Efficiency,intrinsic competition and interspecific host discrimination of Copidosoma desantisi and Trichogramma evanescens,two parasitoids of Phthorimaea operculella

Trichogramma evanescens West. (Hymenoptera: Trichogrammatidae) and Copidosoma desantisi Annecke & Mynhardt (Hymenoptera: Encyrtidae) are potential parasitoids of the potato tuber moth (PTM), Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae) in Egypt. Discrimination of a parasitized host from an unparasitized host would prevent wasting of time, eggs and reduce competition with conspecifics or heterospecifics. Therefore, we evaluated interspecific host discrimination, multiparasitism and intrinsic competition between the two wasp species. In a choice test, females of T. evanescens showed high interspecific host discrimination only when they were offered 2-day-old C. desantisi parasitized and unparasitized PTM eggs. In contrast, C. desantisi showed high host discrimination and preferred unparasitized eggs to PTM eggs harboring 2-h- or 2-day-old T. evanescens’ eggs. We also evaluated the effect of different introduction sequences on the efficacy of the two wasps. Dissection data indicated that the two parasitoids had a negative impact on each other. There was a significant reduction in the total number of deposited eggs as well as total number of parasitized hosts by each parasitoid. Regarding the rearing experiment, the total number of T. evanescens-induced black eggs or C. desantisi formed mummies in combined treatments was significantly lower than in single parasitoid treatments (control). Moreover, C. desantisi was inferior and did not develop from any multiparasitized host regardless of oviposition order. It was suggested that combined release of the two wasps would not elevate rate of parasitism over that of single parasitoid treatments and competition between them would reduce their efficacy.     

Larval amphibian growth and development under varying density: are parasitized individuals poor competitors?

Population density and infection with parasites often are important factors affecting the growth and development of individuals. How these factors co-occur and interact in nature should have important consequences for individual fitness and higher-order phenomena, such as population dynamics of hosts and their interactions with other species. However, few studies have examined the joint effects of density and parasitism on host growth and development. We examined the co-influences of rearing density and parasitism, by the trematode Echinostoma trivolvis, on the growth and development of larval frogs, Rana (=Lithobates) pipiens. We also examined the potential role of parasite-mediated intraspecific competition by observing how unparasitized individuals performed when housed with other unparasitized tadpoles, versus housing with a combination of unparasitized and parasitized hosts. Mean mass and mean developmental stage were reduced under high rearing densities. The presence of parasitized conspecifics had no significant effect, but there was a significant interaction of density and parasitism presence on host mass, due to the fact that parasitized conspecifics grew poorly at high densities. Unparasitized individuals reared with parasitized and unparasitized conspecifics fared no better than unparasitized individuals reared only with one another. This result indicates that infected hosts compete as much as uninfected hosts for resources, even though infected individuals have reduced mass under high-density conditions. Resource acquisition and resource allocation are different processes, and parasitism, if it only affects the latter, might not have a discernible impact on competitive interactions.     

Host Handling Time in a Polyembryonic Wasp is Affected both by Previous Experience and by Host State (Parasitized or Not)

Foraging behavior for hosts in parasitoids resembles that of predators with respect to finding, evaluating and manipulating of the prey. Host handling time may depend on the life history of the parasitoid and can be affected by oviposition experience. Additionally, handling time can be affected by host aggregation, species, size and state (parasitized or not). We studied handling times in the egg-larval parasitoid wasp Copidosoma koehleri. We allowed naïve female wasps to oviposit into three consecutive unparasitized hosts, and measured time until oviposition, and the duration of ovipositor insertion. We recorded the same data for naïve females ovipositing into already parasitized hosts. We found that both previous experience by females and previous parasitism of hosts reduced handling time. The results suggest that host handling durations reflect the interplay between host state and parasitoid internal state.     

Interactions between parasitized and unparasitized conspecifics: parasitoids modulate competitive dynamics

Parasitism influences many aspects of a host's behavior and physiology. Therefore, parasitism is also likely to influence the competitive ability of the host. Field populations of phytophagous insects are often a mix of parasitized and unparasitized conspecifics and the inclusion of parasitism in their competitive dynamics may alter expected outcomes. We investigated the influence of parasitism by the hymenopteran parasitoid Phanerotoma franklini Gahan on the competitive interactions among larvae of its host Acrobasis vaccinii Riley. We found that parasitized larvae were poorer competitors and required less food to complete development compared to unparasitized larvae. To examine the influence of parasitism on the competitive dynamics of this system, we constructed an individual-based model parameterized with our laboratory data. The model examined the role of resource availability and parasitism rate on larval survival. The model suggests that parasitized larvae (and, hence parasitoids) experience higher levels of mortality from competition than unparasitized larvae. Further, the model also suggests that the decreased consumption of resources by parasitized larvae results in a decline in the occurrence of competition as the parasitism rate increases. We suggest that these observations may be general to many parasitoid-host systems.     

Host location and discrimination mediated through olfactory stimuli in two species of Encyrtidae

The role of olfactory stimuli in host detection and evaluation was studied in two encyrtid Hymenoptera. The first, Epidinocarsis lopezi De Santis, is a monophagous parasitoid of the cassava mealybug Phenacoccus manihoti Matile-Ferrero, itself feeding exclusively on cassava, Manihot esculenta Crantz. The second, Leptomastix dactylopii Howard, is a monophagous parasitoid of the Citrus mealybug, Planococcus citri Risso, but this latter is highly polyphagous. The behaviour of females of both parasitoids (attaction and locomotion) was compared in a tubular olfactometer for the odours of their respective hosts on cassava and poinsettia. Tests were made using: 1) healthy host-plant alone; 2) host-plant infested with unparasitized mealybugs; 3) unparasitized mealybugs only; 4) host-plant infested with parasitized mealybugs and 5) parasitized mealybugs only. Only E. lopezi was attracted by the odour of the host-plant alone, but both species were attracted by the odour of an infested host-plant and that of unparasitized mealybugs. The odour of parasitized mealybugs, alone or on host-plant, induced an undirected activity. The attraction of E. lopezi to the odour of the host-plant alone could be linked to the monophagous diet of its host, whereas the attraction of the two species of parasitoids to the odours of infested host-plants and unparasitized mealybugs could be due to the fact that both parasitoids are specialists. The behavioural response of both species to the odour of parasitized mealybugs revealed a new aspect in host discrimination: the identification of parasitized hosts could be partly mediated through olfactory stimuli, and not only through gustatory stimuli.     

A large-scale rearing method for Peristenus digoneutis (Hymenoptera: Braconidae), a biological control agent of Lygus lineolaris (Hemiptera: Miridae)

Releases of Peristenus digoneutis against Lygus spp. in North America have been conducted for many years; however, no published procedures for mass production of the biological control agent were available. A laboratory rearing method was developed using Lygus lineolaris as the host to enhance establishment efforts and provide large numbers of wasps for inundative releases into high value fruit crops. Experiments were conducted to determine optimum host:parasitoid density and rearing temperature. The effects of nymph:wasp ratios and temperature on parasitism and wasp survival showed a 20:1 ratio at 20°C provided high parasitism (256 parasitized nymphs/wasp over lifetime) and excellent wasp survival of 27 days. Experiments on diapause-inducing conditions for P. digoneutis demonstrated that fluctuating temperatures of 23°C (day) and <16°C (night) and corresponding photo phases of 16 h light, for rearing parasitized nymphs, produced 100% diapausing parasitoids whereas non-diapausing parasitoids were only produced at more than 16 h light. Furthermore, parasitized Lygus nymphs need to be transferred to short day conditions no later than 10 days after parasitism to produce diapausing parasitoids. Critical life stages for exposure to conditions inducing diapause, the egg, first and second instar parasitoid larva, occurred from 0 to 10 days at 24°C constant temperature. Increased time in cold storage reduced the number of days to first emergence of parasitoids from diapausing cocoons when transferred to warm temperatures. The optimum storage time for diapausing P. digoneutis is between 25 and 44 weeks, depending upon the length of time that cocoons remain at warm conditions prior to chilling.     

Intraguild predation and interference by the mealybug predator Cryptolaemus montrouzieri on the parasitoid Leptomastix dactylopii

The compatibility of the encyrtid parasitoid Leptomastix dactylopii with the coccinellid beetle Cryptolaemus montrouzieri against the citrus mealybug, Planococcus citri, is determined by the extent of intraguild predation and interference by the predator. We tested the preference of the adults and fourth-instar larvae of C. montrouzieri for healthy mealybugs and parasitized mealybugs harboring 1-, 4-, 7- and 14-day-old parasitoid larvae. The experiments were conducted in no-choice (only unparasitized mealybugs or parasitized mealybugs of one parasitoid age were offered at one time) and choice (unparasitized and parasitized mealybugs of a specific age were offered simultaneously) tests. Both the adults and larvae of C. montrouzieri fed on unparasitized and parasitized mealybugs but strongly discriminated against the hardened mummies (14 days old). We also investigated the influence of the presence of C. montrouzieri to the foraging effectiveness of L. dactylopii. The level of parasitism by L. dactylopii was reduced from about 13 to 6% when the number of the C. montrouzieri was increased from zero to four. We recommended that the releases of C. montrouzieri should be conducted 14 days after the releases of L. dactylopii to reduce intraguild predation on the parasitized mealybugs and to avoid interference with the foraging parasitoids.     

Facultative gregarious development in a solitary koinobiont parasitoid,Ephedrus californicus: implications of larval ecology and host constraints

In solitary parasitoids, the mandibulate first instars behave aggressively towards potential competitors so that generally only one larva survives per host. A ‘failure of competition’ may result in facultative gregarious development, however. We used Ephedrus californicus Baker (Hymenoptera: Braconidae: Aphidiinae), a solitary koinobiont parasitoid of aphids, to test two hypotheses in the laboratory that could explain facultative gregarious development. Gregarious development increased with the intensity of parasitism, with two (rarely three) parasitoids successfully developing in a single aphid. In heavily superparasitized hosts, interference between surviving larvae often caused abnormal pupation behaviour and inability to emerge from the mummy. The hypothesis that the survival of more than one larva per host is dependent on differences in larval age was not supported. The total body size in terms of dry mass of two males or two females developing together in the same host was higher than that of same‐sex counterparts developing singly. Females were larger than males with which they shared a host. Hypotheses about the evolutionary transition from a solitary to a gregarious lifestyle in parasitoid Hymenoptera have focused on lethal fighting between first instars but have ignored other constraints including immature mortality during later development and limiting host resources. Especially in species that pupate inside the dead host, specific requirements for pupation and emergence may determine whether one or several offspring per host can develop to adult.     

Costs of reproduction and geographic variation in the reproductive tactics of the mosquito Aedes triseriatus

Intraspecific host discrimination is widespread in solitary parasitoids whose adult females forage for and evaluate host suitability, whereas interspecific discrimination is less common. In some parasitoid species, mostly Diptera and Coleoptera, the larva performs the last step of host searching. It has been suggested that host discrimination will rarely occur in such host-seeking larvae because their low mobility results in a low host encounter rate. We determined the extent to which the larvae of Aleochara bilineata Gyllenhal (Coleoptera: Staphylinidae), a solitary parasitoid of aggregated Diptera pupae: (1) discriminated between unparasitized hosts and hosts parasitized by conspecifics; (2) used semiochemical cues to discriminate; (3) were influenced by life expectancy, presence of conspecifics and host availability in their host acceptance decision; and the extent to which (4) A. bilineata and A. bipustulata L., a species exploiting the same hosts and occurring sympatrically, showed interspecific host discrimination. A. bilineata larvae were able to discriminate between unparasitized hosts and hosts parasitized by conspecifics in a choice experiment. Such behavior has never previously been described for a coleopteran parasitoid or for a parasitoid species whose larvae perform host searching. Host discrimination in this species was not based on the presence of visual or tactile cues (e.g., entrance holes) but rather on chemical cues. The life expectancy of A. bilineata larvae was significantly shorter in the presence than in absence of hosts, and older larvae had lower parasitism success than young larvae in a 24-h experiment. However, the host acceptance decision of A. bilineata larvae was not influenced by larval age or the presence of conspecifics when the ratio of hosts per larva was greater than or equal to 1. When hosts were scarce, the degree of superparasitism increased significantly with the number of foraging conspecifics and the age of the larvae. Both species of Aleochara showed intra- and interspecific host discrimination in a choice experiment. In contrast to A. bipustulata, A. bilineata larvae more frequently parasitized hosts parasitized by A. bipustulata than those parasitized by conspecifics. We suggest that host discrimination will be frequent in solitary parasitoids with host-seeking larvae when hosts are aggregated. Received: 4 June 1998 / Accepted: 1 September 1998     

Impact of mating status on egg-laying and superparasitism behaviour in a parasitoid wasp

Most parasitoid female wasps can distinguish between unparasitized and parasitized hosts and use this information to optimize their progeny and sex allocation. In this study, we explored the impact of mating on oviposition behaviour (parasitism and self‐ and conspecific superparasitism) on both unparasitized and already parasitized hosts in the solitary parasitoid wasp Eupelmus vuilleti (Crw.) (Hymenoptera: Eupelmidae). Virgin and mated females had the same oviposition behaviour and laid eggs preferentially on unparasitized hosts. The sex ratio (as the proportion of females) of eggs laid by mated females in parasitism and conspecific superparasitism was 0.67 ± 0.04 and 0.57 ± 0.09, respectively. Likewise, females laid more eggs in conspecific superparasitism than self‐superparasitism under our experimental conditions. These experiments demonstrate that E. vuilleti females can (i) discriminate between unparasitized and parasitized hosts and adapt the number of eggs they lay accordingly, and (ii) probably discriminate self from conspecific superparasitized hosts. Finally, mating does not appear to influence the host discrimination capacity, the ovarian function, or the oviposition behaviour.     

Host nutritional status mediates degree of parasitoid virulence

Parasitic organisms rely on the resources of their hosts to obtain nutrients essential for growth and reproduction. Insect parasitoids constitute an extreme condition since they develop in a single host from which they typically consume all available resources. As a result, the host is killed following parasitism. However, a few intriguing cases of host survival have been reported wherein hosts resume foraging and may even reproduce following parasitoid emergence. Yet, the ultimate and proximate mechanisms responsible for host recovery remain unresolved. We tested the impact of host nutrition on host fate and parasitoid fitness, using the association between Dinocampus coccinellae and the spotted lady beetle Coleomegilla maculata. Under laboratory conditions, we fed parasitized ladybirds on different aphid diets, with or without pollen. In the field, we followed the fate of parasitized ladybirds during seasonal variations in pollen and aphid abundance. We found that ladybirds fed on aphids or a combination of aphids and pollen recovered more frequently from parasitism (from 65 to 81%) than those eating only pollen (48%). Field data suggest that the fate of parasitized ladybirds is also related to food availability. On the other hand, when hosts fed on a combination of aphids and pollen, consequences for parasitoid fitness were often ‘all‐or‐nothing’: parasitoid emergence rate was the lowest of all host nutrition regimes (～50%), but parasitoids that did emerge were larger than individuals emerging from other host nutrition regimes. Laboratory and field results concur to show that host nutritional status during parasitoid development significantly influences both host fate and parasitoid fitness.     

Foraging behaviour of the whitefly parasitoidEncarsia formosa on tomato leaflets

IndividualEncarsia formosa parasitoids were observed continuously until the parasitoids flew away, either on clean tomato leaflets, on leaflets with honeydew, or on leaflets with unparasitized and parasitized whitefly larvae. Encounters with unparasitized and parasitized whitefly larvae, and contact with honeydew arrested the parasitoids on the leaflet. The walking speed increased linearly from 0.179 to 0.529 mm/s between 15 and 25–30°C. The walking activity showed another relationship with temperature: it was below 10% at 15 and 18°C, and increased to about 75% at 20, 25 and 30°C. It was not affected by host encounters or by 1 to 4 ovipositions. The total handling time of hosts was between 1.8–21.8% of the total time on the leaflet. Self-superparasitism was not observed. Conspecific-superparasitism did occur in 14% of the encounters with hosts containing a parasitoid egg, but was not observed anymore when the parasitoid egg had hatched. Experienced parasitoids superparasitized as often as naive females. The foraging behaviour ofE. formosa from landing on a leaf until departure has now been quantified and is discussed.     

Fitness consequences of superparasitism and mechanism of host discrimination in the stemborer parasitoid Cotesia flavipes

The fitness consequences of superparasitism and the mechanism of host discrimination in Cotesia flavipes, a larval parasitoid of concealed stemborer larvae was investigated. Naive females readily superparasitized and treated the already parasitized host as an unparasitized host by allocating the same amount of eggs as in an unparasitized host. However, there was no significant increase in the number of emerging parasitoids from superparasitized hosts due to substantial mortality of parasitoid offspring in superparasitized hosts. Furthermore, the developmental time of the parasitoids in a superparasitized host was significantly longer than in a singly parasitized host and the emerging progeny were significantly smaller (body length and head width). Naive females entered a tunnel in which the host was parasitized 4 h previously and accepted it for oviposition. Experienced females (oviposition experience in unparasitized host) refused to enter a tunnel with a host parasitized by herself or by another female. In experiments where the tunnel and/or host was manipulated it was demonstrated that the female leaves a mark in the tunnel when she parasitizes a host. The role of patch marking in C. flavipes is discussed in relation to the ecology of the parasitoid.     

Influence of egg load and oviposition time interval on the host discrimination and offspring survival of Anagyrus pseudococci (Hymenoptera: encyrtidae), a solitary endoparasitoid of citrus mealybug, ++Planococcus citri (Hemiptera: pseudococcidae)

Oviposition and host discrimination behaviour of unmated Anagyrus pseudococci (Girault), an endoparasitoid of the citrus mealybug Planococcus citri (Risso), were investigated in the laboratory. Female parasitoids were able to discriminate between parasitized hosts and healthy ones. The mean number of ovipositions was significantly higher in unparasitized than in parasitized hosts. Conspecific-superparasitism occurred more often than self-superparasitism. Changes in consecutive ovipositions over three hours by A. pseudococci suggested that egg load influenced the discrimination behaviour of the parasitoids, with females which had low egg loads mostly avoiding oviposition in already parasitized hosts at time intervals ranging from 0 h to 96 h, and distributing their eggs in the high quality (unparasitized) hosts. The parasitized hosts were rejected more commonly through antennal perception of external markers than during ovipositor probing which could have encountered internal markers but this relationship changed with increasing time after oviposition. The parasitoid's oviposition rate in unparasitized and conspecific-parasitized hosts varied at the different oviposition time intervals when the females had fewer eggs in the ovaries. Percentage emergence of parasitized offspring was not significantly influenced by whether they developed in single or superparasitized mealybugs. The significance of host discrimination by A. pseudococci is discussed.     

Inter-larval Competition and its Subsequent Effect on Pediobius furvus (Hym.: Eulophidae) Broods for the Management of Graminaceous Stem Borers

The effects of superparasitism on the size, rate of development, progeny production, sex ratio and percentage parasitism of the parasitoid Pediobius furvus (Gah.) and the number of parasitized hosts producing mature parasitoids were investigated. High parasitoid egg densities in hosts were obtained using two host densities (1 and 2) and three arbitrary parasitoid densities (1, 2 and 5) under two exposure times (12 and 24 h). Superparasitism in P. furvus resulted in a reduction in the number of adults that matured and a reduction in the size and preponderance of male production. Dissections of the parasitized host pupae of the stem borer Chilo partellus (Swinhoe) showed no combat among developing wasps. Immature supernumeraries were eliminated by starvation and/or suffocation. The results showed that P. furvus larvae are not harmed by surplus food material in their hosts and that the female parasitoid preferred to lay more eggs into unparasitized than into parasitized host pupae.     

Complementary effects of resident natural enemies on the suppression of the introduced moth Epiphyas postvittana

Generalist predators may disrupt or complement biological control by parasitoids. Past studies have examined how predators and parasitoids interact to affect aphid suppression, but more information is needed from other host taxa. Here, we explore the interactive effects of a spider (Cheiracanthium mildei) and a generalist parasitoid (Meteorus ictericus) on the light brown apple moth (Epiphyas postvittana), a recent introduction to North America. The spider negatively affected adult parasitoids in a field experiment, and reduced numbers of parasitized larvae in the laboratory. Nonetheless, the combined effects of parasitoids and spiders on larval mortality of the moth were additive. Percent parasitism was not affected by the presence of the spider in field or laboratory experiments, and results were similar when single or multiple larval instars of the moth were included. The spider’s lack of prey preference for unparasitized or parasitized larvae likely precluded any disruptive effects on parasitism. Results suggest that resident generalist parasitoids and predators can work in conjunction to hinder the invasion success of a novel herbivore prey species.     

Effect of expected offspring survival probability on host selection in a solitary parasitoid

Optimal Foraging Theory predicts that parasitoid females should optimize their host selection to maximize their lifetime fitness gain and parasitize the most profitable hosts. In particular, in solitary parasitoids, females should avoid superparasitism, at least when sufficient unparasitized hosts are available. However, when unparasitized hosts are scarce, they should prefer, among already parasitized hosts, those that provide the best survival probability to their progeny, which depends on the age and the developmental stage of the first parasitoid. To test this hypothesis in a solitary ectoparasitoid, Pachycrepoideus vindemmiae Rondani (Hymenoptera: Pteromalidae), we first assessed the survival probability of a second parasitoid according to the time elapsed since initial parasitism. We then analyzed the female selection behavior in patches containing a mixture of hosts parasitized over various time intervals. Our results showed that the older the opponent larva was, the lower the survival probability of the second parasitoid was. However, when the first individual had reached the prepupal stage, both individuals could complete their development. At this stage, the survival probability of the second parasitoid was surprisingly high but such individuals were reduced in size. Our study also showed that host acceptance by females was strongly correlated with the survival probability of their progeny when the first parasitoid was from 0 to less than 10 days‐old. When the first parasitoid had reached the prepupal stage, females usually rejected these hosts, although the survival probability of the offspring was quite high. This discordance between female host selection behavior and progeny survival probability is discussed.     

Errors associated with estimating parasitism using a modified version of Southwood's graphical method

A simulation model is used to examine the errors in estimating parasitized and nonparasitized host densities independently with Southwood's graphical technique. This technique is accurate when parasitoid attack occurs prior to the sampling period (e.g. the previous life stage of the host). When this is not the case, the parasitized host density is estimated accurately, but the unparasitized host density is over estimated by those individuals that are sampled as healthy prior to attack. This error is neglible at low levels of parasitism (<20% parasitized), but increases with increasing parasitism. Of the biological parameters tested, only the parasitoid attack pattern (shape of the parasitoid attack curve) has a significant influence on the magnitude of this error. A generalized simulation model is presented for evaluating errors in estimates of seasonal parasitism for specific host-parasitoid interactions. University of Rhode Island, Agricultural Experiment Station Journal, Article Number 2479.     

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。     

Interspecific competition between Diadegma semiclausum Hellen (Hym., Ichneumonidae) and Cotesia plutellae (Kurdjumov) (Hym., Braconidae) in parasitizing Plutella xylostella (L.) (Lep., Plutellidea)

Abstract:  Interspecific competition between Diadegma semiclausum and Cotesia plutellae was investigated at 25°C in the laboratory, by exposing the third instar larvae of the diamondback moth, Plutella xylostella to both species together, either species alone or by exposing the host larvae already parasitized by one species, at different intervals, to the other. When host larvae were exposed simultaneously to two species in one arena, parasitism rates of the host by each species were not reduced by the presence of the other species; joint parasitism rate by two species was not significantly higher than that by either parasitoid alone. Both parasitoids could lay eggs into the host larvae which had previously been parasitized by the other species, leading to the occurrence of multiparasitized hosts. When host larvae were parasitized first by D. semiclausum and then being followed within 1–2 h by exposing to C. plutellae , or vice versa, ensuing parasitoid cocoons from the multiparasitized host larvae were nearly all C. plutellae . When host larvae were parasitized initially by C. plutellae and then being followed by D. semiclausum two or more days later, all parasitoids ensued from the multiparasitized hosts were C. plutellae . In contrast, when host larvae were parasitized initially by D. semiclausum and then being followed by C. plutellae two or more days later, most host larvae could not survive to prepupae and most of the ensuing parasitoid adults from the surviving hosts were D. semiclausum . Dissections of host larvae at various time intervals after parasitization by the two parasitoids showed that development of both parasitoids in multiparasitized hosts were somewhat arrested, and that the first instar larvae of C. plutellae could initiate a physical attack on the larvae of D. semiclausum and remove the latter.