Effects of host age and host availability on developmental period,adult size,sex ratio,longevity and fecundity inLariophagus distinguendus Förster (Hymenoptera: Pteromalidae)

The effects of host age on parasitoid reproductive capacity are studied using the pteromalid parasitoid Lariophagus distinguendusFörster and its bruchid hosts, Callosobruchus chinensis (L.) and C. maculatus (F.). A series of experiments were performed to investigate relationships between age and size of host parasitized and the developmental period of pre-imaginal progeny, sex ratio, female size, longevity, fecundity and oviposition rate. There was no effect of host size on preimaginal parasitoid developmental period. Sex ratio varied from less than 5% females from young (small) hosts to 60% females from mature (large) hosts. Adult size, female longevity, fecundity, and oviposition rate were also positively related to host age. Females provided mature hosts lived longer than those provided either young hosts or no hosts, possibly because of an increased ability to host-feed from the larger hosts. The implications of these findings to parasitoid population reproductive capacity and host-parasitoid synchrony are discussed.     

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.     

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.     

Preference and performance of the hyperparasitoid Syrphophagus aphidivorus (Hymenoptera: Encyrtidae): fitness consequences of selecting hosts in live aphids or aphid mummies

Abstract.  1. Theoretical models predict that ovipositional decisions of parasitoid females should lead to the selection of the most profitable host for parasitoid development. Most parasitoid species have evolved specific adaptations to exploit a single host stage. However, females of the aphid hyperparasitoid Syrphophagous aphidivorus (Mayr) (Hymenoptera: Encyrtidae) display a unique and atypical oviposition behaviour by attacking either primary parasitoid larvae in live aphids, or parasitoid pupae in dead, mummified aphids.
2. In the laboratory, the correlation between host suitability and host preference of S. aphidivorus on the host Aphidius nigripes Ashmead parasitising the aphid Macrosiphum euphorbiae (Thomas) was investigated.
3. The relative suitability of the two host stages was determined by measuring hyperparasitoid fitness parameters (survival, development time, fecundity, sex ratio, and adult size of progeny), and calculating the intrinsic rate of population increase ( r _m). Host preference by S. aphidivorus females and the influence of aphid defence behaviour on host selection was also examined.
4. Hyperparasitoid offspring performance was highest when developing from hosts in aphid mummies and females consistently preferred this host to hosts in parasitised aphids. Although aphid defensive behaviour may influence host selection, it was not a determining factor. Ecological and evolutionary processes that might have led to dual oviposition behaviour in S. aphidivorus are discussed.     

Host-feeding and oviposition strategies of parasitoids in relation to host stage

Until now, mathematical models of parasitoid-host interactions have not incorporated the tendency for destructively host-feeding parasitoids to partition their feeding and oviposition behaviour in relation to different host stages. A literature survey reveals a trend for female parasitoids to feed preferentially or exclusively on earlier host stages and to oviposit preferentially or exclusively in/or later ones. We explore the relative advantages to host-feeding parasitoids of a number of possible host stage selection strategies. We develop hypotheses, formalizing and testing them using modifications to our earlier simulation model of host-feeding strategies (Jervis and Kidd, 1986). We conclude from our modelling that the advantage to be gained from feeding on early host stages and ovipositing in late ones is likely to be associated with: 1) reduced handling times when feeding on early stage hosts; 2) reduced wastage of progeny from mortality factors other than host-feeding by the parent parasitoid, achieved by confining oviposition to late host stages; and 3) reduced probability of progeny mortality resulting from the parent's host-feeding activities.     

Host size induced variation in progeny sex ratio of an aphid parasitoid Lysiphlebia mirzai

A study aimed at determining the effect of host size on the pattern of sex allocation by an aphid parasitoid Lysiphlebia mirzai Shuja-Uddin (Hymenoptera: Braconidae: Aphidiinae) was undertaken to test the validity of the following three hypotheses: (1) larger hosts will yield a greater proportion of daughters, (2) differential mortality of the sexes of the parasitoid during development is a function of host-size, and (3) daughters emerging from larger hosts are more fecund. The results suggested a propensity in the parasitoid to deposit fertilised (diploid) eggs in large hosts (third instar nymph) and unfertilised (haploid) eggs in small hosts (first and second instar nymphs). Unpreferred fourth instar nymphs and apterous aphid adults also received more haploid eggs despite being larger in size than the preferred third instar nymphs. However, the perception of host size by the mother was dependent on the extent of temporal variation in the host size distribution and on her previous experience of host size. Developmental period, longevity, mating potential, fecundity and progeny sex ratio of L. mirzai emerging from small versus large hosts indicated that the host size affected the fitness of the daughters more than that of sons. No differential mortality of the sexes during development of L. mirzai was observed in small versus large hosts. This shows that L. mirzai, while ovipositing in growing stages of the host, adjusts progeny sex ratio according to the host size and by doing so she tends to contribute maximally to progeny fitness without knowing about the future host quality.     

Host-size-dependent sex ratio in a parasitoid wasp

Charnov's host-size model explains parasitoid host-size-dependent sex ratio as an adaptive consequence when there is a differential effect of host size on the offspring fitness of parasitoid males versus females. This article tests the predictions and the assumptions of the host-size model. The parasitoid wasp Pimpla nipponica Uchida (Hymenoptera: Ichneumonidae) laid more female eggs in larger or fresher host pupae when choice among hosts of different sizes or ages was allowed. Then, whether an asymmetrical effect of host size and age on the fitness of females versus males existed in P. nipponica was examined. Larger or fresher host pupae yielded larger wasps. Larger females lived longer, whereas male size did not influence male longevity. Large males mated successfully with relatively large females but failed with small females, whereas small males could mate successfully either with small or with large females. Thus, small-male advantages were found, and this held true even under male–male competition. Ovariole and egg numbers at any one time did not differ among females of different sizes. Larger females attained higher oviposition success and spent less time and energy for oviposition in hosts. Larger females produced more eggs from a single host meal. Taken together, females gained more, and males lost more, by being large. Host size and age thus asymmetrically affected the fitness of offspring males versus females through the relationships between host size or hast age and wasp size, which means the basic assumption of the host-size model was satisfied. Therefore, sex ratio control by P. nipponica in response to host size and age is adaptive. Received: November 13, 1998 / Accepted: January 18, 1999     

Influence of parasitoid-host density and host availability on the laboratory propagation ofMicroctonus aethiops [Hym.: Braconidae] parasitoid ofHypera postica [Coleop.: Curculionidae]

Laboratory studies of the arrhenotokous braconid wasp,Microctonus aethiops (Nees) were conducted relative to maximizing parasitoid production without adversely affecting sex ratios of the progeny. The influence of hostHypera postica (Gyllenhal) and parasitoid densities on parasitoid productivity was evaluated. Maximum production or a mean 25.8M. aethiops progeny were obtained per female when the host parasitoid ratio was 50∶1. More than one parasitoid per rearing cage resulted in fewer parasitoid progeny per female and increased host mortality. Oviposition significantly increased when female parasitoids were exposed to 25 different weevils every 24 hours. The most intense ovipositional activity usually occurred during the first 48 hours of exposure. However, superparasitism occurred if exposure to 25 hosts continued for more than 24 hours. Continuous and discontinuous oviposition patterns did not affect the numbers of parasitoid progeny produced. However, fewer females were produced when new weevils were provided every 24 hours as in the continuous oviposition group.     

Sex ratios in field populations of Epidinocarsis lopezi, an exotic parasitoid of the cassava mealybug, in Africa

Abstract.

• 1 In cassava fields in Africa, population sex ratios of Epidinocarsis fopezi vaned from 0.44 (males to total parasitoids) at low host densities to highly male-biased ratios of 0.70 at high host densities.
• 2 This variability is caused by the difference in allocation of sons and daughters to hosts of different sizes, through the following mechanisms: (a) small, i.e. second instar, hosts are mainly used for the production of male offspring, whereas in large, i.e. third instar, hosts a variable, female-biased sex ratio is produced; (b) E.fopezi does not selectively oviposit into large hosts but always accepts both small and large hosts for oviposition upon encountering; (c) in the field, this parasitoid is time-limited, and not egg-limited. On the basis of an optimal diet model, such general host acceptance is shown to be the best strategy.
• 3 Thus, sex ratio increases with host density for three reasons: the proportion of small hosts encountered in the field increases with increasing host density, small hosts are used for male production, and hosts are always accepted when encountered.

     

Host age and fitness-related traits in a koinobiont aphid parasitoid

Abstract.  1. Trade-offs play a key role in species evolution and should be found in host–parasitoid interactions where the host quality may differ between host age categories.
2. The braconid wasp, Aphidius ervi , is a solitary endoparasitoid that allows its aphid hosts to continue to feed and grow after parasitisation. The hypotheses that host age influences their quality and that female parasitoids exploit their hosts based on that quality were tested under laboratory conditions using no-choice tests.
3.  Aphidius ervi females accepted the aphid Myzus persicae for oviposition and their progeny developed successfully in all host ages. The fitness-related traits of parasitoids did not increase linearly with the host age in which they developed. Host quality was found to be optimal at intermediate host ages and the females preferred to parasitise these hosts. The shortest progeny development time and a more female-biased sex ratio were observed in hosts of intermediate age.
4. This study suggests the existence of multiple interactive trade-offs occurring during host–parasitoid interactions according to host age related quality.     

Combined effects of host quality and local mate competition on sex allocation inLariophagus distinguendus

Summary Many parasitoid wasps are known to adjust sex ratio in response to either local mate competition (LMC) or host quality. Nevertheless, few studies have investigated the combined effects of these two factors on sex allocation. The sex allocation pattern inLariophagus distinguendus, a parasitoid of granary weevil larvae, is contrasted to the expectations of Werren's (1984) model combining LMC and host quality. Several predictions of the model are confirmed, but others are not. Sex ratio on both large and small hosts declines with proportion of small hosts attacked in a manner consistent with the model. However, when only one host size is parasitized, sex ratio is not independent of that host size, as predicted by the model. Various possibilities for the deviation between expected and observed are discussed. A partial LMC/host quality model is developed which allows for some matings outside the natal patch, and predictions of this model conform more closely to the pattern observed inL. distinguendus. Finally, the application of parasitoid studies to basic questions in evolutionary ecology is briefly discussed.     

Host Stage Selection and Sex Allocation by Gyranusoidea tebygi (Hymenoptera: Encyrtidae), a Parasitoid of the Mango Mealybug, Rastrococcus invadens (Homoptera: Pseudococcidae)

Host stage selection and sex allocation by Gyranusoidea tebygi Noyes (Hym,, Encyrtidae) were studied in choice and no-choice experiments in the laboratory. The parasitoid reproduced on first, second, and third instars of the mango mealybug, Rastrococcus invadens Williams (Hem., Pseudococcidae), and it avoided hosts that were already parasitized. Host feeding was occasionally observed. Sex ratios of the offspring produced by individual wasps were highly biased in favor of females, whereas the sex ratio of groups of wasps foraging under crowded conditions varied from male biased in smaller hosts to female biased in larger hosts. Females had longer developmental times than males, developed faster in larger mealybugs than in smaller ones, and were always larger than males emerging from the same host instar. Their size increased with the instar of the host at oviposition. About 90% of all ovipositions in second and third instar nymphs resulted from an attack with multiple stings, starting with a sting in the head of the host for the most part. The function of these head stings is either to assess quality of the host or to subdue hosts prior to oviposition. Encounter rates, number of attacks, and number of stings during one attack increased, while ovipositions decreased with host instar. Time investment per oviposition and time spent preening increased with increasing host age because older hosts defended themselves more vigorously than younger ones. Thus, while fitness of the parasitoid increased with host size, fitness returns per time decreased. The implications of this host selection behavior for the biological control of the mango mealybug are discussed.     

The influence of prior experience on preference and performance of a cryptoparasitoid Scleroderma guani (Hymenoptera: Bethylidae) on beetle hosts

1. Numerous studies have reported the effects of learning or experience on parasitoid host preference and location. However, the integration of pre‐imaginal and adult experiences on the subsequent host preference and adult/offspring performance has been rarely tested in host–parasite interactions. 2. We present direct evidence that theses two kinds of experiences affect host preference and related fitness in the polyphagous parasitoid, Scleroderma guani. Two colonies of parasitoids were reared on Monochamus alternatus and Saperda populnea (Cerambycidae: Lamiinae). Individuals from the two colonies were given host‐switching experience for one generation (pre‐imaginal experience) while other individuals were given prior ovipositing experience on the two species, respectively (adult experience). 3. Scleroderma guani females demonstrated that their experiences determined adult behavioural responses and their subsequent performance to hosts. Females maximised both adult fitness (fecundity and longevity) and offspring fitness (survival and sex ratio) when they encountered hosts similar to their maternal hosts. Behavioural plasticity in host choice was affected by adult experience, resulting in improved adult feeding and ovipositing behaviour and further modifying adult fecundity and the offspring sex ratio. There was a positive correlation between oviposition preference and adult fecundity. 4. The results indicated that S. guani exhibited positive preference–performance correlations. This is most likely due to an adaptation to maternal hosts over multiple generations. However, foraging potential of adults to available cues from hosts may be driven quickly by an experience‐induced learning process rather than by natural selection processes shaped over many generations.     

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.     

Fecundity and development of the ectoparasitic waspNasonia vitripennis are dependent on host quality

The gregarious, ectoparasitoidNasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) was offered pupae representing seven fly species, but only members of two families (Sarcophagidae and Muscidae) were parasitized. Host acceptance as an oviposition site did not imply host suitability for parasitoid growth:N. vitripennis produced fewer progeny, a higher proportion of males, required a longer development time, and produced smaller adult wasps onMusca domestica L. (Diptera: Muscidae) than on the three sarcophagid species tested [Sarcophaga bullata Parker,S. crassipalpis Macquart, andPeckia abnormis (Enderlein) (Diptera: Sarcophagidae)]. The physiological and nutritional status of a preferred host,S. bullata, influenced oviposition behavior and development ofN. vitripennis. Progeny allocation and sex ratio, which were regulated by the female parasitoid during oviposition, differed on living and dead nondiapausing hosts and on diapausing pupae. Differences in the host's nutritional condition was reflected in changes of the wasp's development time and adult body size. Envenomation was essential for successful development of the parasitoid on nondiapausing hosts, but venom injection byN. vitripennis did not increase the suitability of diapausing or dead pupae. The results suggest that wasp development is enhanced by changes induced in the host by parasitism.     

Factors influencing brood sex ratios in polyembryonic Hymenoptera

Copidosoma sp. is a polyembryonic encyrtid wasp which parasitizes isolated hosts. Most broods of this wasp are unisexual, but some contain both sexes and the secondary sex ratio of these is usually highly female biased. The overall population secondary sex ratio is female biased. Walter and Clarke (1992) argue that because the majority of individuals must mate outside the natal patch, the bias in the population secondary sex ratio contradicts predictions made by Hamilton's (1967) theory of local mate competition (LMC). We suggest that the primary sex ratio is unbiased and that Walter and Clarke's results do not cast doubt on LMC. Instead these results imply that ovipositing females make a combined clutch size and sex ratio decision influencing whether individuals developing from a particular brood will outbreed or largely inbreed; for each case the predictions of LMC theory are not violated. If this interpretation is correct, what is of interest is the basis on which this decision is made rather than the population secondary sex ratio. We show that host encounter rate influences the proportions of mixed and single sex broods laid by Copidosoma floridanum, a related polyembryonic parasitoid. Among single-sex broods the primary sex ratio is female biased, but our results are in agreement with LMC theory since offspring developing from these broods will probably mate with siblings from adjacent hosts. We consider the egg load of females to be of major influence on oviposition behaviour, and that the mating structure of parasitoid offspring, potentially differential costs of male and female broods and the natural distributions of hosts both at oviposition and eclosion, require further study.     

Using hosts of mixed sizes to reduce male-biased sex ratio in the parasitoid wasp, Diglyphus isaea

Overproduction of males in mass rearing of parasitic Hymenoptera contributes to higher costs for biological control because only females directly kill pests. We present a technique, based on manipulating host composition, to generate less male‐biased sex ratios in parasitoid species that adjust their sex allocation in response to relative host size. Our system consisted of chrysanthemum, Dendranthema grandiflora Tzvelev var. ‘Miramar’; a leafminer, Liriomyza langei Frick (Diptera: Agromyzidae); and a commercially available parasitoid, Diglyphus isaea (Walker) (Hymenoptera: Eulophidae). We compared the offspring sex ratios of D. isaea females presented with different compositions of L. langei larvae on chrysanthemum. Presenting individual females with only large hosts increased mean sex ratio from 32 to 67% male over 2 days. However, presenting individual females with progressively larger hosts over 1 or 2 days reduced mean sex ratio from 90 to 100% male to less than 30% male. Groups of females produced sex ratios around 58% male if presented with both plants infested by only small hosts and plants infested by only large hosts. In comparison, groups of females produced sex ratios around 48% male if presented with plants infested by both small hosts and large hosts. We compared the use of both small hosts and large hosts to only large hosts for simulated mass rearing of wasps over 8 weeks. Using both small hosts and large hosts produced similar numbers of wasps as using only large hosts, but reduced mean sex ratio of weekly cohorts from 66% male to 56% male. The two techniques produced females of similar size, but using both large hosts and small hosts produced slightly smaller males than using only large hosts. The use of both small hosts and large hosts for mass rearing of D. isaea could reduce actual costs of females by 23%.     

Active defence of herbivorous hosts against parasitism: Adult parasitoid mortality risk involved in attacking a concealed stemboring host

Phytophagous insects have several defence strategies to defend themselves against attack by parasitic wasps. Larval lepidopteran hosts can defend themselves actively to prevent oviposition by the parasitoid. Among the aggressive kinds of behaviour exhibited by hosts against parasitoids are violent wriggling, biting and spitting. The behaviour of the braconid parasitoid Cotesia sesamiae attacking stemboring larvae inside their feeding tunnel in the plant stem was investigated in maize and sugarcane stem pieces and transparent artificial tunnels. Attacking a defending stemborer host inside the confined space of a tunnel was shown to be risky for the female parasitoid. A considerable proportion (25%) of female wasps were killed in their attempt to attack the spitting and biting host. No difference was found in the behaviour of C. sesamiae attacking the suitable host Sesamia calamistis or the unsuitable host Eldana saccharina. The consequences of this high mortality risk involved in each host attack is discussed in relation to the ecology of the parasitoid.     

Sex ratio regulation in Coccygomimus turionella Linnaeus (Hymenoptera: Ichneumonidae) and its ecological implications

Abstract. 1. Sex regulation and its relevance to the ecology of a k-selected pupal parasitoid was investigated in Coccygomimus turionellae.
2. The ratio varied with host size, females predominating in large hosts.
3. Egg transfer experiments and comparative mortality rates among the progeny of virgin and inseminated females demonstrated that the phenomenon was due to parental behaviour.
4. The study of the mechanism confirmed Aubert's hypothesis of host size estimation with the addition that the host had to be exposed from the poles and had to contain the proper host kairomones.
5. Host size was highly correlated to parasitoid fecundity via influences on size and longevity but not the number of ovarioles per female or daily egg production.
6. Host encounters are more limiting than egg production and host size had no effect on host acceptance.
7. Sex regulation allows a maximal host encounter rate at the same time that it maximizes the reproductive potential of female progeny.
8. Low host density increased the production of female progeny resulting in more offspring searching for the limited resource. Concealed hosts and a high host density resulted in a shift towards a Fisher 1:1 sex ratio.     

拟寄生蜂搜索产卵过程中对寄主的竞争

综述拟寄生蜂搜索产卵过程中对寄主竞争的最新研究进展.这类竞争具有四种方式,即标记寄主、杀卵和杀幼、守护寄主和捕食寄主.（1）标记寄主常涉及寄主标记信息素,这是由雌蜂在产卵前、产卵时或产卵后分泌的化学物质.寄主标记信息素常介导拟寄生蜂对已寄生和健康寄主的辨别、减少过寄生和多寄生、减轻种内和种间竞争压力.（2）雌蜂遇到已寄生寄主时,很多种类杀死前一雌蜂遗留的卵和幼虫,再产下自己的卵.雌蜂使用三种方法杀卵和杀幼,即产卵器穿刺、取食和使用有毒物质.通过杀卵和杀幼,产卵雌蜂清除了前一雌蜂遗留的后代,主动改善了寄主品质,从而有利于自身后代的生存.（3）守护寄主在肿腿蜂科、缘腹细蜂科、金小蜂科、缨小蜂科和茧蜂科中均有报道,守护者驱逐入侵者以保护后代及健康寄主.（4）捕食寄主不仅减少了健康寄主数量,且直接导致已寄生寄主中拟寄生蜂卵和幼虫的死亡.雌蜂一般在体内成熟卵量较少时捕食寄主.讨论了研究拟寄生蜂搜索产卵过程中竞争寄主的理论意义和实际应用价值.