Testing predictions of small brood models using parasitoid wasps

Behavioral Ecology, 2009

Competition between siblings occurs in many taxa including parasitoid wasps. Larvae of solitary species eliminate competitors by engaging in aggressive behavior, thus restricting brood size to a single individual. In gregarious species, more than one offspring can develop per host. There are 2 models by which gregariousness can arise in a population of solitary individuals: 1) through a reduction in larval mobility (with the retention of aggressive behavior) or 2) through a reduction in fighting behavior or ability. When more larvae are present than can be supported by available host resources, these 2 models make opposing predictions regarding the process of brood size reduction: Mortality occurring early in larval development under the reduced mobility hypothesis versus mortality occurring throughout larval development under the reduced aggression hypothesis. Here, we measure changes in brood size over the course of larval development of the gregarious parasitoid, Cotesia flavipes. Superparasitized hosts contained approximately twice as many C. flavipes eggs as hosts parasitized by a single parasitoid female. Brood sizes in superparasitized hosts declined gradually as C. flavipes individuals developed, whereas brood sizes remained constant during larval development in singly parasitized hosts. An absence of wounded or destroyed larvae suggested no aggressive behavior. Collectively, these results support the reduced aggression hypothesis.

Functional Ecology, 2003

The ovigeny index, previously identified as both a significant fitness variable in parasitoid wasps and an important factor in parasitoid-host population dynamics, is the proportion of the maximum potential lifetime complement of eggs that is mature when the female emerges into the environment following pupal development. We tested the hypothesis that ovigeny index varies with female body size in parasitoid wasps. Body size measurements were obtained for 40 species in 13 families, representing a broad taxonomic and morphological diversity of parasitoid wasps. There was an almost 18fold difference in size between the smallest and the largest species. 2. Ovigeny index is shown to be negatively correlated with body size across speciessmaller wasps have a higher proportion of eggs mature at emergence than do larger wasps-a result supporting the hypothesis. This relationship has previously been observed within species. 3. The previously reported cross-species negative correlation between lifespan and ovigeny index is robust, as it still holds when variation in body size is controlled for. 4. We discuss the likely selective factors in the evolution of a link between ovigeny index and body size across species.

Functional Ecology, 2012

1. Life-history traits that shift in response to environmental conditions are well studied; however, allocation patterns of traits in the context of a trade-off shifting with the environment are hardly examined. 2. Using a polyembryonic parasitoid wasp system, we revealed features of the plasticity in allocation by observing the trade-off outcome between individual body mass and the number of offspring in a wasp brood in response to different host conditions. 3. We manipulated food availability to the host caterpillar at two different phases of development: early host instars during wasp embryo division and late host instars after completion of wasp embryo division. The food manipulation allowed us to shift the rate of development and the total mass of the wasp brood both separately and together through altered host development time and final size. 4. With greater host mass, the trade-off relationship between wasp body mass and brood size shifted, to the same extent for both sexes, towards higher wasp brood size and individual body mass. On the other hand, with faster host development, the trait combinations shifted towards greater wasp body mass but smaller brood size along the same trade-off relationship. 5. These shifts indicate a reduction of wasp brood size when resource level is reduced late in host development. This adjustment yields wasp brood size and body mass in rough proportion to each other and positively related to resource availability late in development. 6. Our demonstration of simultaneous shifts in traits under the trade-off relationship indicates that wasp broods respond differently to the different environmental conditions by altering body mass and the number of wasp individuals, suggesting that the plastic response in the trade-off allocation pattern may be adaptive.

Behavioral Ecology, 2009

Brood size has important implications for the fitness of both parents and offspring. In polyembryonic parasitoid wasps, each egg develops into many genetically identical embryos through clonal division inside the host. Thus, offspring may have the potential to affect brood size by adjusting the degree of embryonic division. In some species, a proportion of embryos develop into soldier larvae, which attack competitors inside the host. This may be another mechanism for offspring to affect final brood size. We investigated the effect of relatedness between competing clones on brood size in the polyembryonic wasp Copidosoma koehleri. We predicted that final brood size would be affected by the number and relatedness between competing clones inside the host. Additionally, we predicted that due to a competitive asymmetry between male and female clones (apparently only female clones produce a soldier larva), this effect would depend on the sex composition of wasps inside the host. We allowed 2 wasp eggs (laid either by 1 female or by different females) to develop in a host and counted the emerging adults. Relatedness between male clones did not affect brood size. However, female-containing broods of related clones were larger than broods of nonrelated clones, suggesting higher aggression of the soldier toward less related individuals. Dissections of hosts parasitized by 2 clones indicate that normally only 1 soldier survives and that it often eliminates unrelated clones. Thus, offspring control over brood size in response to relatedness is probably mediated by soldier aggression and not by clonal division.

Proceedings of the Royal Society B: Biological Sciences, 2007

Nest-building Hymenoptera have been a major testing ground for theories of parental investment and sex allocation. Investment has usually been estimated by the likely costs of offspring provisioning, ignoring other aspects of parental care. Using three experimental treatments, we estimated the costs of egg-laying and provisioning separately under field conditions in a digger wasp Ammophila pubescens . In one treatment, we increased the provisioning effort required per offspring by removing alternate prey items as they were brought to the nest. In two other treatments, we reduced parental effort by either preventing females from provisioning alternate nests or preventing them from both ovipositing and provisioning. Our results indicate that both egg-laying and provisioning represent significant costs of reproduction, expressed as differences in productivity but not survival. A trade-off-based model suggests that other components of parental care such as nest initiation may also repre...

Physiological …, 2011

Maternal pre-reproductive experience can impose phenotypic changes on offspring traits. These modifications may result from physiological constraints, although they can also increase the adaptation of offspring to their anticipated environment. Distinguishing between the two interpretations is often difficult. The effects of virgin female rearing density on their longevity and the characteristics of their male offspring are explored in the polyembryonic parasitoid wasp Copidosoma koehleri (Blanchard) (Encyrtidae: Hymenoptera). High rearing density may adversely affect maternal physiology or, alternatively, act as a cue for anticipated competition during the lives of the mothers and their offspring. Male offspring of group-reared females reach pupation significantly sooner than male offspring of females reared alone. This accelerated development may provide an advantage when competition from superparasitising individuals is expected. The lifespan of high-density females is longer than that of singly-reared females, and their male offspring survive longer, suggesting that crowded rearing does not reduce the fitness of females or offspring. The shortened development time of male offspring may reflect an adaptive epigenetic response to predicted competitive conditions.

The Open Ecology Journal, 2009

The trade-off between the number of offspring in a brood and the sizes of those offspring has been documented in a diverse array of species. Here we consider the factors that might account for the way that the trade-off achieves a particular size-number balance for female offspring. In particular, we determine whether bet hedging, along with traits influencing the expected short-term reproductive success of the brood, could select for a brood size and body mass that maximize long-term fitness. We also evaluate whether the optima based on these characteristics agree qualitatively with documented responses to brood resource levels. To develop a model incorporating these phenomena and keep it as simple and general as possible, we consider an organism that produces clonal broods, eliminating the complexities associated with parent-offspring and sibling conflict. We use the polyembryonic parasitoid wasp Copidosoma bakeri as the focal example. We find that bet hedging effects and key reproductive traits (the number of searching females, their host-finding efficiency, and survival prospects) are often sufficient to produce an optimal size-number balance and can account for the frequent tendency of both brood size and body mass to increase with brood resources (host size). Under some conditions, however, balancing the trade-off requires an additional minimum body-size constraint. Future empirical work and spatially explicit models must better establish the parameter magnitudes and functional relationships so that a deeper understanding and more precise predictions are obtained.

Evolutionary Ecology …, 2010

Offspring of polyembryonic parasitoid wasps (in which each egg divides clonally to produce several individuals inside a host body) adjust their numbers according to the host carrying capacity.

Behavioral Ecology, 1997

Mass-provisioning wasps package maternal investment into brood cells, sealed structures that contain all the provisions necessary for an offspring's growth and development. Optimal sex-allocation theory predicts that if maternal provisions determine die size of each offspring, and the amount of provisions available to each offspring varies, females should allocate well-stocked brood cells to the sex that benefits most from being large. I tested this hypothesis using observations of organ-pipe wasps, Trypoxylon politum, and dissections of their nests. A Mississippi population of T. politum was intensively studied from 1993 to 1995. This population fit the assumptions of optimal iex-al]ocation models by Green and Brockmann and Grafen. Female weight at emergence was 1.29 times that of males, and wing length was 1.15 times that of males. This discrepancy in size occurred because the volume of parental provisions strongly influenced adult body size, and better-stocked brood cells were preferentially allocated to daughters. Brood-cell volume correlated with both wing length and weight at emergence in both sexes, and the chance that a given brood cell contained a female offspring increased with increasing brood-cell volume. Fitness was positively related to body size for females, but I found no evidence of an advantage to large males. Although there was evidence of stabilizing selection for male wing length in one year, there was no evidence of an increasing relationship between body size and fitness (directional selection) for males in either 1993 or 1994. Female fecundity was positively related to body size in both years, indicating that larger females have increased reproductive success. The rate at which females provisioned brood cells was also correlated with body size. Observed patterns of investment in brood cells are quantitatively consistent with the predictions of optimal sex-allocation theory, but certain aspects of female provisioning behavior suggest females are not following a single "optimal" strategy. Patterns of provisioning were variable among different females at the study site during the same year. Large females tended to produce largef offspring. Although Brockmann and Grafen's model predicts a single, populationwide "switchpoint" from the production of male to female offspring, there was no evidence for such a switchpoint

Entomologia Experimentalis et Applicata, 2013

Body size can strongly influence the components of fitness. We investigated the role of body mass in the reproductive success of the parasitoid Copidosoma bakeri (Howard) (Hymenoptera: Encyrtidae). Copidosoma bakeri develops polyembryonically, producing thousands of genetically identical individuals from one egg. In this species, the brood size and body mass of clonal individuals developing within a host are in a trade-off relationship, which flexibly responds to the environment while maintaining female-biased sexual size dimorphism. We addressed the following questions regarding several possible effects of body mass on male and female reproductive success. (1) Is longevity positively related to body size in both sexes? (2) Is potential fecundity positively related to female body size? (3) Is mating competitiveness positively correlated with male body size? We found that body size increased with longevity of females, but not of males. Copidosoma bakeri was weakly synovigenic; a few eggs were produced after female maturation. Also, egg volume increased with age of the females. We did not detect a relationship between body size and potential fecundity, but mated females produced more eggs than unmated females. Thus, larger females may increase their opportunities to mate, find hosts, and oviposit primarily by living longer. Male mating competitiveness was not related to body mass. Mating reduced male longevity, suggesting a cost of mating that was independent of body mass. Differences between males and females in the effects of body mass on longevity suggest sex-specific selection pressures on body mass that may explain female-biased sexual size dimorphism.