Influence of aphid size,age and behaviour on host choice by the parasitoid wasp Ephedrus californicus: a test of host-size models

Summary When host quality varies, parasitoid wasps are expected to oviposit selectively in high-quality hosts. We tested the assumption underlying host-size models that, for solitary species of wasps, quality is based on host size. Using Ephedrus californicus, a solitary endoparasitoid of the pea aphid, we evaluated the influence of aphid size (= mass), age and defensive behaviours on host selection. Experienced parasitoid females were given a choice among three classes of 5-day-old apterous nymphs: small aphids that had been starved daily for 4 h (S4) and 6 h (S6) respectively, and large aphids permitted to feed (F) normally. Wasps attacked more, and laid more eggs in, small than large aphids (S6>S4>F). This rank-order for attack did not change when females could choose among aphids of the same size that differed in age; however, wasps oviposited in all attacked aphids with equal probability. Host size did not influence parasitoid attack rates when aphids were anaesthetized so that they could not escape or defend themselves. As predicted by host-size models, wasp size increased with host size (F>S4; S6), but large wasps required longer to complete development than their smaller counterparts (S4E. californicus reflects a trade-off between maximization of fitness gains per egg and the economics of search-time allocation. Because large aphids are more likely to escape parasitization, a wasp must balance her potential gain in fitness by ovipositinng in a high-quality (large) aphid against her potential cost in terms of lost opportunity time if the attack fails.     

Insect-repelling behaviour in bovids: role of mass, tail length, and group size

Biting insects are costly to hosts, and insect-repelling movements of the tail, ears, head, and feet are widespread in mammals and effective in reducing bites. We investigate whether the 'peripheral stimulation model' can explain the regulation of this widespread behaviour pattern in a comparative study of bovids. The peripheral stimulation hypothesis predicts: (1) a positive association between insect-repelling rates and body size because larger hosts produce more of the sensory cues that attract biting insects; (2) that individuals in larger groups will exhibit a higher rate of insect defense behaviour if group size and insect attraction follows a linear function; and (3) larger species will evolve proportionately longer tails in response to higher rates of insect attack. To test these predictions, we observed insect-repelling behaviour in 26 species of bovids at a zoological park, and controlled for common ancestry with formal phylogenetic analyses (independent contrasts). Consistent with the peripheral stimulation hypothesis, rates of tail-switching and all insect-repelling behaviours combined were positively associated with body mass, whereas ear-flicking was positively associated with proportional tail length. Larger bovids had proportionately longer tails for more effective fly swatting. There was no significant association between insect-repelling rate and group size, suggesting that a nonlinear relationship exists between group size and insect attacks whereby individuals in larger groups do not experience an increased attack rate.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 383–392.     

Superparasitism and host size effects in Oomyzus sokolowskii, a parasitoid of diamondback moth

Many aspects of a parasitoid's biology may be affected by its host. Host size, for example, could affect parasitoid fitness, especially in gregarious parasitoids, in which the resource is used by multiple siblings. Oomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae) is a gregarious larval–pupal endoparasitoid of the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), a major pest of crucifers worldwide, and is able to superparasitize the host. This study focuses on the hypothesis that because resource availability is higher in larger hosts, parasitoids developing in larger hosts will fare better. However, superparasitized hosts are expected to yield larger numbers of parasitoid offspring of smaller body size. Results showed that superparasitism increased the number of parasitoid offspring produced per host and increased offspring longevity, but decreased offspring body size. However, developmental time and sex ratio of parasitoid offspring was similar among hosts parasitized once, twice, or three times. Regardless of superparasitism, parasitoids emerging from larger hosts that were fed honey solution lived longer than similarly fed progeny from smaller hosts (36.4 vs. 22.1 days). The results partially support the hypothesis that Oomyzus gained fitness from an increase in host size; moreover, superparasitism seems advantageous for Oomyzus due to increased offspring numbers and longevity.     

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.     

Host-size-dependent sex ratios among parasitoid wasps: does host growth matter?

Summary Waage's (1982) hypothesis that host-size-dependent sex ratios will occur in parasitoids of nongrowing hosts and not in parasitoids of growing hosts is examined using published data on parasitoid wasps. Waage's hypothesis is supported as a general, but not absolute, rule: among solitary parasitoid wasps, a significantly greater proportion of parasitoids of nongrowing than of growing hosts show some evidence of host-size-dependent sex ratios (85% versus 42%, G=6.54, P< 0.05). The premise of Waage's hypothesis-that for parasitoids which develop in a growing stage, host size at oviposition is not a good predictor of the amount of resources available to the developing parasitoid-is also examined. It is suggested that across host species Waage's premise will hold for some, but not all, parasitoids of growing hosts. Likely exceptions to Waage's premise, and thus his prediction, are discussed. Parasitoids of growing hosts which are expected to have evolved hostsize-dependent sex ratios include parasitoids which utilize a narrow size range of host species, parasitoids which can distinguish among host species by some criterion other than size, and parasitoids which utilize host species whose susceptible instars do not overlap in size.     

Parasitic Attachments by Overwintering Silver Lampreys, Ichthyomyzon unicuspis, and Chestnut Lampreys, Ichthyomyzon castaneus

We present evidence that at least some parasitic-phase silver lampreys, Ichthyomyzon unicuspis, and chestnut lampreys, I. castaneus, remain attached to host fish during the winter. Lake sturgeon, Acipenser fulvescens, harvested through the ice by spearfishers in the Lake Winnebago system in Wisconsin may bear silver lampreys or fresh lamprey wounds, and sturgeon with lamprey marks were significantly larger than sturgeon without them. Silver lampreys collected on paddlefish, Polyodon spathula, in the Wisconsin River in March were not significantly longer than silver lampreys collected previously in late October, but they were significantly heavier, an indication that they were feeding to at least some extent during the intervening period. Other large fish species, including northern pike, Esox lucius, and flathead catfish, Pylodictus olivaris, have been collected or observed during the winter with silver or chestnut lampreys attached. Although energy and nutrient intake by parasitic lampreys may be reduced during the winter, lampreys attached to hosts may also benefit from the hosts' mobility and ability to avoid potentially harmful situations.     

Viral Richness is Positively Related to Group Size,but Not Mating System,in Bats

Characterizing host traits that influence viral richness and diversification is important for understanding wildlife pathogens affecting conservation and/or human health. Behaviors that affect contact rates among hosts could be important for viral diversification because more frequent intra- and inter-specific contacts among hosts should increase the potential for viral diversification within host populations. We used published data on bats to test the contact-rate hypothesis. We predicted that species forming large conspecific groups, that share their range with more heterospecifics (i.e., sympatry), and with mating systems characterized by high contact rates (polygynandry: multi-male/multi-female), would host higher viral richness than species with small group sizes, lower sympatry, or low contact-rate mating systems (polygyny: single male/multi-female). Consistent with our hypothesis and previous research, viral richness was positively correlated with conspecific group size although the relationship plateaued at group sizes of approximately several hundred thousand bats. This pattern supports epidemiological theory that, up to a point, larger groups have higher contact rates, greater likelihood of acquiring and transmitting viruses, and ultimately greater potential for viral diversification. However, contrary to our hypothesis, there was no effect of sympatry on viral richness and no difference in viral richness between mating systems. We also found no residual effect of host phylogeny on viral richness, suggesting that closely related species do not necessarily host similar numbers of viruses. Our results support the contact-rate hypothesis that intra-specific viral transmission can enhance viral diversification within species and highlight the influence of host group size on the potential of viruses to propagate within host populations.     

The daily timing of lamprey attacks

Synopsis Laboratory and field data suggest that several species of parasitic lampreys forage primarily at night, but data forPetromyzon marinus are equivocal. Nocturnal foraging may minimize predation risk. This may be particularly important to those species of lampreys that inhabit shallow streams. Nocturnal foraging also may enhance foraging efficiency if potential hosts at night are quiescent and more easily approached. Future experiments should be designed to avoid several potential sources of bias.     

The interplay between plant traits and herbivore attack: a study of a stem galling midge in the neotropics

1. The relationship between plant traits and the frequency of attack by a stem galling midge, Neolasioptera sp. (Diptera: Cecidomyiidae), on Eremanthus erythropappus (Asteraceae) was studied. The morphological changes of the host after a galler attack and the potential effects of these changes on attacks by the next generation of gallers were analysed. The study was conducted in the Serra do Japi, São Paulo, south-eastern Brazil. 2. Galled branches were significantly longer, thicker, and had more leaves than ungalled branches. Accordingly, gall establishment was higher in the longer and more foliose branches. Hence, it is suggested that ovipositing females are maximizing their performance by selecting larger branches. 3. Galled branches were larger than ungalled branches of the same age. Two hypotheses, not necessarily exclusive, can explain this pattern: (1) the plant vigour hypothesis that the females are choosing the more vigorous, fast-growing branches, which still remain more vigorous after galling; or (2) the resource regulation hypothesis that galling increases branch growth rates, thus increasing resource quality for forthcoming conspecifics. 4. Co-occurrence frequencies of current and past generation galls showed that the likelihood of a branch being galled increased when it, or the branch from which it stemmed, had been galled before. The data indicated that this preference was conditioned by the number of previous attacks. Heavier attack intensities, such as one gall in the same branch and another in the branch from which it stemmed, decreased the probability of further galling. 5. The suggested links between herbivore attack and plant traits indicate that studies on host selection by phytophagous insects must take into account that herbivory itself may change the plant traits that are postulated to be selected by the insects.     

The effect of host size on quality attributes of the egg parasitoid,Trichogramma pretiosum

In a study of the quality ofTrichogramma pretiosum Riley (Hymenoptera: Trichogrammidae), we compared female wasps emerging from natural hosts, parasitized in the laboratory or the field with those emerging from factitious hosts used for commercial mass production. Females from the natural hosts were larger, more fecund, and longer lived than those from the factitious hosts. Compared to small females, large female wasps are substantially more fecund when honey (carbohydrate) is available but marginally more fecund when honey is unavailable. The size of a femaleT. pretiosum depends on two factors: the size of the host egg from which it emerges even when the wasp was gregarious, and the number of conspecifics that emerge with it. The similarities in the size distribution of female wasps emerging from natural hosts, in conjunction with the mechanism by whichTrichogramma measure host size and allocate eggs accordingly, suggests the hypothesis that size related components of fitness in femaleT. pretiosum are under strong selection in the field.     

Egg allocation by Bracon hylobii Ratz., the principal parasitoid of the large pine weevil (Hylobius abietis L.), and implications for host suppression

1 The braconid parasitoid Bracon hylobii Ratz. is one of the few specialist natural enemies of the large pine weevil, Hylobius abietis L., a destructive pest of conifer transplants. An assessment of its role as an agent of biological control requires a detailed knowledge of the allocation of its reproductive effort. 2 Parasitoid females were continuously observed in laboratory culture with individually reared host larvae in bark discs. The outcome of sequential parasitoid–host encounters was recorded by subsequent examination of hosts and by rearing all parasitoids. 3 Parasitoids avoided ovipositing on host larvae < 100 mg fresh weight, even though such larvae represented sufficient biomass for complete parasitoid development. All larger larvae were vulnerable to attack, which leaves a window of vulnerability for parasitoids of about 90% of weevil larval life. 4 Parasitoids presented with a range of host sizes showed no preference above 100 mg for the size of host first attacked, but allocated more eggs and a greater total handling time to larger hosts. 5 Most eggs were deposited on the first host attacked, with progressively fewer allocated to subsequent hosts. However, oviposition experience did not affect the time spent on the next host. 6 From these results it is anticipated that when weevil larval size is reduced by less favourable feeding substrates, fewer parasitoid eggs will be allocated to each but more host larvae will ultimately be attacked. 7 Generation time, host finding, oviposition rate, clutch size, life expectancy and diapause induction are strongly affected by temperature. Life expectancy is substantially shorter for parasitoids deprived of non‐host food supplement. At 15 and 20 °C the number of hosts attacked and the number of eggs deposited decreased with female age. 8 Bracon hylobii is inevitably poorly synchronized with a variable life‐cycle host; it is egg‐limited and can enter diapause at a relatively high field temperature. None of these characteristics suggest that it could stabilize the abundance of its host below an economically acceptable threshold density. However, the reproductive potential of the parasitoid suggests that it could make a significant contribution to larval mortality and suppress adult recruitment, thus complementing other control strategies.     

Growth of coyote willow and the attack and survival of a mid-rib galling sawfly,Euura sp.

We studied the relationship between variation in age and shoot characteristics of the host plant Salix exigua Nuttall (coyote or sandbar willow) and the attack and survival of Euura sp. (an unnamed leaf-midrib galling sawfly). Variation in shoot characteristics resulted from reduced growth as willow ramets aged. Mean shoot length per ramet and mean longest leaf length per shoot decreased by 95% and 50% respectively between 1- and 9-year-old willow ramets. All measured shoot characteristics-shoot length, longest leaf length, number of leaves per shoot, and mean internode length-were significantly negatively correlated with ramet age (r ² ranged from –0.23 to –0.41). Correlations between shoot characteristics were highly positive, indicating that plants also grew in a strongly integrated fashion (r ² ranged from 0.54 to 0.85). Four hypotheses were examined to explain sawfly attack patterns. The host-plant hypothesis was supported in explaining enhanced larval sawfly survival through reduced plant resistance. As willow ramets aged, the probability of Euura sp. attack decreased over 10-fold, from 0.315 on 1-year-old ramets to 0.024 on 2- to 9-year-old ramets. As shoot length increased, the probability of sawfly attack increased over 100-fold, from 0.007 on shoots <100 mm, to 0.800 on shoots in the 1001–1100 mm shoot length class. These attack patterns occurred even though 1-year-old ramets and shoots >500 mm each represented less than 2% of the total shoots available for oviposition. Host plant induced mortality of the egg/early instar stage decreased by 50% on longer leaves and was the most important factor determining survival differences between vigorous and non-vigorous hosts. Sawfly attack was not determined by the resource distribution hypothesis. Although shoots <200 mm contained 82% of the total leaves available, they contained only 43% of the galls initiated. The attack pattern also was not explained by the gall volume hypothesis. Although gall volume increased on longer shoots, there was no significant variation in mid or late instar mortality over shoot length, as would be expected if food resources within smaller galls were limited. The natural enemy attack hypothesis could not explain the pattern of oviposition since predation was greater on longer shoots and leaves. In addition, larval survival was related to oviposition behavior. Due to a 69% reduction in late instar death and an 83% reduction in parasitism, survival of progeny in galls initiated close to the petiole base was 2.8 times greater than in galls initiated near the leaf tip. A 75% reduction in gall volume over this range of gall positions may account for the observed increases in late instar mortality and parasitism.     

Diadegma mollipla parasitizing Plutella xylostella: host instar preference and suitability

Oviposition decisions (i.e., host selection and sex allocation) of female parasitoids are expected to correspond with host quality, as their offspring fitness is dependent on the amount and quality of resources provided by a single host. The host size model assumes that host quality is a linear function of host size, with larger hosts believed to contain a greater quantity of resources, and thus be more profitable than smaller hosts. We tested this assertion in the laboratory on a solitary larval–pupal parasitoid Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae) developing on three instars (second–fourth) of one of its hosts, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). In a no‐choice test, parasitism levels and sex ratio (i.e., proportion of female progeny) were significantly high in hosts attacked in the second instar followed by third then fourth instars. However, the few parasitoids that completed a generation from the fourth instars did so significantly faster than conspecifics that started development in the other two instars. In direct observations, however, the parasitoids (i) randomly attacked the various host instars, (ii) spent a similar period examining the various host instars with their ovipositors, (iii) subdued all three host instars with about the same effort, and (iv) no statistical differences were observed in the attack rates on the three host instars. In a choice test, the females parasitized significantly more third instars followed by second then fourth instars. However, total parasitism in this experiment was 43% lower compared to parasitism of only second instars in the no‐choice test. No significant differences were detected in progeny sex ratios. In both choice and no‐choice tests, significantly more fourth instars died during the course of the experiments than second instars, while third instars were intermediate. The higher parasitism of third than second instars in the choice test indicates that the females perceived larger hosts as higher quality than smaller hosts, despite their lower suitability for larval development.     

Selection Strategies of Parasitized Hosts in a Generalist Parasitoid Depend on Patch Quality but Also on Host Size

Host rejection, superparasitism, and ovicide are three possible host selection strategies that parasitoid females can adopt when they encounter parasitized hosts. These differ in costs (in terms of time and energy required) and benefits (in terms of number and quality of offspring produced). Their relative payoff should vary with patch quality, (i.e., proportion of parasitized hosts present), and female choice between them should be adapted accordingly. We conducted behavioral observations to test the effect of the ratio of parasitized/unparasitized hosts present in a patch on the host selection strategies of Pachycrepoideus vindemmiae Rondani (Hymenoptera: Pteromalidae). This species being a generalist known to attack hosts of a great range of size, we also tested the impact of host size on female decisions with two host species differing greatly in size (Drosophila melanogaster and Delia radicum). We evaluated the adaptive value of each strategy in relation to host parasitization status and host size by measuring their duration and the potential number of offspring produced.     

Sex Allocation by a Parasitoid Wasp (Hymenoptera: Ichneumonidae) to Different Host Species: A Question for the Mechanism of Host Size Estimation

Sex allocation by the polyphagous solitary pupal parasitoid wasp Pimpla luctuosa Smith to a small host species, Galleria mellonella (L.), and a large host species, Mamestra brassicae L., was investigated to test whether female wasps responded to hosts of different sizes across different host species. In the experiments, both host species were presented to each test female wasp. Primary and secondary sex ratio experiments revealed that female wasps laid more female eggs in larger pupae of each host species, indicating that female wasps recognized size differences within host species. The wasp sex ratio (male ratio) from M. brassicae, however, was much higher than that expected on the basis of the sex ratio curve from different-sized G. mellonella. Larger hosts of each host species yielded larger wasps, indicating that the host size estimation by female wasps across different host species was incomplete or was not simple. These results suggested that P. luctuosa evaluated host size not only by physical measures such as dimension but also by other unknown measures. A possible explanation for the adaptiveness of different sex ratio responses by Pimpla luctuosa to different host species was discussed.     

Do sex ratio and development differ in sexually size‐dimorphic shiny cowbirds (Molothrus bonariensis) parasitizing smaller and larger hosts?

Two possible patterns of bias in primary sex ratio have been proposed for size‐dimorphic brood parasites that do not evict host chicks: (1) larger males should be laid at greater frequency in hosts larger than the parasite because they compete better (increasing their survival) than females with large host nest‐mates, and (2) more costly males (i.e. the larger sex) should be laid at greater frequency in hosts smaller than the parasite because, in these hosts, parasite nestlings are provisioned at a higher rate and grow faster than in larger hosts. We tested these hypotheses in two hosts of the sexually size‐dimorphic shiny cowbird, Molothrus bonariensis, one smaller (house wren, Troglodytes aedon) and one larger (chalk‐browed mockingbird, Mimus saturninus) than the parasite. We measured: (1) sex ratio at laying; (2) development of sexual differences in body mass during the nestling stage; and (3) chick survival and sex ratio of chicks before fledging. In both hosts, we found sexual differences in body mass of nestlings from 7 days of age onwards, although we did not find a bias in the sex ratio of eggs laid and chicks fledged. The results of the present study do not support the hypothesis that shiny cowbird females benefit from biasing the primary sex ratio depending on the size of the hosts they parasitize. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 442–448.     

Using superparasitism by a stem borer parasitoid to infer a host refuge

Abstract. 1. Macrocentrus cingulum Reinhard (Hymenoptera: Braconidae), a parasitoid of the European corn borer, Ostrinia nubilalis, and the Asian corn borer, O. furnacalis (Lepidoptera: Pyralidae), has high fecundity but has been reported to parasitize a low proportion of host larvae. This was corroborated in field collections: in Hebei (China) and Delaware (U.S.A.), M. cingulum parasitized only 15 and 25%, respectively, of hosts collected. 2. Because M. cingulum females cannot oviposit through plant tissue, they must parasitize hosts either before they have bored into stalks or while they are near entrance holes, so that at any one time, many Ostrinia larvae may be unavailable to M. cingulum. This refuge, together with fluctuations in abundance of foraging M. cingulum females, may explain why M. cingulum parasitizes relatively few Ostrinia larvae. 3. To test this hypothesis, levels of superparasitism were measured in the field. Low parasitism resulting from a refuge for host larvae should cause high rates of superparasitism in hosts outside the refuge. 4. Because M. cingulum is polyembryonic, the number of parasitoids per host does not indicate the level of superparasitism. Random amplified polymorphic DNA markers were used to determine the number of different genotypes emerging from each host. The resulting frequency distributions were fitted to those expected under random oviposition to estimate the proportion of Ostrinia larvae unavailable to M. cingulum. 5. In the samples from Hebei and Delaware, the level of superparasitism was much higher than expected by chance if all hosts were available. Fitting the frequencies of genotypes per host to a Poisson distribution, the authors estimated that 74–82% and 69–74% of host larvae were unavailable to M. cingulum in these collections, respectively. This means that M. cingulum parasitized 60–84% and 82–95% of available hosts in these collections, respectively. These levels of parasitism contrast strongly with the 15–25% found when all hosts were assumed available for oviposition. 6. Genetic distances of M. cingulum within and between hosts did not differ, allowing rejection of the hypothesis that high levels of superparasitism resulted from a female laying several eggs in the same host. 7. The hypothesis that M. cingulum parasitizes few Ostrinia larvae because many larvae are in a refuge explains these data and previously published information better than other hypotheses that have been suggested.     

外寄生性花绒寄甲的寄生选择及其发育表现

"选择-表现"假说认为,成虫应该选择有利于子代发育的高品质寄主,但在寄主选择中,除了寄主品质外,其他因素也可能影响寄主选择决策。寄主选择研究通常以成虫为对象,而对那些初龄幼虫选择寄主的寄生性昆虫很少关注。以1龄幼虫积极搜寻寄主的寄生性花绒寄甲为模式生物,采用双选试验设计,观察了花绒寄甲初孵幼虫在不同体重青杨天牛幼虫之间、在已被寄生与健康的黄粉虫蛹之间的寄生选择性;然后采用回归设计,观察了花绒寄甲寄生若干不同体重的青杨天牛幼虫后的发育表现。研究结果表明,花绒寄甲1龄幼虫对体型较大的青杨天牛幼虫的选择偏好显著大于对体型较小的寄主幼虫的选择,选择大体型幼虫的比值比是选择小体型幼虫的4.55倍;对已被寄生的寄主黄粉虫蛹的选择偏好显著大于对健康寄主蛹的选择,选择已被寄生寄主的比值比是选择健康寄主的12.57倍。寄生青杨天牛幼虫的花绒寄甲幼虫发育历期平均为11.49 d、蛹历期为26.67 d、幼虫发育至成虫的羽化率50%,这些发育表现与寄生时青杨天牛幼虫的体重没有显著关系。但刚羽化寄甲成虫体重与寄生时寄主的体重存在显著的正直线关系:寄生时的寄主体重每增大0.01 g,羽化出的寄甲成虫体重增大近0.08%;方差分析寄甲成虫体重在不同寄主体重水平之间的差异表明,从体型较大寄主中羽化的寄甲成虫体重显著大于从体型较小寄主中羽化的成虫。研究结果说明,花绒寄甲初孵幼虫在寄主选择决策时,在寄主体型大小与被寄生状态之间可能采取折衷对策,而且对体型大小不同的寄主选择与子代发育适合度表现存在一致性,从而支持"选择-表现"假说。     

Competitive Growth Strategies in Intermediate Hosts: Experimental Tests of a Parasite life-History Model Using the Cestode, Schistocephalus solidus

In parasites with a complex life cycle, the fitness of an individual depends on its probability of reaching the final host and on its fecundity. Because larval growth in intermediate hosts may affect both transmission and adult size, selection should optimize growth patterns that are conditional on the presence and number of conspecific competitors. A recent model predicts that the total parasite volume per host should increase with intensity if larvae are able to vary growth depending on the number of conspecifics in the host (Life History Strategy hypothesis, i.e. LHS). Further, we would here expect growth rates to increase with intensity. By contrast, under the simplest alternative hypothesis of Resource Constraints (i.e. RC), the total parasite volume should remain constant. We experimentally infected copepods Macrocyclops albidus with the cestode Schistocephalus solidus to achieve 1, 2 or 3 parasites per host taking care that hosts had similar quality status at each infection level, and compared larval growth trajectories at the three intensity levels. The asymptotic total parasite volume was larger in double and triple infections than in single infections. Furthermore, the asymptotic total parasite volume was significantly larger in triple than in double infections but only in larger copepods that were less constrained by a host-size ceiling effect. These results, together with the fact that growth rates increased with intensity, support the LHS hypothesis: procercoids of a tapeworm may “count” their conspecific competitors in their first intermediate host to harvest its resources strategically until the next step in their complex life cycle. Co-ordinating editor: A. Biere     

A comparative study of host selection in the European cuckoo Cuculus canorus

Certain kinds of hosts are commonly regarded as being more suitable than other for rearing European cuckoos (Cuculus canorus) – insectivores that lay small eggs and have open, shallow nests – although empirical tests of cuckoo host selection are lacking. We analysed host use by the European cuckoo in 72 British passerines that are potential hosts and for which there was information available on life-history variables and variables related to cuckoo-host coevolution, such as rate of parasitism, rejection rate of non-mimetic model eggs and degree of cuckoo-egg mimicry of host eggs. The relative population size of the host species affected parasitism rate most strongly, followed by relatively short duration of the nestling period, and the kind of nest, with cuckoos selecting open-nesting hosts. However, the effect of the nestling period could be related to host body size and the kind of nest used, because hole-nesting species normally have longer nestling periods than open-nesters. We re-analysed the data excluding hole nesters and corvid species (species with larger body mass), but the results remained identical. The European cuckoo may benefit from selecting hosts with short nestling periods because such hosts provide food for their nestlings at a very high rate. When only those species known as cuckoo hosts were analysed, the variable that best accounted for the parasitism rate was duration of the breeding season. Therefore, availability of potential hosts in both time and space is important for cuckoos in selecting hosts. Received: 16 July 1998 / Accepted: 27 October 1998