Age-dependent clutch size in a koinobiont parasitoid

Abstract.  1. The Lack clutch size theory predicts how many eggs a female should lay to maximise her fitness gain per clutch. However, for parasitoids that lay multiple clutches it can overestimate optimal clutch size because it does not take into account the future reproductive success of the parasitoid.
2. From egg-limitation and time-limitation models, it is theoretically expected that (i) clutch size decreases with age if host encounter rate is constant, and (ii) clutch size should increase with host deprivation and hence with age in host-deprived individuals.
3. Clutch sizes produced by ageing females of the koinobiont gregarious parasitoid Microplitis tristis Nees (Hymenoptera: Braconidae) that were provided daily with hosts, and of females ageing with different periods of host deprivation were measured.
4. Contrary to expectations, during the first 2 weeks, clutch size did not change with the age of the female parasitoid, neither with nor without increasing host-deprivation time.
5. After the age of 2 weeks, clutch size decreased for parasitoids that parasitised hosts daily. The decrease was accompanied by a strong decrease in available eggs. However, a similar decrease occurred in host-deprived parasitoids that did not experience egg depletion, suggesting that egg limitation was not the only factor causing the decrease in clutch size.
6. For koinobiont parasitoids like M. tristis that have low natural host encounter rates and short oviposition times, the costs of reproduction due to egg limitation, time limitation, or other factors are relatively small, if the natural lifespan is relatively short.
7. Koinobiont parasitoid species that in natural situations experience little variation in host density and host quality might not have strongly evolved the ability to adjust clutch size.     

How two different host species influence the performance of a gregarious parasitoid: host size is not equal to host quality

1. Hyssopus pallidus Askew (Hymenoptera, Eulophidae) is a gregarious ectoparasitoid of the two tortricid moths species Cydia molesta Busck and C. pomonella L. (Lepidoptera, Tortricidae). It paralyses and parasitizes different larval instars of both species inside the apple fruit, which leads to the death of the caterpillar. 2. We assessed the influence of host species characteristics and host food on the performance of the parasitoid female in terms of clutch size decisions and fitness of the F(1) generation. 3. A comparison of clutch size revealed that female parasitoids deposited similar numbers of eggs on the comparatively smaller C. molesta hosts as on the larger C. pomonella hosts. The number of parasitoid offspring produced per weight unit of host larva was significantly higher in C. molesta than in C. pomonella, which is contrary to the general prediction that smaller hosts yield less parasitoid offspring. However, the sex ratio was not influenced by host species that differed considerably in size. 4. Despite the fact that less host resources were available per parasitoid larva feeding on C. molesta caterpillars, the mean weight of emerging female wasps was higher in the parasitoids reared on C. molesta. Furthermore, longevity of these female wasps was neither influenced by host species nor by the food their host had consumed. In addition we did not find a positive relationship between adult female weight and longevity. 5. Parasitoid females proved to be able to assess accurately the nutritional quality of an encountered host and adjust clutch size accordingly. These findings indicate that host size is not equal to host quality. Thus host size is not the only parameter to explain the nutritional quality of a given host and to predict fitness gain in the subsequent generation.     

Sex allocation and clutch size in the gregarious larval endoparasitoid wasp, Cotesia glomerata

The ability of the gregarious larval endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) to adjust progeny sex ratio and clutch size was investigated. The sex ratios (proportion of males) of field clusters were diverse, but many (70%) were female-biased. Nearly 10% yielded males only, suggesting a low percentage of unmated females in the field. In over half of the clusters containing females, the sex ratio was below 0.3. Superparasitism was common in the field, and females were believed to increase progeny sex ratio when attacking previously-parasitized hosts. However, in a single oviposition bout, sex allocation was not precisely controlled both in the field and laboratory. In the laboratory, the number of eggs laid in a day tended to decrease with increasing female age. For females which were offered two hosts per day and for those offered three hosts per day, this value became nearly the same several days after the start of oviposition. The total number of hosts which a female could parasitize during her lifetime was often less than 40. Some of the old females which attacked more than 40 hosts produced male-biased clutches; this was due to sperm depletion, because sperm remained viable throughout a female's lifetime. The amount of sperm used in a single oviposition bout seemed fixed and was not dependent on the number of eggs laid. Females with much oviposition experience did not produce new eggs to compensate for deposited eggs, and the efficiency of egg use (deposited eggs/total eggs) was more than 80%.     

Fitness effects of sex ratio response to host quality and size in the parasitoid wasp Spalangia cameroni

The parasitoid wasp Spalangia cameroni oviposited a greaterproportion of daughters in stable fly pupae than in house flypupae, even when I controlled for stable flies being smallerthan house flies. Sex ratio manipulation in response to hostquality has been modeled as being adaptive through an effectof host quality on the size and hence offspring production ofdaughters. 5. cameronis response to host species may insteadbe adaptive through an effect on larval survivorship, the developmenttime of daughters, and the size of sons. There was greater survivalof daughters than sons on stable flies. Controlling for hostsize, I found that development time of daughters was about 2%less on stable flies than on house flies. The decrease in developmenttime corresponds to a 2% increase in fitness as estimated byr, the intrinsic rate of increase, and is equivalent to abouta 9% increase in offspring production. Sons were about 2% largerfrom house flies than stable flies, which may increase offspringproduction by up to 3%. Host species had no consistent effecton size of daughters or development time of sons. In additionto the response to host species, mothers oviposited a greaterproportion of daughters in larger stable fly hosts. Whetherthis behavior is adaptive is unclear. Although offspring werelarger when they developed on larger stable flies, the rateof increase was less for daughters dian for sons. Effects ofstable fly size on offspring development time were negligible.     

Reproductive strategy as a major factor determining female body size and fertility of a gregarious parasitoid

The generally known “adult size‐fitness hypothesis” (ASFH) is applied to the gregarious parasitic wasp Anaphes flavipes (Foerster, 1841) (Hymenoptera: Mymaridae). ASFH is dependent on the reproductive strategy of the mother, which means the larger females have more offspring compared to smaller females. Two main factors, the mother's body size and food quantity received during larval development, can affect the body size of the offspring. For the first time, we present a study on the relative effect of both factors on fitness of the same species, wasp A. flavipes. Our data confirmed that females of A. flavipes with larger body sizes had more offspring compared to smaller ones. At the same time, mother's body size does not seem to affect the body size of the offspring. The other studied factor, quantity of food received during larval development, can be influenced by reproductive strategy (number of parasitoids developing in one host egg), host quality or the duration of development. We found only the reproductive strategy to have a statistically significant effect on body size. We demonstrated that the variable reproductive strategy (VRS) of wasp A. flavipes causes a plasticity in body size and future number of offspring. The generally known “trade‐off” scheme (more small offspring or fewer bigger offspring) does not apply to A. flavipes, because their large females have more offspring and it is their reproductive strategy that determines body size.     

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.     

The influence of competition between foragers on clutch size decisions in an insect parasitoid with scramble larval competition

The effect of competition between ovipositing females on theirclutch size decisions is studied in animals that lay their eggsin discrete units of larval food (hosts). In such species theeffect of competition depends on the form of the larval competitionwithin such units. In insect parasitoids, there might eitherbe contest (solitary parasitoids) or scramble competition (gregariousparasitoids) between larvae within a host For gregarious parasitoids,a decreasing clutch size with increasing competition betweenforagers is predicted. This prediction is tested in experimentsusing the parasitoid Aphaertta minuta. Parasitoids were eitherkept alone or in groups of four before the experiment, in whichthey were introduced singly in a patch containing unparasitizedhosts. Animals kept together laid on average clutches of 0.74eggs smaller than females kept alone (average clutch is 5.3),thereby confirming the prediction. Clutch size decreased withencounter number, which might be due to the adjustment of thefemale's estimate of the encounter rate with hosts. Finally,the results are compared with those reported for solitary parasitoids(that have scramble larval competition), for which it is predictedthat the clutch size will increase with increasing levels ofcompetition between females.     

聚寄生性蝶蛹金小蜂雌蜂体型大小对产卵策略的影响

在寄生蜂行为生态学研究中, 通常将寄主体型大小作为寄主品质的主要性状来探究寄生蜂的搜寻行为机理, 而忽略寄生蜂体型大小的意义。为揭示聚寄生蜂雌蜂体型大小对其产卵决策的影响, 在严格控制寄主菜粉蝶Pieris rapae蛹体型大小（体重）的情况下, 于室内观察了不同体型大小的蝶蛹金小蜂Pteromalus puparum雌蜂的产卵行为, 并调查了子代蜂数量（窝卵数）、 性比和体型大小的变化。结果表明： 雌蜂在寄主上的驻留时间随其自身体型增大而缩短, 但随寄主体重增大而延长。窝卵数和余卵量受到雌蜂体型大小的显著影响, 均随雌蜂体型增大而显著增加（P<0.05）; 但子代蜂性比不受雌蜂体型大小的显著影响 (P>0.05)。子代雌、 雄性体型大小均与雌蜂体型大小无关, 但子代雌蜂体型随寄主体重增大而增大。结果证实, 雌性蝶蛹金小蜂体型大小影响其部分产卵决策。因此, 在建立聚寄生蜂产卵决策模型中应考虑雌蜂体型大小这一重要变量因素。     

Encountering competitors reduces clutch size and increases offspring size in a parasitoid with female-female fighting

Understanding the size of clutches produced by only one parent may require a game-theoretic approach: clutch size may affect offspring fitness in terms of future competitive ability. If larger clutches generate smaller offspring and larger adults are more successful in acquiring and retaining resources, clutch size optima should be reduced when the probability of future competitive encounters is higher. We test this using Goniozus nephantidis, a gregarious parasitoid wasp in which the assumption of size-dependent resource acquisition is met via female-female contests for hosts. As predicted, smaller clutches are produced by mothers experiencing competition, due to fewer eggs being matured and to a reduced proportion of matured eggs being laid. As assumed, smaller clutches generate fewer but larger offspring. We believe this is the first direct evidence for pre-ovipositional and game-theoretic clutch size adjustment in response to an intergenerational fitness effect when clutches are produced by a single individual.     

On the evolution of clutch size and nest size in passerine birds

Summary I examined the hypothesis that the clutch size of some altricial birds may be limited by over-crowding of the nestlings in the nest, by comparing data on different species of European passerines. Large-sized birds build, relative to the body, larger nests than small-sized birds, both as regards the inner and the outer nest widths and as regards edge breadth; only inner nestcup depth did not change relatively to body size. Nest size also varied in relation to nesting place. Birds with open nests built off the ground had a rather narrow nestcup, whereas those with a domed nest, or which nest in a cavity, had a wide nestcup. When only open-nesters were compared, birds nesting on, or close to, the ground tended to have a wider nestcup than birds nesting above the ground. Inner nestcup width was correlated with the amount of mosses and lichens used in building the nest; the more of such materials the narrower the nestcup. The three variables: standardised body size, nesting place, and type of nesting material used accounted for 92% of the overall variation observed in inner nestcup width. When controlling for adult body size, clutch size was positively correlated with the size of the nestcup. A multiple regression analysis showed that relative nestcup depth, nest site, and type of nesting materials used, accounted for 64% of the overall variation in clutch size.     

Factors affecting sequential sex allocation in the parasitoid wasp Gryon japonicum

Female wasps of the solitary egg parasitoid Gryon japonicum (Hymenoptera: Platygastridae) allocate male and female offspring in a particular sequence to successive hosts. Male eggs are typically laid in the second host, and the sex allocation sequence is reset after a certain period of time. The present study aimed to examine the underlying mechanism to hold information and reset the sequence by using eggs of Riptortus pedestris (Heteroptera: Alydidae) as hosts. After completion of initial oviposition, a female wasp was treated by cold anesthesia for 1 h, exposure to a parasitized host for 3 h, or being kept at 15°C in darkness for 24 h, and then presented with three host eggs. Cold‐anesthetized females did not reset the sex allocation sequence, indicating that cold anesthesia did not block the mechanism of holding information about oviposition order. Frequent encounters with parasitized hosts were also insufficient to reset the sequence. However, being kept in cool, dark conditions significantly affected resetting, suggesting that low temperature lengthened the time required to reset the sequence. This implies that it is probable that the mechanism to hold information and reset sex allocation sequence in G. japonicum involves metabolism.     

The influence of female age and host size on the sex ratio of the parasitoid Opius concolor

The sex ratio of the progeny of single females parasitizing large hosts favoured the females (sex ratio=0.26); but on small hosts favoured the males (0.73). No differences in mortality of the sexes were detected. The sex ratio was independent of female age when large hosts were used. The percentage of males observed in the progeny of the first day of female oviposition was significantly greater than the mean, irrespective of the age at which female oviposition began. When females were exposed to small hosts, a greater percentage of females was observed in the progeny from the last days of oviposition.

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Résumé L'influence de la taille de l'hôteret de l'âge de la femelle sur le taux sexuel de la descendance a été étudiée sur le parasitoïde Opius concolor Szépl.Le taux sexuel de la descendance des femelles isolées est favorable aux femelles (t.s.=0,26) quand elles ont à leur disposition des hôtes de grande taille, tandis qu'avec des hôtes petits le taux sexuel est favorable aux mâles (t.s.=0.73). On n'a pas détecté de mortalité différentielle des sexes.Les pourcentage de mâles obtenu le premier jour de ponte des femelles sur les hôtes de grande taille est significativement différent de la moyenne, indépendamment de l'âge de la femelle à ce moment. Cependant, sur des hôtes petits, bien que restant favorable aux mâles dans l'ensemble, une plus grande proportion de femelles à partir des premiers jours de ponte a pu être observée.

     

Hatching order and sex ratio in Southern Grey Shrike Lanius meridionalis in relation to clutch size

Despite major advances in sex ratio theory, how offspring sex should vary with hatching order remains unclear. We examine nestling sex ratio in the Southern Grey Shrike Lanius meridionalis according to hatching order and clutch size. Southern Grey Shrike nestlings present a different sex ratio with body‐mass rank order depending on clutch size. When the clutch size was five eggs (with a very low risk of brood reduction; 13%) the less costly sex (male) was found at the end of the body mass hierarchy. However, when clutch size was six eggs (with a high risk of brood reduction; 42%) the larger sex (female) was found at intermediate positions in the hatching order, possibly to decrease competitive asymmetries.     

Variability in diapause propensity within populations of a temperate insect species: interactions between insecticide resistance genes and photoperiodism

Temperate insects generally use day length as a reliable cue for long-term seasonal changes in their environment. Significant variation in photoperiodism between and within populations is thought to be associated with genetic variation resulting from local adaptation. In this study, we investigated whether genetic variation associated with selection for insecticide resistance may be a source of divergence in the photoperiodic timing of diapause through pleiotropic interactions. Critical photoperiods for diapause induction were estimated in one susceptible and two insecticide-resistant homozygous strains of the codling moth Cydia pomonella , as well as in their reciprocal F₁ progeny. Diapause responses to naturally decreasing day length were subsequently followed in the laboratory strains and in two field populations of C. pomonella in south-eastern France. We found higher critical photoperiods for diapause induction in homozygous resistant individuals than in both homozygous susceptible and heterozygous ones. This partly explained the significantly earlier timings of diapause found in homozygous resistant individuals of both laboratory and field populations of C. pomonella under natural photoperiods, relative to those found in both homozygous susceptible and heterozygous ones. We assume that adaptive genetic changes associated with selection for insecticide resistance may generate substantial variation in the seasonal timing of diapause, an essential ecological feature of the fitness of insecticide resistance genes in this species.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 341–351.     

Host instar, body size and fitness in the koinobiotic parasitoid Aphidius nigripes

In aphidiine parasitoids, resources for growth and adult body size increase with host instar used by ovipositing females, but the fitness consequences of body size on fitness are poorly documented. We compared the fitness of male and female A. nigripesadults that varied in size as a consequence of developing in different instars of their host Macrosiphum euphorbiae. When reproductive fitness was measured without considering time, female wasps from small and large hosts performed similarly, contributing 125–175 foundresses plus 100–180 sons to the next generation. However, when expressed as the innate capacity for increase (r _m), female fitness correlated with host-induced variation of wasp size, indicating that micropopulations initiated by large wasps would increase faster. In a wind-tunnel, a sex pheromone plume from large female wasps induced more males to fly upwind when released at a distance of 50 cm downwind than small females, indicating that large females were sexually more attractive. With respect to male body size effects on fitness, large individuals performed similar to small ones, whether fitness was measured by lifetime mating frequency, fertile inseminations, or proportion of daughters among progeny born from their mates. When young naive males of unequal size were directly competing for mating with a virgin female, small and large males had equal mating success, and large individuals were no more successful than small ones at displacing a competitor already positioned on a receptive female. In a wind-tunnel test where males were scored on their ability to reach a female pheromone source, small and large males were equally affected by wind speed but reached the source located 50 cm downwind in equal proportions, suggesting similar capacity for finding mates by flying upwind. Our results indicate that despite host resources not being fixed at the time of attack for the koinobiont A. nigripes, fitness consequences of resource limitation by the mother may be perceived to be greater for daughters than sons, which would explain male-biased sex ratio in early-instar hosts.     

Host stage selection of the mealybug parasitoid Anagyrus spec. nov near sinope

The mealybug parasitoid Anagyrus spec. nov near sinope (Hymenoptera: Encyrtidae) is an undescribed parasitoid of the Madeira mealybug, Phenacoccus madeirensis Green (Homoptera: Pseudococcidae). We investigated the preference of Anagyrus spec. nov near sinope for six developmental stadia (first‐ and second‐instar nymphs, third‐instar immature females, third‐ or fourth‐instar immature males, pre‐reproductive adult females, and ovipositing adult females) of P. madeirensis and the fitness consequences of the host stage selection behavior. In the no‐choice test, Anagyrus spec. nov near sinope parasitized and completed development in all host stadia except third‐instar immature males. When all host stadia were offered simultaneously, the parasitoids preferred third‐instar immature and pre‐reproductive adult females. Dissection of the stung mealybugs revealed that the clutch size (number of eggs per host) was approximately four and three in the third‐instar and pre‐reproductive females, respectively, and one egg per first‐instar nymph. Parasitoids emerged from P. madeirensis parasitized at third‐instar or pre‐reproductive adult female completed development in the shortest duration, achieved a higher progeny survival rate, larger brood and body size, and the lowest proportion of males. We showed that the continued development of mealybugs had significant influence on the fitness of the parasitoids. Although deposited as eggs in first‐ or second‐instar nymphs, parasitoids emerged from mummies that had attained third‐instar or adult development achieved similar progeny survival rate, brood size, body size, and sex ratio as those parasitoids deposited and developed in third‐instar or adult mealybugs. By delaying larval development in young mealybugs, Anagyrus spec. nov near sinope achieved higher fitness by allowing the parasitized mealybugs to grow and accumulate body size and resources. We suggest that the fitness consequence of host stage selection of a koinobiont parasitoid should be evaluated on both the time of parasitism and the time of mummification.     

Offspring allocation in externally ovipositing fig wasps with varying clutch size and sex ratio

The simultaneous optimization of clutch size and sex ratio isa tricky problem. Unless parameters such as host size or fecundityexist to pin down the optimal clutch size, this problem remainselusive to analytical analysis. This is because the fitnesslandscape with respect to clutch size and sex ratio does nothave one single evolutionarily stable peak toward which thepopulation can evolve. To solve this problem, I used a computeremulation to optimize both clutch size and sex ratio using externallyovipositing fig wasps as a model taxon. The simulation approachallows the use of integer numbers of eggs rather than assumingthat females can produce any sex ratio between 0 and 1. Whenfemales have no information about the patches on which theyoviposit, they produce either large clutches with a strong femalebias or clutches of a single male egg. When females have completeknowledge of their oviposition site, a set of conditional substrategiesis evolutionarily stable. Again, these substrategies are eitherlarge clutches with a female bias or dutches consisting of asingle male egg. This dichotomous oviposition pattern resultsin unrelated males sharing a fig, a condition conducive to theevolution of fatal fighting. Selection on female ovipositionstrategies may therefore be an important driving force behindhigh levels of fighting observed between male fig wasps.     

转基因在苹果蠹蛾不育昆虫释放技术中的应用

使用不育昆虫释放技术是一项新兴的苹果蠹蛾Cydia pomonella(L.)防治方法,使用转基因得到不育雄蛾具有比传统辐射方法更多的优点。转基因技术通过使用基于piggyBac等转座子的质粒载体,并插入显性条件致死基因以培育遗传性别品系,同时插入荧光蛋白等标记基因来显示转基因的效果;在人工控制的发育条件,建立成熟的稳定苹果蠹蛾品系,用以最终的田间释放以达到防治的目的。     

Effects of predation on clutch size and egg dispersion in the codling moth Laspeyresia pomonella

ABSTRACT.

• 1 The effects of predation on clutch size and egg dispersion in the codling moth Laspeyresia pomonella (L.) were experimentally studied in an orchard by recording survival of eggs glued onto different substrates.
• 2 Egg mortality due to sucking insects was readily distinguishable from that due to chewing insects.
• 3 In both the field and laboratory, females preferred to deposit eggs singly on the uppersides of leaves.
• 4 Eggs on the uppersides of leaves and on fruit survived significantly better than did those on the undersides of leaves or on twigs.
• 5 Egg losses were highest during the early part of the flight season.
• 6 The smallest (one egg) and largest (nine eggs) clutches, associated with one egg-bearing leaf per twig, both suffered the lowest rates of predation. Factors selecting against oviposition on fruits and production of large clutch sizes are presented and discussed.