Guy Boivin

Host age and fitness‐related traits in a koinobiont aphid parasitoid

Abstract.  1. Trade‐offs play a key role in species evolution and should be found in host–parasitoid interactions where the host quality may differ between host age categories.

Modeling the impact of plant structure on host-finding behavior of parasitoids

For many parasitoid species, the final step of host location occurs on plants whose structure varies in time and space, altering the capacity of parasitoids to exploit hosts. Plant structure can be defined by its size, heterogeneity and connectivity. We tested the hypothesis that these three components and all possible interactions affect the level of parasitism of Trichogramma evanescens and that parasitism can be predicted if the structure of a plant is measured. We quantified and varied the structure of three-dimensional artificial plants to determine which component(s) of plant structure explain variability of parasitism and to develop a model that predicts parasitism by Trichogramma females. This model was validated with three natural tritrophic systems. The experiment with artificial plants revealed that plant structure affected host-finding success, which was higher on plants with a simple structure and low on plants with a complex structure. A response surface regression showed that the linear and quadratic terms of connectivity were highly significant, indicating that connectivity best explained the variability in the rate of parasitism obtained. The interaction between connectivity and heterogeneity was also significant. Observed values of parasitism from experiments with three natural tritrophic systems fit predicted values of parasitism generated by the model, indicating that parasitism can be predicted if heterogeneity and connectivity of a plant are known. Consequences of these results in regard to population dynamics, evolution and biological control are discussed.

Comparative Dispersal of Trichogramma evanescens and Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) in Relation to Environmental Conditions

Abstract Insect parasitoids have to disperse to locate host habitat and host, and individuals of small species, such as egg parasitoids, are expected to be very sensitive to climatic conditions during dispersal. The effect of 16 environmental variables on the spatio-temporal pattern of dispersion of Trichogramma evanescens Westwood, originating from Egypt, and Trichogramma pretiosum Riley, originating from Quebec, was quantified. In T. evanescens, the accumulation of solar radiation at temperatures >15°C had a significant effect on its dispersion. When >15,000 kJ/m2 were accumulated, more parasitism was observed in a larger area. When wind blew above 15 km/h for >4 h in a day, dispersion also decreased significantly. In T. pretiosum, accumulated solar radiation had no significant effect, whereas it took 8 h of wind >15 km/h to significantly reduce dispersion. T. evanescens, a warm climate species, thus appears to be more sensitive to temperature. Its relatively higher sensitivity to wind condition may indicate that this species is adapted to aggregated or rare hosts, which require more flight control for location.

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.

Differential prey utilization by the generalist predator Coleomegilla maculata lengi according to prey size and species

Prey utilization by the generalist predator Coleomegilla maculata lengi Timb. (Coleoptera: Coccinellidae) of three sympatric lepidopterous species was quantified in relation with prey size (age) and prey species. Based on optimal foraging theory, we argued that costs associated with the utilization of small and large prey are higher than those of intermediate prey size. As a result, we expected a higher prey consumption rate on intermediate prey size leading to a convex prey consumption curve. Laboratory experiments showed that, within a given prey instar, Coleomegilla maculata lengi preyed more on Plutella xylostella (L.) compared to Artogeia rapae (L.) and Trichoplusia ni (Hübner). Generally, prey consumption rate by Coleomegilla maculata lengi on the three prey species decreased with increasing immature prey size (age). The predation efficacy of Coleomegilla maculata lengi adults and fourth instar larvae was higher compared to younger coccinellids (L2). Although, Coleomegilla maculata lengi showed a higher level of predation on smaller immature prey, we demonstrated that it is not the optimal size range for this predator. As predicted, prey weight consumption rate by Coleomegilla maculata lengi was higher at intermediate prey size leading to a convex prey utilization curve. The beneficial impact of Coleomegilla maculata lengi predation on the host plant was also estimated by using a Protection Index that considers the differential predation caused by the coccinellids and the relative importance of each pest species in terms of plant injury. Coleomegilla maculata lengi has a more significant beneficial impact when it preys on T. ni immatures.

Intra- and interspecific host discrimination in two closely related egg parasitoids

Intraspecific host discrimination is frequently found in solitary parasitoids, but interspecific host discrimination, where female parasitoids recognize hosts already parasitized by females of other species, is rare. This particular behaviour appears to be adaptive only under specific circumstances. In this paper, we quantified intraspecific host discrimination in Anaphes n. sp. (Hymenoptera: Mymaridae), an endoparasitoid of the eggs of Listronotus oregonensis (LeConte) (Coleoptera: Curculionidae) and interspecific host discrimination toward eggs parasitized by Anaphes sordidatus (Girault), a sympatric species competing for the same resource in similar habitats. To examine host discrimination, choice experiments were used where the females had to choose between different categories of eggs (unparasitized, parasitized by Anaphes n. sp. or A. sordidatus). Superparasitism and multiparasitism were avoided in experiments where the female had a choice between unparasitized hosts and hosts parasitized by the same female, by a conspecific or by a female A. sordidatus. When all hosts available were parasitized, conspecific superparasitism occurred more often than self-superparasitism or multiparasitism. These results indicated that females Anaphes n. sp. were capable of self-, conspecific and interspecific discrimination. Self-discrimination followed recognition of an external marking while interspecific discrimination occurred mostly after insertion of the ovipositor. Interspecific discrimination could result from the recent speciation of these species and could be associated with a genotypic discrimination. This behavior appears to be adaptive because of the competition for common hosts between the two parasitoid species.

Optimal patch time allocation for time-limited foragers

The Charnov Marginal Value Theorem (MVT) predicts the optimal foraging duration of animals exploiting patches of resources. The predictions of this model have been verified for various animal species. However, the model is based on several assumptions that are likely too simplistic. One of these assumptions is that animals are living forever (i.e., infinite horizon). Using a simple dynamic programming model, we tested the importance of this assumption by analysing the optimal strategy for time-limited foragers. We found that, for time-limited foragers, optimal patch residence times should be greater than those predicted from the classic, static MVT, and the deviation should increase when foragers are approaching the end of their life. These predictions were verified for females of the parasitoid Anaphes victus (Hymenoptera: Mymaridae) exploiting egg patches of its host, the carrot weevil Listronotus oregonensis (Coleoptera: Curculionidae). As predicted by the model, females indeed remained for a longer time on host patches when they approached the end of their life. Experimental results were finally analysed with a Cox regression model to identify the patch-leaving decision rules females used to behave according to the model’s predictions.

Male reproductive strategy in Trichogramma evanescens: sperm production and allocation to females

Abstract.  The production and allocation of sperm among successive mates during a males life largely determine its fitness. The sperm production pattern and sperm allocation to females is studied in Trichogramma evanescens, a short‐lived egg parasitoid of several lepidopteran species. At emergence, virgin males have an average of 1607 ± 249 sperm stored in the seminal vesicles and no further production occurs during adult life. These males are able to mate with at least 20 females in rapid succession. During the first 10 matings, the males transfer approximately 100 sperm each time and then transfer fewer and fewer sperm per mating. The number of sperm stored in spermathecae of successively mated females remains relatively constant for the first 10 females, and decreases slowly for the subsequent females. The relationship between male reproductive strategy and some life‐history traits are discussed.

Effect Of Plant Structure, Host Density And Foraging Duration On Host Finding By Trichogramma evanescens (Hymenoptera:Trichogrammatidae)

Abstract While searching for hosts on plants, female parasitoids meet different plant structures and host densities that will influence their host finding success. In this study, we determined whether plant structure, host density, and foraging duration influenced the percentage of Ephestia kuehniella Zeller eggs parasitized by Trichogramma evanescens Westwood. One female was introduced to forage either 4 or 24 h on artificial plants of three different structures (simple, intermediate, and complex) on which 4 or 16 host eggs were glued. Plant structure influenced rate of parasitism in both foraging durations, whereas host density was found to be significant only when female had 4 h to forage. Mean rate of parasitism generally decreased with an increase in complexity of plant structure and host density in both foraging durations. Almost twice as many eggs were parasitized on simple plants compared with complex plants for all host densities and foraging durations. Overall, female parasitoids parasitized, on average, <1 egg/h, except when they foraged 4 h in the presence of 16 eggs. Plant structure therefore affects host-encountering success. Physical structure of a plant mediates ecological interactions and is involved in parasitoid-host population dynamics. It operates by increasing costs associated with host finding.

Manipulation of parasitoid size using the temperature-size rule: fitness consequences.

The phenotypic effects of rearing temperature on several fitness components of the koinobiont parasitoid, Aphidius colemani, were examined. Temperatures experienced during development induced a plastic linear response in the dry and fat masses of the immature stage and a non-linear response in the growth rate as well as in the size of adults. We investigated if the phenotypic morphometrical plasticity exhibited by parasitoids reared at different temperatures can induce variations in fitness-related traits in females. We did not find any difference in immature (pupal) mortality in accordance to rearing temperature. However, when examining adult longevity, we found an inverse linear relation with developmental temperature, confirming the usual rule that larger and fatter wasps live longer than smaller ones. The pattern of female fecundity was non-linear; wasps that developed at high and low temperatures were less productive. We suggest that when development is short, the accumulated reserves are not adequate to support both fecundity and survival. By manipulating adult size through changes in the rearing temperature, we showed that the usual shape of the size/fitness function is not always linear as expected. Developmental temperature induced a plasticity in energy reserves which affected the functional constraints between survival and reproduction.