariyai, and C. ruficrus, in their common host Mythimna separata. We found that MpVLP alone (or with venom) deleteriously affected the development of the two gregarious species. Similarly, CkPDV plus venom had toxic effect on M. pulchricornis eggs and immature larvae, although they were not harmful to immature stages of C. ruficrus. Cotesia kariyai and C. ruficrus were able to coexist mainly through the expression of regulatory factors and both could successfully emerge from a multiparasitized host. The injection of CkPDV plus venom after oviposition in L5 host larvae facilitated C. ruficrus development and increased the rate of successful parasitism from 9% to 62%. This suggests that the two gregarious parasitoid wasps exhibit strong phylogenetic affinity, favoring their coexistence and success in multiparasitized hosts.     

小菜蛾主要寄生性天敌——菜蛾绒茧蜂与菜蛾啮小蜂间的相互作用

在28℃下,以小菜蛾3龄幼虫作寄主,研究了菜蛾绒茧蜂与菜蛾啮小蜂间的相互关系．当寄主供2种蜂同时产卵寄生时,与只供1种蜂时相比。绒茧蜂的寄生率无显著变化,而啮小蜂的寄生率则显著下降;2种蜂的合计寄生率与任一种蜂单独存在时相比无显著差异．当寄主先供绒茧蜂寄生,再供啮小蜂寄生时,绒茧蜂的成功寄生率不受影响,而啮小蜂的寄生率仅为8％～13％;啮小蜂能寄生在寄主体内的绒茧蜂高龄幼虫．绒茧蜂能寄生已被啮小蜂寄生的寄主幼虫,其子代部分个体能正常发育至成虫羽化．当已被绒茧蜂寄生和未被寄生的寄主同时存在时,啮小蜂主要寄生未被寄生的寄主．表明绒茧蜂具有竞争优势。但这种优势可因啮小蜂的寄生而被削弱．     

Parasitic castration of Plutella xylostella larvae induced by polydnaviruses and venom of Cotesia vestalis and Diadegma semiclausum

In the present study, we used gamma-ray to irradiate the female parasitoids to make wasp eggs infertile, resulting in pseudoparasitization, which allowed the analysis of maternal secretions such as polydnaviruses (PDVs) and venom in the absence of larval secretions or teratocytes by the growing parasitoids. We then investigated the spermatogenesis and components of testicular proteins of male Plutella xylostella larvae pseudoparasitized by two endoparasitoids (Cotesia vestalis and Diadegma semiclausum). The results showed that pseudoparasitism by the two endoparasitoids at the early third instar host larvae both induced smaller testes in size than those of nonparasitized host larvae. Both of them caused parasitic castration, and the degree of castration is almost as severe as in naturally parasitized hosts. This suggested that PDVs and venom played a major role in the degeneration of host testes. There are significant differences in the degree of castration induced by the two endoparasitoids, with respect to testicular growth, testicular protein concentrations, and histological changes of germ cells. Cotesia vestalis bracovirus always has a significantly stronger effect on host testicular growth and development than D. semiclausum ichnovirus. SDS-PAGE analysis indicated that synthesis of P 65 and P 67 proteins were clearly inhibited in testes of hosts that were pseudoparasitized by C. vestalis while reduction in synthesis of other proteins was not evident.     

Interspecific Relations Between Two Sympatric Species of Hymenoptera, Dinarmus basalis (Rond) and Eupelmus vuilleti (Crw), Ectoparasitoids of the Bruchid Callosobruchus maculatus (F)

Dinarmus basalis (Rond) and Epelmus vuilleti (Crw) are two Hymenopteran species, which are solitary ectoparasitoids of bruchid larvae. In the presence of seeds of Vigna unguiculata (Walp) containing hosts parasitized by E. vuilleti, a high percentage of D. basalis females avoided multiparasitism whatever the age of the eggs or the larvae present on the host. The least avoidance was observed when the hosts were parasitized by E. vuilleti 30 min beforehand. This avoidance behavior is adaptive and is related to the low survival chances of the D. basalis larvae when they are in interspecific competition with E. vuilleti larvae. The analysis of the behavior of D. basalis demonstrated that the avoidance of multiparasitism could be due to the perception of two signals; an external signal deposited on the surface of the seeds during the E. vuilleti oviposition phase and an internal signal due to the presence of the eggs and larvae at the surface of the hosts. E. vuilleti females did not avoid multiparasitism and multiparasitized the hosts bearing D. basalis eggs or larvae. The behavior of E. vuilleti females was not disturbed by the presence of its competitor. Under these conditions of interspecific competition, the survival chances of E. vuilleti larvae were very high whatever the age of its competitor D. basalis. The two species of parasitoids could move in a column containing healthy seeds of V. unguiculata and patches with seeds containing parasitized or unparasitized larvae. The distribution of D. basalis females introduced into these columns depended on the host quality. They avoided the patches containing the hosts parasitized E. vuilleti and were found in the patches with healthy hosts. The behavior of E. vuilleti females was very different; the distribution of the females and the parasitism and multiparasitism rates were not affected by the quality of the hosts present in the patches. The adaptive significance of the behaviors of these two species was analyzed in relation to the survival chances of their offspring.     

Inter- and intra-specific host discrimination in gregarious and solitary endoparasitoid wasps

In nature, most species of Lepidoptera are attacked by parasitoids, and some species may be hosts for several parasitoid species. When hosts are parasitized by more than one female of the same species (=superparasitism) or females of different species (=multiparasitism), then intrinsic competition occurs for control of host resources. To reduce competition, some parasitoids are able to recognize the difference between parasitized and unparasitized hosts. Inter- and intra-specific host discrimination were investigated in the two sympatric species, the gregarious Cotesia kariyai (Watanabe) and solitary Meteorus pulchricornis (Wesmael), endoparasitoids of the Oriental armyworm Mythimna separata (Walker). To measure host discrimination, choice experiments were conducted in which females of both species foraged and chose between healthy host larvae and hosts initially parasitized by either C. kariyai or M. pulchricornis. An olfactory test was also performed to examine the discrimination behavior of the two parasitoids. Our results showed that, in oviposition choice tests, both braconid female wasps were able to discriminate between unparasitized hosts and from four to seven day-old hosts previously attacked by conspecific and heterospecific wasps. On the other hand, superparasitism and multiparasitism occurred even in host larvae that were parasitized two days earlier. However, once the immature parasitoids hosts are at larval stage (1st and 2nd instar), super- and multiparasitism were avoided in the two-choice test, but the latter often occurred in the multiple-choice experiment. Host discrimination abilities may have been based on plant volatile signals incurred from damaged plants and internal mechanisms from four to seven post-parasitized hosts.     

Inhibition of juvenile hormone esterase activity in Lymantria dispar (Lepidoptera, Lymantriidae) larvae parasitized by Glyptapanteles liparidis (Hymenoptera, Braconidae)

Glyptapanteles liparidis is a gregarious, polydnavirus (PDV)-carrying braconid wasp that parasitizes larval stages of Lymantria dispar. In previous studies we showed that parasitized hosts dramatically increase juvenile hormone (JH) titers, whereas JH degradation is significantly inhibited in the hemolymph. Here we (i) quantified the effects of parasitism on JH esterase (JHE) activity in hemolymph and fat body of penultimate and final instars of L. dispar hosts and (ii) assessed the relative contribution of individual and combined wasp factors (PDV/venom, teratocytes, and wasp larvae) to the inhibition of host JHE activity. The effects of PDV/venom was investigated through the use of gamma-irradiated wasps, which lay non-viable eggs (leading to pseudoparasitization), while the effects of teratocytes and wasp larvae were examined by injection or insertion of these two components in either control or pseudoparasitized L. dispar larvae. Parasitism strongly suppressed host JHE activity in both hemolymph and fat body irrespective of whether the host was parasitized early (premolt-third instar) or late (mid-fourth instar). Down-regulation of JHE activity is primarily due to the injection of PDV/venom at the time of oviposition, with only very small additive effects of teratocytes and wasp larvae under certain experimental conditions. We compare the results with those reported earlier for L. dispar larvae parasitized by G. liparidis and discuss the possible role of JH alterations in host development disruption.     

The endoparasitoid Psyllaephagus arenarius benefits from ectoparasitic venom via multiparasitism with the ectoparasitoid Tamarixia lyciumi

As solitary nymphal parasitoids of Paratrioza sinica, the ectoparasitoid Tamarixia lyciumi and the endoparasitoid Psyllaephagus arenarius act as effective biocontrol agents. Thus, it is necessary to facilitate mass productions of both species. Despite showing an excellent parasitic ability, Ps. arenarius is often trapped fatally inside 5th‐instar nymphs of Pa. sinica due to strong host immunity. To improve the emergence rate of Ps. arenarius, we evaluated whether Ps. arenarius could utilize T. lyciumi venom via multiparasitism, so the parasitism characteristics of both species were examined between separate‐existence (monoparasitism only) and co‐existence (mono‐ and multiparasitism) systems. Further, the parasitism characteristics of Ps. arenarius on venom‐injected hosts with/without T. lyciumi eggs were tested to further identify the facilitator. The results showed the parasitism rate of T. lyciumi was increased while that of Ps. arenarius did not change from separate‐existence to co‐existence systems. The intrinsic performances of two species in monoparasitism did not differ between separate‐ and co‐existence systems. From monoparasitism (separate‐existence) to multiparasitism (co‐existence), no differences were detected in the intrinsic performances of T. lyciumi, but those of Ps. arenarius were greatly improved. After T. lyciumi venom injection, the parasitism characteristics of Ps. arenarius did not differ between venom‐injected hosts with T. lyciumi eggs and those without, further indicating Ps. arenarius benefited from the venom of T. lyciumi females rather than T. lyciumi egg/larval secretions. Instead of negative effects, multiparasitism with ectoparasitoids improves endoparasitoids due to ectoparasitic venom. The study increases host resource utilization and provides creative ways for mass production of endoparasitoids.     

The classification, evolution and biology of the Costa Rican species of Cryptophion (Hymenoptera: Ichneumonidae)

The six known Costa Rican species of the campoplegine ichneumonid genus Cryptophion Viereck are described and keyed. The distribution of species throughout Costa Rica is detailed based on data gathered by an intensive Malaise trap survey of the ichneumonids of the country. Five new species are recognized: C. espinozai. C. guillermoi, C. manueli. C. moragai and C. tickelli , and a sixth, C. inaequalipes (Cresson) redescribed. The monophyly of the genus is demonstrated and the phylogeny of the Costa Rican species reconstructed. Host relationships have been established for all species in Costa Rica; they develop as koinobiont endoparasitoids of first to third instar larvae of Sphingidae or Saturniidae. Analysis of the host relationships from a phylogenetic perspective suggested that the genus first evolved using macroglossine sphingid larvae feeding on rubiaceous understorey plants as hosts, and subsequently diversified to utilize sphingine sphingids and ceratocampine saturniids feeding on a variety of food-plants. Most species appear to be monophagous and oligophagy is apparently a derived feature of one sister-species pair, C. espinozai and C. manueli . In Santa Rosa National Park, Costa Rica, only a small proportion of the species of Saturniidae and Sphingidae present are used as hosts by Cryptophion species. No one host species is parasitized by more than one Cryptophion species. No species of Cryptophion is known to parasitize more than one host species feeding on any one plant species. The monophagous species C. inaequalipes is only known to develop in its sphingid host when that host is feeding on one of its two alternative food-plants.     

Intra- and interspecific competition by Fopius arisanus and Diachasmimorpha tryoni (Hymenoptera: Braconidae), parasitoids of tephritid fruit flies

Fopius arisanus (Sonan) and Diachasmimorpha tryoni (Cameron) are two important solitary endoparasitoids of tephritid fruit flies. The former species attacks host eggs while the latter attacks host larvae, and both species emerge as adults from the host puparium. This study investigated intrinsic competition between these two parasitoids, as well as aspects of intraspecific competition within each species in the Mediterranean fruit fly, Ceratitis capitata (Wiedemann). Parasitization by F. arisanus resulted in direct mortality of host eggs and prolonged development of host eggs and larvae. Superparasitism by F. arisanus was uncommon when mean parasitism per host patch was <50%, but increased with rising rates of parasitism. Superparasitism by D. tryoni was more common. In superparasitized hosts, supernumerary individuals of F. arisanus were killed through physiological suppression, while supernumerary larvae of D. tryoni were killed mainly through physical attack. In multiparasitized hosts, dissections showed that 81.6% of D. tryoni eggs in the presence of F. arisanus larvae died within 3 days, indicating physiological inhibition of egg hatch. Rearing results further showed that F. arisanus won almost all competitions against D. tryoni. The ratio of D. tryoni stings to ovipositions was lower in hosts not previously parasitized by F. arisanus than in parasitized hosts, suggesting that D. tryoni can discriminate against parasitized hosts. The mechanism that F. arisanus employs to eliminate D. tryoni is similar to that it uses against all other larval fruit fly parasitoids so far reported. The results are discussed in relation to the competitive superiority of early acting species in fruit fly parasitoids, and to a possible competitive-mediated mechanism underlying host shift by D. tryoni to attack non-target flies following the successful introduction of F. arisanus in Hawaii.     

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.     

Larval competition between Aphidius ervi and Praon volucre (Hymenoptera: Braconidae: Aphidiinae) in Macrosiphum euphorbiae (Hemiptera: Aphididae)

Interspecific competition between parasitoid larvae may influence the size, structure, and stability of the population, leading to a reduction in total parasitism and thus restricting the pest control. Aphidius ervi (Haliday) and Praon volucre (Haliday) are endoparasitoids that possess a wide host range and present considerable potential for the biological control of the aphid Macrosiphum euphorbiae (Thomas). The larval competition between A. ervi and P. volucre, and the possible intrinsic competitive superiority of one of the parasitoids in M. euphorbiae, have been studied. In single parasitism experiments, mated parasitoid females (n=10) were maintained individually in contact with M. euphorbiae hosts (n=30) inside petri dishes containing lettuce leaf discs and maintained in environmental chamber at 22 ± 1°C, 70 ± 10% RH, and 12-h photophase. The multiple parasitism experiments consisted of exposing single parasitized aphids (n=120) to the second parasitoid species. Two oviposition events were performed with a 4-h interval between them, namely the following: sequence A (oviposition by A. ervi, followed by P. volucre) and sequence B (oviposition by P. volucre, followed by A. ervi). Oviposition sequence A generated 24 A. ervi and 55 P. volucre adults, whereas oviposition sequence B generated 23 and 49 adults. P. volucre is an intrinsically superior competitor compared with A. ervi, and the use of the two species simultaneously may result in competitive exclusion and influence the stability of the parasitoid population.     

闭弯尾姬蜂与菜蛾盘绒茧蜂寄生菜蛾幼虫时的种间竞争

在室内25℃下,以菜蛾3龄初幼虫作寄主,研究了菜蛾盘绒茧蜂Cotesia plutellae和半闭弯尾姬蜂Diadegma semiclausum的种间竞争。当寄主供2种蜂同时产卵寄生时,2种蜂各自的寄生率与其单独寄生时无显著差异,合计寄生率比一种蜂单独存在时有所提高,但差异不显著。2种蜂均能产卵寄生已被另一种蜂寄生了的寄主幼虫。当寄主被2种蜂寄生的间隔时间很短（少于10 h）时,所育出的蜂绝大部分（80%以上）为绒茧蜂;当寄主先被绒茧蜂寄生,并饲养2天以上再供弯尾姬蜂寄生时,所育出的全为绒茧蜂;当寄主先被弯尾姬蜂寄生,并饲养2天以上再供绒茧蜂寄生时,寄主幼虫绝大部分不能存活,只有少部分能育出寄生蜂,且多为弯尾姬蜂。当2种蜂的幼虫存在于同一寄主体内时,2种蜂的发育均受到另一种蜂的抑制;绒茧蜂1龄幼虫具有物理攻击能力,能将弯尾姬蜂卵或幼虫致死。这些结果表明,菜蛾盘绒茧蜂与半闭弯尾姬蜂在同一寄主中发育时,前者具有明显的竞争优势。     

Mechanism of reduction in the number of the circulating hemocytes in the Pseudaletia separata host parasitized by Cotesia kariyai

Larval endoparasitoids can avoid the immune response of the host by the function of polydnavirus (PDV) and venom. PDV infects hemocytes and affects the hemocyte function of the host. In this paper, we investigated how PDV and venom affect the hemocyte population of the host. Cotesia kariyai, the larval endoparasitoid, lowers the hemocyte population of the noctuid host larvae soon after parasitization. The reduction in the number of circulating hemocytes is caused by the breakdown of the circulating hemocytes and of the hematopoietic organ which generates the circulating hemocytes. The decrease in the number of hemocytes shortly after parasitization is a response to the venom. However, the decrease in hemocyte population on and after 6 h post-parasitization appears to be caused by the PDV. Apoptosis in circulating hemocytes was observed on and after 6 h post-injection of PDV plus venom. It was revealed through cytometry that mitosis of circulating hemocytes was halted within 24 h after the injection of PDV plus venom. Apoptosis in the hematopoietic organ was induced 12 h after the injection of PDV plus venom. Furthermore, the plasma from the hosts injected with PDV plus venom depressed the number of hemocytes released from the hemotopoiteic organs.     

二种寄生蜂寄生对不同虫龄小菜蛾精巢的影响

为探明寄生蜂引起寄主寄生性去势的机制,本文选取携带不同多分DNA病毒(Polydnavirus,PDV)的2种内寄生蜂与共同寄主小菜蛾Plutella xylostella(L.)为寄生体系,研究不同虫龄小菜蛾被寄生后雄性生精细胞、精子束形态和精巢发育体积变化,系统比较寄生性去势程度和分别拥有2类PDV的寄生蜂在寄主精子发生和形成过程中的作用。结果表明:携带Bracovirus PDV的菜蛾盘绒茧蜂Cotesiave stalis(Haliday)或拥有Ichvovirus PDV的半闭弯尾姬蜂Diadegma semiclausum Hellén寄生对不同虫龄小菜蛾的精子发生和形成过程均产生明显的抑制作用,表现为不能产生精子束或精子束数量减少,但抑制程度以寄生低龄寄主时最明显。2种蜂寄生均能抑制小菜蛾精巢体积的增长,但对低龄寄主的抑制程度明显强于高龄寄主,寄生性去势程度取决于寄生时寄主虫龄。相比而言,寄生不同虫龄小菜蛾时,茧蜂引起小菜蛾寄生性去势的程度均强于姬蜂。     

The biology of Diadromus collaris (Hymenoptera: Ichneumonidae), a pupal parasitoid of Plutella xylostella (Lepidoptera: Plutellidae), and its interactions with Oomyzus sokolowskii (Hymenoptera: Eulophidae)

The ichneumonid Diadromus collaris (Gravenhorst) has been recorded in many parts of the world as an important parasitoid of the diamondback moth, Plutella xylostella (Linnaeus), a serious pest of brassica vegetable crops worldwide. Some aspects of its biology and its interactions with Oomyzus sokolowskii (Kurdjumov), another major parasitoid of the same pest, were studied in the laboratory. At 25 degrees C, female wasps did not have mature eggs in their ovaries until about 12 h after emergence. Both males and females mated successfully 24-48 h after emergence, and females started to oviposit one to two days after emergence. Unmated females produced male progeny only; mated females produced progeny of both sexes. The development rate of the parasitoid increased linearly with temperature from 15 to 30 degrees C, with an estimated low temperature threshold of 7.4 degrees C and a thermal constant of 225.1 day-degrees for development from egg to adulthood. Rates of survival from larva to adulthood were about 90% between 20 and 28 degrees C and decreased as temperature decreased or increased. No immatures survived to adulthood at 35 degrees C. When provided with honey solution, the females lived on average 8.3, 11.5 and 7.0 days, and parasitized 26, 44 and 46 host pupae at 20, 25 and 30 degrees C, respectively. Female wasps could be stored at 15 degrees C for up to four weeks without detrimental effects on reproduction. Females of D. collaris attacked host pupae already parasitized by O. sokolowskii, inserting their ovipositor into the hosts at a similar frequency as into unparasitized host pupae, but they did not lay eggs inside the hosts.     

Egg Size, Intrinsic Competition, and Lethal Interference in the Parasitoids Encarsia pergandiella and Encarsia formosa

Recent population dynamic theory predicts that disruption of biological control may occur when one parasitoid species' superiority in intrinsic competition is associated with a lower ability to find and exploit hosts (i.e., ability in extrinsic competition). One might expect such a trade-off, for instance, if parasitoids with larger (and fewer) eggs are more likely to prevail in intrinsic competition than species with smaller (and more numerous) eggs. We tested the idea that relative egg size could be used to predict the outcome of intrinsic competition in two closely related endoparasitoids, Encarsia pergandiella Howard and Encarsia formosa Gahan. Contrary to expectation, the parasitoid species with smaller eggs, E. pergandiella, prevailed in intrinsic competition, regardless of the order that hosts were exposed to the two species. In a literature survey, we found four studies of competing pairs of endoparasitoid species for which: (a) egg size estimates were available and (b) one species was consistently superior in intrinsic competition. In three of the four studies, the small-egged species prevailed in intrinsic competition, as we also found. Although E. formosa lost in intrinsic competition, this species negatively affected E. pergandiella's progeny production by host feeding on and killing hosts containing E. pergandiella eggs. E. formosa females also host fed on conspecific-parasitized hosts. As a mechanism of both intra- and interspecific interference competition, host feeding on parasitized hosts contradicts assumptions about the nature of interference competition in existing population dynamics models.     

Interspecific host discrimination and within-host competition between Encarsia formosa and E. pergandiella (Hymenoptera: Aphelinidae), two endoparasitoids of whiteflies (Hemiptera: Aleyrodidae)

Interspecific host discrimination and within-host competition between Encarsia formosa Gahan and Encarsia pergandiella (Howard), two endoparasitoids of whiteflies, were studied under laboratory conditions. Interspecific host discrimination was studied at two time intervals (0 h and 72 h after the first species had oviposited). Parasitized and unparasitized Trialeurodes vaporariorum (Westwood) hosts were accepted for oviposition at the same rate by the two parasitoid species. Host type did not affect the handling time of the two parasitoids. The outcome of within-host competition was investigated after females of the two species parasitized the hosts at various time intervals. In four treatments, E. pergandiella was allowed to oviposit 0, 24, 48 and 72 h after E. formosa while in the other two, E. formosa was allowed to oviposit 0 and 72 h after E. pergandiella. In four of these treatments: E. formosa following E. pergandiella at 0 and 72 h, and E. pergandiella following E. formosa at 0 and 24 h, E. pergandiella prevailed. In the host discrimination experiment (72 h interval), 20% of E. pergandiella eggs were killed by E. formosa females. Interspecific ovicide was also observed in the within-host competition experiment, in which 6% of 72-h-old E. pergandiella eggs were killed by E. formosa females.     

Phenology of Parasetigena silvestris (Diptera: Tachinidae), gypsy moth (Lymantria dispar) (Lepidoptera: Lymantriidae) larval parasitoid and its efficiency for parasitisation

Parasetigena silvestris is a univoltine, solitary, larval endoparasitoid which lays its eggs on the surface of gypsy moth larvae. Field collection of the host larvae (2nd through 5th instar) from an artificially established gypsy moth population were made to compare stage specific parasitism between larvae without and with P. silvestris tachinid eggs. The tachinid oviposition rate detected was highest in second instar larvae, and then decreased as larvae developed toward full maturity. The opposite was true for tachinid parasitoid emergence which had no emergence from second through third host instar larvae. Fourth instar gypsy moth larvae, however, experienced significantly higher parasitism by P. silvestris in the larvae with eggs than those without the eggs. The braconid wasp Cotesia melanoscelus caused significantly higher parasitism in early instar larvae with P. silvestris eggs than in those without the eggs. The tachinid prefers to lay more eggs on parasitised larvae by the braconid even though the braconid is a superior competitor to the fly during multiparasitism. Factors influencing parasitism rates by P. silvestris such as host-parasitoid synchronisation and the multiparasitism interaction with C. melanoscelus are discussed.     

Seasonal abundance of the parasitoid complex associated with the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) in Hangzhou, China

An investigation of insect parasitoids of the diamondback moth, Plutella xylostella (Linnaeus), in brassica vegetable crops in the suburbs of Hangzhou was conducted during five periods from 1989 to 1997. Eight species of primary parasitoids were recorded: Trichogramma chilonis Ishii, Cotesia plutellae Kurdjumov, Microplitis sp., Oomyzus sokolowskii Kurdjumov, Diadromus collaris (Gravenhorst), Itoplectis naranyae (Ashmead), Exochus sp. and Brachymeria excarinata Gahan. Seven species of hyperparasitoids were also collected. Rates of parasitism of eggs of P. xylostella were usually very low. However, rates of parasitism of larvae and pupae were substantial and showed two peaks each year, around June-July and September-November respectively. Rates of parasitism during peaks were usually 10-60% and reached over 80% on a few occasions. Cotesia plutellae, O. sokolowskii and D. collaris were the major larval, larval-pupal and pupal parasitoids respectively. In the field, C. plutellae was active throughout the year. Oomyzus sokolowskii was active from May to October, entered a quiescent pupal stage in October-November to overwinter and did not emerge until next April-May. Diadromus collaris was recorded from April to July and October. Rates of parasitism of P. xylostella in radish and mustard fields were usually higher than those in cabbage and Chinese cabbage fields in the same locality. Negative correlations of parasitism rates between C. plutellae and O. sokolowskii indicate a competitive relationship for host larvae between these two larval parasitoids.