Host-feeding strategies of parasitoid wasps

Summary Three models of the evolution of host-feeding behaviour in parasitoid wasps are developed. The first assumes that the wasp host feeds purely to obtain resources to mature eggs (limited resource model) while the second assumes that host feeding provides energy for maintenance (pro-ovigenic model). The third model assumes that host feeding provides resources for both maintenance and egg maturation (resource pool model). Two variants of the third model are examined: the first assumes that the risk of mortality is constant and state-independent, the second that resource-depleted individuals suffer a higher risk of mortality. The models are analysed using a combination of stochastic dynamic programming and analytical techniques. The models make different predictions about the relationships between the probability of host feeding and egg load and host density. The available experimental evidence best supports the resource pool model.     

Sib-mating and sex ratio strategies in scelionid wasps

Abstract. 1. Theoretical models by Hamilton and others predict that the optimal sex ratio (proportion males) for a species should decrease with increasing levels of sib-mating. A test of this hypothesis is made for members of the parasitoid family Scelionidae.
2. Scelionid wasps differ in levels of sib-mating as a consequence of differences in host egg mass size, male—male aggression and female—female aggression.
3. Sex ratios for thirty-one species vary with inferred changes in sib-mating as predicted by the models.
4. Interspecific differences in sex ratio may represent different fvted strategies or different manifestations of a single variable strategy (sex ratio game) in response to parasitoid-produced stimuli.
5. Males and females are assigned non-randomly to egg masses in a manner which ensures mixed broods in the proper proportions for a particular species.     

Life-history strategies of a freshwater snail in response to stream permanence and predation: balancing conflicting demands

Summary Life-history parameters for the freshwater snail Physella virgata virgata were estimated in temporary and permanent streams with and without crayfish (known snail predators). Snails from the permanent stream with crayfish exhibited higher age and size at first reproduction, as well as higher size and age at death, compared to snail populations from both temporary streams and the permanent stream without crayfish. Life-history patterns of snails from the permanent and temporary streams without crayfish were indistinguishable, with small sizes at first reproduction and at death, as well as lower age at first reproduction and at death. The snail population from the temporary stream with crayfish showed an intermediate life-history pattern. The results suggest that snail life-history patterns vary with both predation pressure and habitat stability, exhibiting phenotypically plastic responses to the variables.     

The evolution of host use and unusual reproductive strategies in Achrysocharoides parasitoid wasps

We studied host selection and exploitation, two crucial aspects of parasite ecology, in Achrysocharoides parasitoid wasps, which show remarkable host specificity and unusual offspring sex allocation. We estimated a molecular phylogeny of 15 Achrysocharoides species and compared this with host (plant and insect) phylogenies. This tri-trophic phylogenetic comparison provides no evidence for cospeciation, but parasitoids do show phylogenetic conservation of the use of plant genera. Patterns of sequence divergence also suggest that the parasitoids radiated more recently (or evolved much faster) than their insect hosts. Three main categories of brood production occur in parasitoids: (1) solitary offspring, (2) mixed sex broods and (3) separate (split) sex broods. Split sex broods are very rare and virtually restricted to Achrysocharoides, while the other types occur very widely. Our phylogeny suggests that split sex broods have evolved twice and provides evidence for a transition from solitary to mixed sex broods, via split sex broods, as predicted by theory.     

Spermatogeny as a life-history index in parasitoid wasps

Reproduction is a major life-history trait but it has been studied mostly in relation to female reproductive effort. Recently, an ovigeny index using the proportion of oocytes ready to be oviposited at eclosion has been proposed for female insect parasitoids. Here, we propose a spermatogeny index for male parasitoid wasps. Prospermatogenic species have an index of 1, have all their spermatozoids mature at emergence and do not produce more later in life. At the other end of the spectrum, synspermatogenic species have no spermatozoids at emergence and produce them later in life. The level of spermatogeny should be linked to several other life-history parameters such as longevity, size, nutrition, distribution of mating opportunities and dispersion before and after mating. Data presented for some parasitoid species support the presence of variability in this male life-history parameter.     

Life-history variation within a taxon: a comparative analysis of birds and mammals

Most studies on life-history evolution discuss the necessity of distinguishing between extrinsic and intrinsic sources of variability in life-history traits. I use log/log plots of yearly neonate production in daugters (b) versus adult mortality (Ma) for 75 bird species and 88 mammal species to compare graphically life-history "fields" arranged by these selective forces along a "slow-fast continuum". Under the assumptions of steady-state and linear relationship between adult mortality and reproductive effort, as well as between juvenile survival and relative neonate weight, it is possible to place additional axes in the two-dimentional plot, and to predict covariations among demographic and individual growth traits. The functional regression analysis shows, that the assumptions are completely fulfilled, at least for birds, but mammals show nonlinear relationship between adult mortality and reproductive effort. This can be explained by peculiarities of metabolism and parental care in small mammals with high reproductive output. Hence, for birds the axis of relative neonate weight approximately coincides in direction with the juvenile survivorship axis, but this is not a case for mammals. In both taxa, the relative neonate weight is an invariant in relation to fecundity and adult mortality (but not in relation to adult body weight). This important feature, together with other intrinsic (energetic and phylogenetic) constraints, explains well-documented close covariations among traits, even when the effect of body size is factored out. It is argued that life-history and body size variations in birds and mammals mainly depend on a pattern of temporal resource deficiency, although this impact cannot be separated from that of extrinsic juvenile mortality.     

Egg maturation strategy and its associated trade-offs: a synthesis focusing on Lepidoptera

Abstract. 1. Insects vary considerably between and within orders, and even within the same genus, in the degree to which the female's lifetime potential egg complement is mature when she emerges as an adult. 2. The ‘ovigeny index’ (OI) – the number of eggs females have ready to lay divided by the lifetime potential fecundity – quantifies variation in the degree of early life concentration of egg production, and also variation in initial reproductive effort. 3. Here, an integrated set of hypotheses is presented, based on a conceptual model of resource allocation and acquisition, concerning trade‐offs at the interspecific level between initial investment in egg production (as measured by OI) and other life‐history traits in holometabolous insects. 4. The evidence supporting each of these hypotheses is reviewed, and particular attention is paid to the Lepidoptera, as relevant life‐history data are rapidly accumulating for this ecologically and economically important group. 5. There is evidence at the interspecific level supporting: (i) a link between OI and a trade‐off between soma and non‐soma in Trichoptera and Hymenoptera (the proportionate allocation to soma decreases with increasing OI); (ii) a negative correlation between OI and dependency on external nutrient inputs (via adult feeding) in Hymenoptera and in Lepidoptera; (iii) a negative correlation between OI and the degree of polyandry (and nuptial gift, i.e. spermatophore, use) in Lepidoptera; (iv) negative correlations between OI and resource re‐allocation capabilities (egg and thoracic musculature resorption) in Hymenoptera and in Lepidoptera; (v) a negative correlation between lifespan and OI in Trichoptera, Hymenoptera, and Lepidoptera, indicating a cost of reproduction; (vi) a link between winglessness and an OI of one in Lepidoptera; (vii) a negative correlation between OI and the degree of female mobility in winged Lepidoptera; and (viii) a negative correlation between OI and larval diet breadth (as mediated by oviposition strategy) in Lepidoptera.     

REVIEW: The evolution of polyembryony in parasitoid wasps

Polyembryony has evolved independently in four families of parasitoid wasps. We review three main hypotheses for the selective forces favouring this developmental mode in parasitoids: polyembryony (i) reduces the costs of egg limitation; (ii) reduces the genetic conflict among offspring; and (iii) allows offspring to adjust their numbers to the quality of the host. Using comparative data and verbal and mathematical arguments, we evaluate the relative importance of the different selective forces through different evolutionary stages and in the different groups of polyembryonic wasps. We conclude that reducing the cost of egg limitation is especially important when large broods are favoured. Reducing genetic conflict may be most important when broods are small, thus might have been important during, or immediately following, the initial transition from monoembryony to polyembryony. Empirical data provide little support for the brood‐size adjustment hypothesis, although it is likely to interact with other selective forces favouring polyembryony.     

Molecular analysis of parasitoid linkages (MAPL): gut contents of adult parasitoid wasps reveal larval host

Metamorphosing insects often have complex and poorly known life histories. In particular, what they feed on during their larval stages remains unknown for the vast majority of species, and its documentation only results from difficult and time-intensive field observations, rearing or dissections. Through the application of a DNA analysis of gut contents in adult parasitoid wasps, we were able to selectively sequence a diagnostic DNA marker that permitted the identification of the host used by these wasps during their larval stages. By reproducing these results in species with different life histories, we excluded other potential sources of host DNA, confirming that after ingestion by the parasitoid larva the host DNA can persist through metamorphosis in the abdominal contents of the adult wasp. Our discovery considerably extends the applicability of molecular analysis of gut contents by enabling the documentation of food used by insects during their larval stages and thus increasing the accuracy and precision of food web studies. The 24% success rate of our approach is surprisingly high considering the challenging context for host DNA preservation, and we discuss the factors possibly affecting this rate. We propose molecular analysis of parasitoid linkages (MAPL) as a new method to document host-parasitoid associations at a faster pace and with unrivalled precision. Because of the key regulatory role of parasitoid wasps in ecosystems, which makes them the most commonly used biological control agents, MAPL will have immediate applications in both basic and applied biological sciences.     

Egg resorption behaviour by the solitary egg parasitoid Anaphes nitens under natural conditions

We investigated under outdoor conditions and inside a climate chamber: (i) whether Anaphes nitens Girault (Hymenoptera: Mymaridae), an egg parasitoid of the Eucalyptus snout beetle, Gonipterus scutellatus Gyllenhal (Coleoptera: Curculionidae), experienced egg resorption, and (ii) how various treatments (location, food, and/or host supply) and body size influenced egg load, egg resorption, fecundity, and longevity. One‐day‐old females were mated and randomly assigned to five groups: (A) honey + hosts, (B) water + hosts, (C) only honey, (D) only water, and (E) control females killed at emergence. We compared the egg load of the newly emerged females, which represent the control group (egg load = ovarian eggs present at emergence), with the lifetime egg load (i.e., ovarian eggs + emerged offspring + not emerged offspring) of the females with various host and diet treatments, by dissection of the ovaries to find evidence of egg resorption. All groups reared outdoors had fewer eggs than the control, while indoors there was no significant difference. Outdoors, starved but host‐provided wasps (B) experienced the highest reduction of the lifetime egg load (51%). Groups without access to food (B + D) resorbed more eggs than groups provided with honey (A + C). Females with honey and hosts (A) had the highest lifetime fecundity, but those with water and hosts (B) showed a higher daily realized fecundity. Host‐deprived females with access to food (C) attained the longest lifespan. Our results suggest that under stress conditions, such as low temperature and food shortage, A. nitens females practice egg resorption, probably to save energy.     

寄生蜂抗药性研究进展

植物-植食性昆虫-寄生蜂三级营养结构之间由于长期相互适应和协同进化,产生了一系列独特的相互关系。选择压力将对害虫和寄生蜂的抗药性演化产生影响,但由于寄生蜂具有与植食性昆虫不同的生物学及生态学特性,选择压力对害虫和寄生蜂抗药性演化的影响作用也是不同的。研究结果表明,除体外杀虫剂对寄生蜂的直接汰选因素外,进入寄主昆虫体内的杀虫剂成分、寄主昆虫取食不同植物的特有成分以及气候因子等均会对寄生蜂的抗药性演化产生影响。     

Alternative male reproductive strategies in the peacock blenny

A 4-year study (1989–1992) of a population of Lipophrys pavo living in a brackish lagoon (Maugio, France) revealed three types of male during the spawning season (April to September): (i) nest-breeder males which are territorial, and have well-developed secondary sexual characteristics; (ii) non-functional males which were abundant only in 1991 when population density was very high and (iii) 1-year-old kleptogamic males with small body size which possess neither secondary sexual organs nor a testicular gland, although their testes are functional and hyperdeveloped (up to 10.5% of carcass weight). The percentage of kleptogamic males was fairly stable in the population (from 7.9 to 10.0%) and highly variable in their age class (from 8.5 to 51.3%). It is suggested that kleptogamy is a conditional strategy adopted by smaller males.     

Patch exploitation strategies of parasitic wasps under intraspecific competition

The optimal residence time of a forager exploiting a resourcepatch in the absence of competitors has been much studied sincethe development of the marginal value theorem. However, severalforagers are frequently observed exploiting the same patch simultaneously,and patch residence time has been surprisingly little studiedin such competitive situations. The few theoretical models developedon this topic predict that foragers should engage in a war ofattrition and stay in the patch longer than when foraging alone.We tested this prediction in Pachycrepoideus vindemmiae (Hymenoptera:Pteromalidae), a solitary parasitoid species in which femalesare known to defend the hosts they are exploiting via intraspecificfighting. By measuring the effect of direct (i.e., presenceof conspecifics) and indirect (i.e., presence of already-parasitizedhosts) competition on patch exploitation strategies, we revealedan apparent polymorphism of strategies. Indeed, in competitivesituations, some members of the population tended to retreatalmost immediately from the patch whereas other members tendedto remain, further exploit the patch, and be more involved ininteractions with conspecifics. The proportion of "retreaters"strongly depended on competition intensity and prior experienceof competition. The forager's physiological state (age) alsoaffects patch exploitation strategy and the intensity of interactionsbetween competing females. Our results highlight the necessityfor further theoretical studies that consider cases where contestsbetween foragers are costly and take into account prior experienceof competition and the forager's physiological state.     

Relative movement patterns of a tephritid fly and its parasitoid wasps

The extent of within-patch dispersal by a tephritid fly and its four major parasitoids was examined over three field seasons. Hosts and parasitoids were marked using acrylic paint and observed as they oviposited into the flowerheads of marsh thistle, Cirsium palustre. The average recapture rate pooled across all species was 22%. The four parasitoids showed consistently greater rates of movement than the host in all three years. In nearly all comparisons, male dispersal was less than female dispersal. There was no evidence that parasitoids moved longer distances after visiting low quality rather than high quality patches. In the one season it was studied, no correlations between movement and insect size were observed. The relevance of these observations to host-parasitoid population dynamics is discussed.     

Life-history evolution in Australian snakes: a path analysis

I recently attempted to investigate interspecific patterns in ecological traits of Australian snakes using univariate statistical techniques (Shine 1994), but high intercorrelations among variables (especially with mean adult body size) made it difficult to interpret the observed patterns. In the present paper, I attempt to tease apart causal factors using multivariate (path) analysis on the same data set (103 species, based on dissection of >22000 museum specimens). Two separate path analyses were conducted: one that treated each species as an independent unit (and thus, ignored phylogeny) and the other based on independent phylogenetic contrasts. Path coefficients from the two types of analyses were similar in magnitude, and highly correlated with each other, suggesting that most interspecific patterns among traits may reflect functional association rather than phylogenetic conservatism. Path analysis showed that indirect effects of one variable upon another (i.e., mediated via other traits) were often stronger than direct effects. Thus, even when two variables appeared to be uncorrelated in the univariate analysis, this apparent lack of relationship sometimes masked strong but conflicting indirect effects. For example, a tradeoff between clutch size and offspring size tends to mask the direct effect of mean adult body size on clutch size. Path analysis may also suggest original causal hypotheses. For example, interspecific allometry of sexual size dimorphism (as seen in Australian snakes, and many other animal groups) may result from a strong effect of another allometrically-tied trait (offspring size) on growth trajectories of females.