State dependent superparasitism in a solitary parasitoid: egg load and survival

In solitary parasitoids, superparasitism (the allocation ofan egg to an already parasitized host) has a payoff, measuredin offspring produced and costs, measured in eggs and time invested.Solitary parasitoids that are capable of host discriminationmust adopt the strategy that ensures the best use of both theiregg load and available lifetime. In this paper, we develop astate-dependent model defining the optimal strategy of superparasitismfor a solitary parasitoid species with overlapping generations.The fitness measure we use is based on the growth rate of thenumber of genotype copies. The model predicts that the tendencyto superparasitize should increase as the egg load of the parasitoidincreases, or as its life expectancy decreases. The model alsopredicts that under particular conditions wasps should showpartial preferences for parasitized hosts. These predictionswere tested with the parasitoid Venturia canescens (Hymenoptera:Ichneumonidae). The tendency of the wasps to superparasitizein the presence of both healthy and parasitized hosts was correlatedto egg load and access to food before the experiment A complementaryexperiment, where parasitized hosts were given sequentiallyto parasitoids, showed that V. canescens exhibits partial preferencestoward superparasitism. These experimental results and a previouswork support the predictions of the model.     

Costs and consequences of superparasitism in the polyembryonic parasitoid Copidosoma koehleri (Hymenoptera: Encyrtidae)

Abstract.  1. Polyembryonic wasps provide dramatic examples of intra-specific developmental conflict. In these parasitoids, each egg proliferates into a clonal lineage of genetically identical larvae. If more than one egg is laid in a host (superparasitism), individuals of different clones may compete for food resources.
2. In the polyembryonic encyrtid Copidosoma koehleri , one larva per clone can differentiate into a sterile soldier. It is shown that soldiers are always females, and that they attack intra-specific competitors.
3. Research hypotheses were that (a) clones that develop in superparasitised hosts suffer heavier mortality than clones that develop in singly parasitised hosts, and (b) female clones cause higher mortality to their competitors than male clones, hence larval survival is lower in superparasitised hosts that contain females than in male-only broods.
4. The potential frequency of superparasitism in C. koehleri was manipulated by varying parasitoid–host ratios and exposure durations.
5. As parasitoid densities and exposure durations increased, the frequency of superparasitism rose, brood sizes increased, but the number of hosts that completed development was reduced. The number of offspring per parasitoid female decreased with increasing parasitoid–host ratios. Offspring size and longevity were inversely correlated with brood size. As superparasitism rates increased, fewer all-male broods were produced. Male–female broods were female-biased, suggesting selective killing of males by female soldiers. All-female broods were significantly smaller than all-male broods at high parasitoid densities only, possibly reflecting aggression among soldiers of competing clones.
6. The results support the working hypotheses, and suggest that female larvae outcompete males in superparasitised hosts.     

Self and conspecific superparasitism by the solitary parasitoid Antrocephalus pandens

Abstract.

• 1 The fitness consequences of superparasitism for a solitary parasitoid depend on whether the host was first parasitized by itself (‘self-superparasitism’) or a different individual (‘conspecific superparasitism’). Self superparasitism is usually expected to be avoided.
• 2 A.pandens females showed no difference in their probability of superparasitism between self-parasitized and conspecifically-parasitized hosts. The probability of superparasitism decreased as time from the laying of the first egg in a host increased, from about 0.29–0.46 at a time interval of 1 h to 0.10–0.14 at 72 h.
• 3 The egg distribution of wasps foraging alone on a patch showed significant avoidance of superparasitism, but that of wasps foraging in the presence of conspecifics was not significantly different from a random distribution. This suggests that wasps switch from avoidance of superparasitism when alone to acceptance of all hosts when in a group.
• 4 When wasps foraged in a group, the hosts had many more ovipositor puncture marks than when wasps foraged singly. This suggests that either hosts were attacked several times per encounter, or that the wasps' encounter rate with hosts was much higher when in a group. If the latter is true, it is possible that, although the egg distribution suggested a higher rate of superparasitism when wasps foraged in a group, the ratio of acceptances to contacts of parasitized hosts may in fact have been lower.

     

Host discrimination modulates brood guarding behaviour and the adaptive superparasitism in the parasitoid wasp Trissolcus semistriatus (Hymenoptera: Scelionidae)

Because hosts utilized by parasitoids are vulnerable to further oviposition by conspecifics, host guarding benefits female wasps. The present study aims to test whether female adults regulate brood guarding behaviour by host discrimination in a solitary parasitoid Trissolcus semistriatus by presenting an intact or parasitized host egg mass to a female adult. Virgin females without oviposition experience have host discrimination ability, which enables them to adjust the number of eggs laid in the hosts. Mating experience increases superparasitism by female adults, whereas mated females achieve a higher discrimination ability as a result of oviposition experience and show a lower superparasitism rate. As expected, females exhibit brood guard after parasitizing an intact host egg mass, whereas those females visiting a previously parasitized host egg mass, do not. Because the survival of eggs in superparasitized hosts is relatively low, regulating brood guarding behaviour by host discrimination is adaptive for female wasps.     

Superparasitism and sex ratio in the solitary parasitoidEpidinocarsis lopezi

Highly variable sex ratios are found in the solitary parasitoidEpidinocarsis lopezi, both in the field and in a mass-rearing situation. Superparasitism is one of a number of factors which can influence sex ratios in parasitoid Hymenoptera. We show that superparasitism inE. lopezi is common in the field. Sex allocation decisions when parasitizing unparasitized hosts are not different from those with parasitized hosts; neither does differential mortality occur between the sexes in superparasitized hosts. Therefore superparasitism does not contribute to the variable sex ratio ofE. lopezi. Both the occurrence of superparasitism and the sex produced when ovipositing are shown to be functional forE. lopezi.     

Oviposition decisions in an endoparasitoid under self-superparasitism conditions

Superparasitism is a widespread phenomenon. Having accepted superparasitism, mated female parasitoids must decide on the sex of each egg they subsequently lay into the same host. Theory predicts that this decision is either based on host quality, when more male eggs are laid in hosts that are already parasitized because they are perceived to be of poorer quality; or more eggs are laid of the sex that is most likely to be a strong larval competitor, i.e. generally females.Anastatus disparis is a facultative endoparasitic egg parasitoid. We used ‘artificial’ hosts to explore outcomes of decision making by A. disparis during superparasitism under a manipulated absence of larval competition. When only one egg was laid it was always female. As the number of eggs laid increased, so more of them were male. This supports the theory that oviposition decisions are based on host quality; more male eggs were laid in hosts that were already parasitized and thus of poorer quality.In a second experiment, eggs were exposed to parasitoids for different periods of time. Half the eggs were dissected to determine the number of parasitoid eggs that had been laid. The remaining eggs were incubated and the number and sex of offspring that ultimately emerged, following larval competition, were recorded. Under superparasitism conditions fierce larval competition ensued; only one offspring survived and they were predominantly female.In conclusion, oviposition decisions by female A. disparis accepting self-superparasitism were made based on host quality.     

The effect of superparasitism on development of the solitary parasitoid wasp, Venturia canescens (Hymenoptera: Ichneumonidae)

Abstract.

• 1 The effects of superparasitism on the rate of development, adult size and mortality of Venturia canescens (Hymenoptera: Ichneumonidae) were investigated. Parasitoids were reared from third (L3) and fifth (L5) instars of one of its hosts, Plodia interpunctella (Lepidoptera: Pyralidae) containing one, two or four parasitoid eggs.
• 2 Superparasitism increased the development time of Venturia reared from both instars, but the developmental delay was more evident in wasps from L5 hosts.
• 3 The size of parasitoids from L3 hosts was unaffected by egg number, but wasps from both superparasitized L5 treatments were significantly smaller than those from singly parasitized hosts.
• 4 Parasitoid mortality was significantly higher in L5 than L3, but within instars did not differ significantly with egg number.
• 5 The results confirm that superparasitism may affect the fitness of both the adult female wasp and her progeny, and should therefore be incorporated into models of superparasitism as an adaptive foraging strategy.

     

Superparasitism and host discrimination in the solitary ectoparasitoid Dinarmus basalis

In the solitary ectoparasitoid, Dinarmus basalis (Hymenoptera: Pteromalidae), the occurrence of superparasitism according to the unparasitised host density, and the nature of the host(s) provided was investigated in laboratory studies. In this species superparasitism was observed whatever the experimental conditions used, but the degree of superparasitism depended on the density of its host, Bruchidius atrolineatus (Coleoptera: Bruchidae). Superparasitism was due to successive egg-laying phases on the same host. However, females were able to discriminate between unparasitised hosts and hosts parasitised from 8 h to 72 h beforehand by themselves or by conspecifics. There was no conclusive evidence that superparasitism in the presence of a host parasitised 30 min before was linked to an absence of host discrimination. Host discrimination in this species is achieved by host-quality markers. These are individual-specific markers since conspecific superparasitism rates were often higher than self superparasitism rates. One deterrent substance is emitted by the females during oviposition onto the egg or released by the 16 to 24 h-old egg itself. Another host-quality marker is associated with the presence of a larva on its host. On the other hand, host discrimination ability did not always imply avoidance of superparasitism. In D. basalis there exists a positive relationship between the survival probability of the second egg and the tendency to superparasitise, and superparasitism could therefore result in a significant fitness gain. Under our experimental conditions, D. basalis females exhibited a wide range of oviposition behavioural plasticity in relation to the parasitoid developmental stage, the type of superparasitism, and the encounter rate with unparasitised hosts.     

Presence of a conspecific increases superparasitism but not infanticide under self‐ and conspecific superparasitism in a semisolitary parasitoid,Echthrodelphax fairchildii (Hymenoptera: Dryinidae)

Female parasitoids are expected to avoid superparasitism (ovipositing in and/or on parasitized hosts) when unparasitized hosts are available. However, when the supply of unparasitized hosts is restricted, they are expected to self‐ as well as conspecifically superparasitize. One of the cues of a reduced availability of unparasitized hosts is the presence of a conspecific. Moreover, if the focal species can perform infanticide, after encountering a conspecific female, the females are expected to kill eggs existing in and/or on hosts when superparasitizing, because the eggs are more likely to be laid by others. In this study we investigated whether females of an infanticidal semisolitary parasitoid, Echthrodelphax fairchildii, increase their frequencies of superparasitism and infanticide after encountering a conspecific female. Echthrodelphax fairchildii females are capable of discriminating between self‐ and conspecific superparasitism until up to 0.75 h after the first egg was laid (self‐superparasitism frequency < conspecific superparasitism frequency). As expected, the female parasitoids were more likely to perform self‐ and conspecific superparasitism after they had encountered a conspecific. In particular, the self‐superparasitism frequency increased highly within a short period after the first oviposition, so that no difference between the self‐ and conspecific superparasitism frequencies was found. In contrast, the infanticidal‐probing frequency remained extremely low, irrespective of whether or not the female parasitoids had encountered a conspecific. Moreover, when superparasitizing, females usually laid female eggs. Possible causes for the low frequency of infanticidal probing and the female‐biased sex ratio are discussed.     

Adaptive self- and conspecific superparasitism in the solitary parasitoid Leptopilina heterotoma (Hymenoptera: Eucoilidae)

One of the foraging decisions facing parasitoids is whetherto accept (superparasitize) or reject hosts that have alreadybeen parasitized. An important distinction is whether the hosthas been parasitized by the female parasitoid herself or bya conspecific. In solitary parasitoids, the pay-off from anegg laid in the latter host type (conspecific superparasitism)is the probability that the second egg wins the competitionfor the host and results in an offspring. The pay-off from anegg laid in the former type (self-superparasitism) increaseswith an increasing probability that another female will superparasitizethe host in the near future. When this probability equals one,self-superparasitism and conspecific superparasitism have thesame payoff. However, conspecific superparasitism will generallyhave a higher pay-off than self-superparasitism. It will thereforebe beneficial for a female parasitoid to be able to distinguishbetween a host she parasitized and one parasitized by a conspecific.The degree of benefit depends on the probability of conspecificsuperparasitism in the near future. Using an optimal diet model,I show that when a parasitoid encounters a patch containinga mixture of unparasitized and already-parasitized hosts, afemale that can distinguish between the two types of parasitizedhosts gains more offspring than a female without this ability.However, when parasitoids search a patch together with conspecifics,it is adaptive to self-superparasitize, and the pay-off fromthis ability may be negligible. It is therefore predicted thatwhen a female parasitoid searches a partially depleted patchalone, it will reject the hosts parasitized by itself more frequentlythan hosts parasitized by conspecifics. In contrast, femaleparasitoids searching together are predicted to accept hoststhat they parasitized themselves much more often. The resultsshow that the solitary parasitoid Leptopilina heterotoma (Hymenoptera:Eucoilidae) is able to distinguish between hosts that it parasitizedand hosts parasitized by conspecifics. The predictions of themodel are met in a second experiment that shows that L. heterotomaself-superparasitizes when the probability of conspecific superparasitismis high.     

Functional response of Trichogramma chilonis to Galleria mellonella and Chilo sacchariphagus eggs

A biological control programme using inundative releases of Trichogramma chilonis Ischii (Hymenoptera: Trichogrammatidae) reared on Galleria mellonella L. (Lepidoptera: Pyralidae) is currently underway to reduce infestations of Chilo sacchariphagus Bojer (Lepidoptera: Crambidae) in sugarcane, Saccharum spp., on Réunion Island. To assess the potential of the parasitoid as an inundative biocontrol agent, the functional response of three T. chilonis strains was tested with G. mellonella and one strain with C. sacchariphagus host eggs in glass tubes in the laboratory. The shape of the functional response (type II or III) was determined using logistic regression, and attack coefficients and handling times (T_h) were determined using non‐linear least‐square regression. The behaviour of all three strains with G. mellonella host eggs corresponded to a type III response. The St Benoît T. chilonis strain had a significantly shorter estimate of T_h than the St Pierre strain (P<0.05) and may, therefore, be more appropriate as a biocontrol agent. The functional response with C. sacchariphagus host eggs was a type II with the St Benoît T. chilonis strain. More T. chilonis wasps developed per host egg from the larger C. sacchariphagus host eggs (2.9) relative to G. mellonella (1.1). Superparasitism at low host egg densities was, therefore, likely to have been less frequent with C. sacchariphagus. Black eggs were chosen as an estimate of number of eggs parasitized, although they represent the number of eggs where parasitism led to complete pupal development. The low rate of detected parasitism at low host densities with G. mellonella eggs may be due to incomplete pupal development due to superparasitism rather than lack of parasitism, thus explaining the type III functional response.     

Discrimination of the age of conspecific eggs by an ovipositing ectoparasitic wasp

The recognition and avoidance of already parasitized hosts is a major issue in parasitoid behavioural ecology. A key factor affecting the fitness reward expected from superparasitism is the probability that the second or subsequent egg laid on a host will win the contest with the first-laid egg. The present study investigated the ability of females of the solitary ecto parasitoid Anisopteromalus calandrae Howard (Hymenoptera: Pteromalidae) to (i) discriminate between unparasitized Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae) hosts and those parasitized by a conspecific, and (ii) discriminate between a host parasitized by an egg just laid (2 h) and one parasitized by an egg about to hatch (28 h). However, they did not adjust their offspring sex ratio on already parasitized hosts compared to unparasitized ones. Our results show that A. calandrae females can discriminate between parasitized and unparasitized hosts, as they lay more eggs on the latter. The probability of the second or subsequent egg laid on a host (superparasitism) winning the contest with a conspecific increases as the time between the two ovipositions decreases. Consequently, parasitoid females should lay more eggs on recently parasitized hosts than on those that have been parasitized for a long time (i.e., when the first eggs are about to hatch), and that is indeed what they were found to do. To increase their fitness in spite of the presence of already parasitized hosts, A. calandrae females have developed highly discriminative capacities regarding the parasitism status of hosts.     

Effects of pupal treatment with Proteus® and Metarhizium anisopliae sensu lato on functional response of Habrobracon hebetor parasitising Helicoverpa armigera in an enclosed experiment system

The ectoparasitoid wasp, Habrobracon hebetor Say (Hymenoptera: Braconidae) is a valuable biocontrol agent attacking larval stages of many lepidopteran pests including Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). The possibility of combined application of this parasitoid wasp with other biological control agents and chemical insecticides is necessary for the success of an Integrated Pest Management (IPM) programme. One of the most important behaviours of a parasitoid refers to the number of hosts attacked by per parasitoid as a function of host density. In this research, the single and joint effects of the insecticide, Proteus® and the entomopathogenic fungus, Metarhizium anisopliae sensu lato (Metsch.) Sorokin (Hypocreales: Clavicipitaceae) (isolate M14) were studied on the functional response of H. hebetor. Newly mated females (<24?h) from treated pupae of H. hebetor were used to study the functional response of this parasitoid wasp to different densities of H. armigera larvae. A type II functional response was observed in all treatments. The results revealed that control and Proteus® treatments developed the lowest (0.5599?±?0.0373?h) and the highest (0.5709?±?0.0443?h) handling time, respectively. The most and the least values of attack rate were observed in control (0.0996?±?0.0164?h^?1) and Proteus® (0.0776?±?0.0119?h^?1) treatments, respectively. The maximum theoretical parasitism rate (T/T_h) was estimated to be 42.86 for female control wasps. After conducting field experiments on functional response along with life table parameters, M. anisopliae sensu lato (isolate M14) may be used as a compatible biocontrol agent with H. hebetor in IPM programmes of H. armigera.     

寄主大小及寄生顺序对蝇蛹佣小蜂寄生策略的影响

寄主大小模型认为寄生蜂后代性比与寄主大小相关,寄生蜂倾向于在大寄主上产出更多雌性后代,在小寄主上产出更多雄性后代.探讨了以家蝇蛹为寄主时,蝇蛹佣小蜂后代产量和性比变化;单次寄生情况下,寄主大小及寄生顺序对寄生蜂后代性比等影响.结果表明,蝇蛹佣小蜂的产卵期为(8.93±3.34)d,单头雌蜂能产雌性后代(34.11±16.34)头和雄性后代(11.04±8.87)头,且雄性百分比为0.24±0.11.随成蜂日龄的增大,寄生蜂产生雄性后代的比率显著增加.蝇蛹佣小蜂在寄生家蝇蛹时,会优先选择寄生个体较大的蛹;在单次寄生的情况下,蝇蛹佣小蜂倾向于在较大的家蝇蛹内产出更多的雌性后代.     

RETRACTED ARTICLE: Foraging of Host-Habitat and Superparasitism in <Emphasis Type="Italic">Cotesia glomerata</Emphasis>: A Gregarious Parasitoid of <Emphasis Type="Italic">Pieris brassicae</Emphasis>

The study investigates differences in the oviposition pattern of a braconid parasitoid, Cotesia glomerata (Linn.) in Pieris brassicae (Linn.) in relation to their use of different cruciferous food plants. The response of P. brassicae to superparasitism and consequences for the parasitoid were examined in order to elucidate the ecological significance of this behaviour. Female parasitoid located various crucifers and searched for host more frequently almost on all the host plants tested i.e. cabbage, cauliflower, Chinese cabbage, broccoli and radish. According to the estimated relative number of female locating hosts, cabbage was the most attractive plant for C. glomerata and total number of eggs laid in host larvae feeding on it was higher than in larvae feeding on other plants. Laboratory experiments demonstrated that superparasitism reduced survivorship of P. brassicae larvae. Superparasitism lengthened parasitoid development and prolonged the feeding period of host larvae. Sex ratio and the body weight of emergent wasps correlated negatively with brood size. Despite a trade-off between maximising brood size and optimising the fitness of individual offspring, two or three ovipositions on P. brassicae larvae resulted in a greater female dry mass than did a single oviposition on the host. Thus, superparasitism might be of adaptive significance under certain circumstances, especially when host density is low and unparasitized hosts are rare in a habitat.     

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.     

Parasitism capacity and searching efficiency of Diaeretiella rapae parasitizing Brevicoryne brassicae on susceptible and resistant canola cultivars

Different cultivars of aplant species can affect the foraging and efficiency of natural enemies, both directly through physical and biochemical properties or indirectly through the herbivore's diet. In this study, the parasitism capacity and functional response of Diaeretiella rapae McIntosh were determined on the cabbage aphid, Brevicoryne brassicae (L.) reared on susceptible (Opera) and resistant (Okapi) canola cultivars under laboratory conditions at 25?±?1?°C, 60?±?5% RH and a16:8?h L:D photoperiod. The parasitoid exhibited Type II and Type III functional responses on the resistant and susceptible cultivars, respectively. The estimated value of searching efficiency (a) was 0.1637?±?0.1095?h^?1 on the resistant cultivar whereas its value was dependent on host density on the susceptible cultivar. The handling times (T_h) on the susceptible and resistant canola cultivars were 0.108?±?0.040 and 0.320?±?0.048?h, respectively. The net parasitism rate (C₀) of the parasitoid wasp varied from 128.09 hosts per parasitoid lifetime on the susceptible to 71.01hosts on the resistant canola cultivar. The transformation rate from host population to parasitoid offspring (Qp) was equal to 1 on both cultivars (C₀?=?R₀). The finite parasitism rate (ω) on the susceptible cultivar (0.819 hosts per parasitoid per day) was significantly higher than that on the resistant one (0.578 hosts per parasitoid per day). In conclusion, canola cultivars affected the performance of D. rapae in controlled small-scale laboratory experiments and compared with the susceptible cultivar, the resistant one had anadverseeffect on the efficiency of the parasitoid.     

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 genotype affects the relative success of competing lines of aphid parasitoids under superparasitism

1. In solitary parasitoids, only one individual can complete development in a given host. Therefore, solitary parasitoids tend to prefer unparasitised hosts for oviposition, yet under high parasitoid densities, superparasitism is frequent and results in fierce competition for the host's limited resources. This may lead to selection for the best intra‐host competitors. 2. Increased intra‐host competitive ability may evolve under a high risk of superparasitism if this trait exhibits genetic variation, and if competitive differences among parasitoid genotypes are consistent across environments, e.g. different host genotypes. 3. These assumptions were addressed in the aphid parasitoid Lysiphlebus fabarum (Hymenoptera: Braconidae: Aphidiinae) and its main host, the black bean aphid, Aphis fabae (Scopoli) (Hemiptera: Aphididae). Three parthenogenetic lines of L. fabarum were allowed to parasitise three aphid clones singly and in all pairwise combinations (superparasitism). The winning parasitoid in superparasitised aphids was determined by microsatellite analysis. 4. The proportions of singly parasitised aphids that were mummified were similar for the three parasitoid lines and did not differ significantly among host clones. 5. Under superparasitism, significant biases in favour of one parasitoid line were observed for some combinations, indicating that there is genetic variation for intra‐host competitive ability. However, the outcome of superparasitism was inconsistent across aphid clones and thus influenced significantly by the host clone in which parasitoids competed. 6. Overall, this study shows that the fitness of aphid parasitoids under superparasitism is determined by complex interactions with competitors as well as hosts, possibly hampering the evolution of improved intra‐host competitive ability.     

Intraspecific competition among first instars of the parasitic wasp Biosteres longicaudatus

Summary Females of the solitary endoparasitic wasp Biosteres longicaudatus sometimes deposit >1 egg (superparasitism) in each larva of the Caribbean fruit fly host, Anastrepha suspensa. As host density increases, there is an inverse relationship between the level of superparasitism and the number of progeny produced/female. Larval parasitoid competition in superparasitized hosts causes an abrupt decline from >1 to 1 or <1 parasitoid/host 24–36 h before the surviving parasitoid larva molts to the 2nd instar. The mechanism by which supernumaries are eliminated was investigated by indirect, in vivo and direct, in vitro methods. There is no apparent competition between parasitoid eggs of the same age. Parasitoid first instars utilize their heavily sclerotized mandibles to eliminate competitors, some of which are subsequently encapsulated by the host. First instars in vitro produce a substance that kills conspecifics. Presumably, this substance is secreted into the surrounding medium. One of each pair of parasitoid first instars, evenly matched for age and size, may live up to 6.4 days longer and grow to 0.13 mm larger than the other. Thus, B. longicaudatus, like other solitary endoparasitoids eliminates competitors by both combat and interference competitions. The latter case, presumably involves allelochemical toxins against conspecifics in the absence of physical encounter.