Host Encounter Rate Research Articles

Factors influencing brood sex ratios in polyembryonic Hymenoptera.

Copidosoma sp. is a polyembryonic encyrtid wasp which parasitizes isolated hosts. Most broods of this wasp are unisexual, but some contain both sexes and the secondary sex ratio of these is usually highly female biased. The overall population secondary sex ratio is female biased. Walter and Clarke (1992) argue that because the majority of individuals must mate outside the natal patch, the bias in the population secondary sex ratio contradicts predictions made by Hamilton's (1967) theory of local mate competition (LMC). We suggest that the primary sex ratio is unbiased and that Walter and Clarke's results do not cast doubt on LMC. Instead these results imply that ovipositing females make a combined clutch size and sex ratio decision influencing whether individuals developing from a particular brood will outbreed or largely inbreed; for each case the predictions of LMC theory are not violated. If this interpretation is correct, what is of interest is the basis on which this decision is made rather than the population secondary sex ratio. We show that host encounter rate influences the proportions of mixed and single sex broods laid by Copidosoma floridanum, a related polyembryonic parasitoid. Among single-sex broods the primary sex ratio is female biased, but our results are in agreement with LMC theory since offspring developing from these broods will probably mate with siblings from adjacent hosts. We consider the egg load of females to be of major influence on oviposition behaviour, and that the mating structure of parasitoid offspring, potentially differential costs of male and female broods and the natural distributions of hosts both at oviposition and eclosion, require further study.

Distinguishing between ‘preference’ and ‘motivation’ in food choice: an example from insect oviposition

Variation among individuals in acceptance of a second-ranked food resource could be caused either by variation in strength of ‘preference’ for the top-ranked over the second-ranked resource type, or by variation in the general readiness to feed. This distinction is important in models of diet evolution. Of the two traits, preference is the more likely to show additive genetic variation and hence be subject to evolutionary change. In the checkerspot butterfly, Euphydryas editha, changes in readiness to oviposit (abbreviated as oviposition ‘motivation’) mediate the relationship between host preference, host encounter rate, and realized diet, and thereby influence the strength of selection on preference. Definitions of acceptance, preference and motivation are given and applied to oviposition behaviour of herbivorous insects. From these definitions, operational measures of these traits are developed for E. editha. These operational measures are then used to investigate a difference between two E. editha populations in acceptance of a second-ranked host species. The results of this study show that this difference was caused partly by variation of preference and partly by variation of motivation. The difference in motivation probably stemmed from the absence of the most-preferred host from one of the study sites, resulting in prolonged search, reduced frequency of oviposition, and higher mean motivation at this site.

Effect of Host Size on Clutch Size in Cotesia glomerata

(1) Optimal clutch size theory predicts that for similar species of parasitoids, smaller hosts should impose more severe density-dependent competition than larger hosts and that, consequently, selection should favour smaller clutches in smaller hosts. (2) These predictions were tested by comparing the strength of density dependence experienced by a native population of Cotesia glomerata attacking a large host species, Pieris brassicae, in Britain, with that experienced by a population of C. glomerata, introduced over 100 years ago into the U.S.A., attacking a smaller host species, P. rapae. (3) No density dependence of adult size, survival or development time was found for the native population attacking P. brassicae. In contrast, strong density dependence of adult size and survival was found for the introduced population attacking P. rapae. (4) Contrary to expectation, the mean clutch size laid by both the native and introduced populations of C. glomerata in P. rapae (26 6 and 28-0 eggs, respectively) were significantly larger than that laid by the native population in P. brassicae (20 4 eggs). The mean clutch size laid by the introduced population in P. rapae was also larger than that predicted to maximize fitness per host. (5) It is suggested that no adaptation to host size has occurred in the introduced population, and that clutch size adaptation in this species is affected by host encounter rate as well as by host size.

Host-cue information processing by foraging jewel wasps: Response-bias tracking of expected host encounter rates, not modified profitabilities

Three hundred forty female jewel wasps were reared as larvae on blowfly pupae and then separately exposed as adults to varying numbers and combinations of previously parasitized and unparasitized housefly pupae. Each adult was tested with 6 fresh housefly pupae. Only foraging experience on unparasitized housefly pupae increased host acceptance for subsequent housefly pupae, producing curvilinear and convex host-acceptance functions; foraging experience on preparasitized housefly pupae had no effect upon subsequent host acceptance. Labile host- acceptance thresholds are thus adjusted by foraging experience to track local unparasitized host abundances for different species by modulating host-species cue response biases. These presum- ably kairomonal response biases were found to be sensitive to psychophysical “subjective proba- bilities of stimulus presentation,” representing expected unparasitized host-species encounter rates, not altered “payoff matrices” of “observer decision outcomes,” representing modified host- species profitabilities.

Search dynamics in fruit-parasitic insects

This paper examines methodology for evaluating and employing forager perceptive abilities into descriptive models for insects that visually search for resources. Using host seeking, female tephritid fruit flies as model parasitic insects a reactive distance envelope model, based upon earlier work of Holling (1966), is developed and tested in the laboratory and field. The laboratory derived values accurately predict the frequency at which flies detect fruit clusters in nature. Further, the envelope is shown to be assymetric wherein flies searching below fruit clusters discover those clusers more readily than similar sized clusters located below searching flies. The concept of the reactive envelope is integrated with forager search paths and host distribution into simple simulation models that test the hypothesis that host encounter rate and patch residence time may be greatly influenced by variation in any of the three aforementioned search parameters. It is concluded that models that include details of: (1) host density and distribution, (2) forager movement patterns, (3) forager perceptual constraints and, (4) forager information processing and memory, provide appropriate tools for evaluating foraging behaviour of animals in nature.

Sex ratio regulation in Coccygomimus turionella Linnaeus (Hymenoptera: Ichneumonidae) and its ecological implications

Abstract. 1. Sex regulation and its relevance to the ecology of a k‐selected pupal parasitoid was investigated in Coccygomimus turionellae.2. The ratio varied with host size, females predominating in large hosts.3. Egg transfer experiments and comparative mortality rates among the progeny of virgin and inseminated females demonstrated that the phenomenon was due to parental behaviour.4. The study of the mechanism confirmed Aubert's hypothesis of host size estimation with the addition that the host had to be exposed from the poles and had to contain the proper host kairomones.5. Host size was highly correlated to parasitoid fecundity via influences on size and longevity but not the number of ovarioles per female or daily egg production.6. Host encounters are more limiting than egg production and host size had no effect on host acceptance.7. Sex regulation allows a maximal host encounter rate at the same time that it maximizes the reproductive potential of female progeny.8. Low host density increased the production of female progeny resulting in more offspring searching for the limited resource. Concealed hosts and a high host density resulted in a shift towards a Fisher 1:1 sex ratio.