Extended larval development time for aphid parasitoids in the presence of plant endosymbionts

Abstract 1. Variation in plant chemistry does not only mediate interactions between plants and herbivores but also those between herbivores and their natural enemies, and plants and natural enemies. 2. Endophytic fungi complete their whole life cycle within the host plant’s tissue and are associated with a large diversity of plant species. Endophytes of the genus Neotyphodium alter the chemistry of the host plant by producing herbivore toxic alkaloids. 3. Here we asked whether the endophyte‐tolerant aphid species Metopolophium festucae could be defended against its parasitoid Aphidius ervi when feeding on endophyte‐infected plants. In a laboratory experiment, we compared life‐history traits of A. ervi when exposed to hosts on endophyte‐infected or endophyte‐free Lolium perenne. 4. The presence of endophytes significantly increased larval and pupal development times, but did not affect the mortality of immature parasitoids or the longevity of the adults. Although the number of parasitoid mummies tended to be reduced on endophyte‐infected plants, the number of emerging parasitoids did not differ significantly between the two treatments. 5. This shows that the metabolism of individual aphids feeding on infected plants may be changed and help in the defence against parasitoids. An increase in parasitoid development time should ultimately reduce the population growth of A. ervi. Therefore, endophyte presence may represent an advantage for endophyte‐tolerant aphid species through extended parasitoid development and its effect on parasitoid population dynamics.     

广大腿小蜂对菜粉蝶蛹体型大小的产卵选择及后代发育表现

产卵选择与后代发育适合度之间的相关性是进化生态学的主要科学问题之一。为探究广大腿小蜂(Brachymeria lasus)对不同体型大小寄主的选择和后代发育表现的关系,采用饥饿方法处理4龄菜粉蝶(Pieris rapae)幼虫以获得体型大小(用体质量表示)差异较大的寄主蛹,供寄生蜂选择寄生。结果表明,广大腿小蜂显著偏好体型较大的寄主蛹,而且在体型较大的蛹内产雌性后代的概率更大;子代蜂体型大小与寄主蛹大小存在显著的正相关性,但子代蜂发育历期与寄主蛹大小无相关性。研究说明,广大腿小蜂对菜粉蝶蛹的寄生选择与后代发育表现,符合寄主大小-质量模型的预测。     

Facultative gregarious development in a solitary koinobiont parasitoid,Ephedrus californicus: implications of larval ecology and host constraints

In solitary parasitoids, the mandibulate first instars behave aggressively towards potential competitors so that generally only one larva survives per host. A ‘failure of competition’ may result in facultative gregarious development, however. We used Ephedrus californicus Baker (Hymenoptera: Braconidae: Aphidiinae), a solitary koinobiont parasitoid of aphids, to test two hypotheses in the laboratory that could explain facultative gregarious development. Gregarious development increased with the intensity of parasitism, with two (rarely three) parasitoids successfully developing in a single aphid. In heavily superparasitized hosts, interference between surviving larvae often caused abnormal pupation behaviour and inability to emerge from the mummy. The hypothesis that the survival of more than one larva per host is dependent on differences in larval age was not supported. The total body size in terms of dry mass of two males or two females developing together in the same host was higher than that of same‐sex counterparts developing singly. Females were larger than males with which they shared a host. Hypotheses about the evolutionary transition from a solitary to a gregarious lifestyle in parasitoid Hymenoptera have focused on lethal fighting between first instars but have ignored other constraints including immature mortality during later development and limiting host resources. Especially in species that pupate inside the dead host, specific requirements for pupation and emergence may determine whether one or several offspring per host can develop to adult.     

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.     

Convergent development of a parasitoid wasp on three host species with differing mass and growth potential

Koinobiont parasitoids develop in hosts that continue feeding and growing during the course of parasitism. Here, we compared development of a solitary koinobiont endoparasitoid, Meteorus pulchricornis Westmael (Hymenoptera: Braconidae), in second (L2) and fourth (L4) instars of three host species that are closely related (Lepidoptera: Noctuidae) but which exhibit large variation in growth potential. Two hosts, Mamestra brassicae L. and Spodoptera littoralis Boisduval, may reach 1 g or more when the caterpillars are fully mature, whereas Spodoptera exigua Hübner is much smaller with mature caterpillars rarely exceeding 200 mg. Parasitoid survival (to pupation) in the two host instars was much higher on the larger hosts than on S. exigua. However, other fitness correlates in M. pulchricornis were very similar in the three host species. Development time was fairly uniform in L2 and L4 hosts of the three host species, whereas wasps were larger in L4 than in L2 hosts. However, M. pulchricornis developmentally arrested each of the hosts differently. The mass of dying L2 and L4 hosts after parasitoid larval egression (i.e., when they emerge from the dying caterpillar) varied significantly, with S. littoralis being by far the largest and S. exigua the smallest. These results reveal that M. pulchricornis is able to adjust its own development in response to species‐specific differences in host resources.     

Size‐induced phenotypic reaction norms in a parasitoid wasp: an examination of life‐history and behavioural traits

The amount of resources available during development often affects body size, causing phenotypic variation in life‐history traits and reproductive behaviours. However, past studies have seldom examined the reaction norms of both life‐history and behavioural traits versus body size. We measured the phenotypic plasticity of several life‐history (age‐specific egg load, egg size, longevity) and behavioural (oviposition rate, host marking rate, walking speed) traits of the egg parasitoid Telenomus podisi Ashmead (Hymenoptera: Scelionidae) in response to body size variation. We predicted that life‐history traits would show more evidence of size compensation than behavioural traits, resulting in fewer positively‐sloped size versus trait reaction norms among the former. As predicted by life‐history models, smaller wasps appear to shift resource allocation towards early‐life reproduction, having a similar egg load to large individuals 9 days after emergence. Surprisingly, longevity was unaffected by body size. However, egg size, the number of offspring produced during oviposition bouts, and the rate of subsequent egg synthesis were greater for larger individuals. In addition, as predicted, the reaction norms of behavioural traits versus body size were all positively sloped. Thus, despite possible adaptive compensatory plasticity of life‐history traits by small individuals, behavioural constraints directly related to body size would contribute to maintaining a positive size–fitness relationship.     

Evidence for association of chromosomal form and development time from complex clines and geographic races in the grasshopper Caledia captiva (Orthoptera: Acridadae)

Evidence for an association between chromosomal form and development time in the grasshopper Caledia captiva (F.) was obtained through comparison of two geographic taxa and analysis of a complex latitudinal cline within one of the taxa. Northern populations of the Moreton taxon possess a metacentric genome and are slow-developing. In contrast, the Torresian taxon, distributed throughout northern, coastal Australia, a region of pronounced seasonality in rainfall, and southern populations of the Moreton taxon, which inhabit a region of pronounced seasonality in temperature, Soth have an acrocentric genome and are fast-developing. The convergence of chromosomal form and development time between Torresian and southern Moreton populations appears to be driven by convergence in life history. Seasonality limits grasshoppers to one generation per year and favours fast development. The transition between relatively acrocentric southern Moreton populations and relatively metacentric northern Moreton populations is gradual but not monotonic. Instead, a shift to a bivoltine life history in the middle of the transect occurs and is associated with shifts in both development time and chromosomal form. These results imply an adaptive role for chromosomal form, although the causative link between chromosomal variation and variation in development time remains to be established.     

Covariance of adult size and development time in the parasitoid waspAphidius ervi in relation to the size of its host,Acyrthosiphon pisum

Summary Adult size (in terms of dry weight; DW) and development time (T _p ) of the solitary parasitoidAphidius ervi varied when reared in different nymphal instars of its host, apterous virginoparae of the pea aphid (Acyrthosiphon pisum). Parasitoid DW increased with an increase in the DW of the host at parasitization, from the first to the third aphid instar. Female wasps gained 1.1 times more in DW than their male counterparts in all four host classes, butT _p did not significantly differ between the sexes. Parasitoid DW was consistently more variable thanT _p . The two traits covaried positively with an increase in host size from the first to the third instar, but they varied independently in parasitoids from fourth-instar hosts. The host size (and stage) at the time of parasitization imposes constraints on the growth and development of immatureA. ervi that are reflected in the pattern of covariation between DW andT _p . When growing in aphids below a certain size threshold, parasitoids can maximize fitness by a trade-off between DW andT _p . Consequently, the assumption implicit in host-size models of parasitoid oviposition decisions — that females incur a relatively greater reduction in size (used as an index of fecundity) than males when developing in poor quality hosts — can be falsified.     

Correlated responses to selection for faster development and early reproduction in Drosophila: the evolution of larval traits

Studies on selection for faster development in Drosophila have typically focused on the trade-offs among development time, adult weight, and adult life span. Relatively less attention has been paid to the evolution of preadult life stages and behaviors in response to such selection. We have earlier reported that four laboratory populations of D. melanogaster selected for faster development and early reproduction, relative to control populations, showed considerably reduced preadult development time and survivorship, dry weight at eclosion, and larval growth rates. Here we study the larval phase of these populations in greater detail. We show here that the reduction in development time after about 50 generations of selection is due to reduced duration of the first and third larval instars and the pupal stage, whereas the duration of the second larval instar has not changed. About 90% of the preadult mortality in the selected populations is due to larval mortality. The third instar larvae, pupae, and freshly eclosed adults of the selected populations weigh significantly less than controls, and this difference appears during the third larval instar. Thereafter, percentage weight loss during the pupal stage does not differ between selected and control populations. The minimum amount of time a larva must feed to subsequently complete development is lower in the selected populations, which also exhibit a syndrome of reduced energy expenditure through reduction in larval feeding rate, larval digging and foraging activity, and pupation height. Comparison of these results with those observed earlier in populations selected for adaptation to larval crowding and faster development under a different protocol from ours reveal differences in the evolved traits that suggest that the responses to selection for faster development are greatly affected by the larval density at which selection acts and on details of the selection pressures acting on the timing of reproduction.     

Testing the role of phenotypic plasticity for local adaptation: growth and development in time-constrained Rana temporaria populations

Phenotypic plasticity can be important for local adaptation, because it enables individuals to survive in a novel environment until genetic changes have been accumulated by genetic accommodation. By analysing the relationship between development rate and growth rate, it can be determined whether plasticity in life-history traits is caused by changed physiology or behaviour. We extended this to examine whether plasticity had been aiding local adaptation, by investigating whether the plastic response had been fixed in locally adapted populations. Tadpoles from island populations of Rana temporaria, locally adapted to different pool-drying regimes, were monitored in a common garden. Individual differences in development rate were caused by different foraging efficiency. However, developmental plasticity was physiologically mediated by trading off growth against development rate. Surprisingly, plasticity has not aided local adaptation to time-stressed environments, because local adaptation was not caused by genetic assimilation but on selection on the standing genetic variation in development time.