Economic Foraging Decisions: The Behavioral Response Of The Parasitoid, Venturia Canescens

Parasitoids are considered as a privileged model to test foraging theories based on a natural selection rationale, as parasitoids' foraging behavior and reproductive success are closely related. In a heterogenous environment, they have to decide where to forage and when to leave the current patch and search for another one to maximize their fitness in the habitat. Frequently, parasitoids are observed exploiting the same patch simultaneously. In this case, the adaptive strategy of patch exploring depends on both the payoff rate of the current patch and the decisions made by the other competitor. Furthermore, the information use of parasitoids is often constrained with their habitat. Therefore, in this study, we focus on mainly how parasitoids acquire and use the information economically and how the information usage relates with their habitats.The ichneumonid wasp Venturia canescens (Gravenhorst) consists of a parthenogenetic (thelytokous) and a sexual (arrhenotokous) type, which often occur sympatrically. Females from both types have a large number of mature eggs right after emergence and new eggs mature throughout their life time. Whereas, females of thelytokous line normally have more eggs than those of arrhenotokous wasps. The thelytokous wasps live mainly inside mills and granaries, where host densities are usually low, but the population size may vary considerably. Walking is usually enough for an individual to find a host. The arrhenotokous wasps live exclusively in field habitats such as orchards where they parasitize mainly pyralid moth larvae in dried fruits. The distribution of hosts across patches is rather uniform with patches containing mostly one, and up to four, host larvae. Flying is necessary for foraging. Using V. canesces females of these two reproductive modes which differ apparently in their habitats, we investigated the relationship between information usage and their habitat difference, during their patch selection and patch exploitation process. The aim of this study is to elucidate how parasitioids display their optimal foraging strategy with different habitat information. The main results are:In a Y-tube olfactometer, temporal dynamics of the decision process in parasitoid patch choice were investigated and the time both thelytokous and arrhenotokous females used to make their choices was measured quantitatively. Both thelytokous and arrhenotokous wasps only chose the higher quality patch based on odour cues when the patch difference was qualitative (i.e., with or without host larvae). When patches differed only with respect to the number of hosts or the presence or absence of competing female parasitoids, no significant preference could be found in females of either strain of the parasitoid. In contrast, both the time until females reached the junction of the y-tube olfactometer (response time) and the time until females made their decision for either patch (decision time) varied with parasitoid strain and odour treatment. Thelytokous wasps were faster than arrhenotokous wasps in their response time until reaching the junction point and in their decision time for either side of the olfactometer. However, females of both strains responded faster with increasing number of total hosts in the setup releasing kairomone. Yet, decision time for patches did not significantly different with patch qualities offered to Venturia wasps. Interestingly, in thelytokous wasps, only those females that moved more slowly to the decision point chose more for the better patch, indicating a trade-off between speed and accuracy of the decision process in foraging parasitoids.When females entered a patch non-simultaneously but forage on a patch together, the females that entered a patch early on also left the patch sooner, those entered the patch later on started superparasitism sooner at high patch encounter rate, but not at low patch encounter rate. Wasps foraging a patch simultaneously exhibited similar or even less patch residence time than that foraging alone, which implied no war of attrition in competitors of V. canescens. Offspring produced by both competitors were similar suggested an evolutionary stable strategy (ESS) existed in the asymmetric game. The effect of patch encounter rate was stronger than the competition in thelytokous line but not in arrhenotokous line.Through a flight mill experiment, we showed that in thelytokous lines, time was a sufficient cue influencing patch exploitation and an additional effect of the energy needed was not visible. In the arrhenotokous wasps, however, only the number of rounds flown in the mill (energy expenditure) influenced subsequent behaviour, while mere time spent during travelling did not.This study investigated the temporal dynamics of the decision process in parasitoid patch choice and the time it took females to make their choices. This was the first report of the functional value of patch response and decision time and it should be a novel approach to consider the searching efficiency of parasitoids in nature. We also verified for the first time, that energy expenditure during patch visit had significant influence on subsequent patch exploitation in parasitoids that flew frequently. Moreover, with a series of delicate design of experiments, we clearly showed that parasitoids of V. canescens could use the information economically and the exploitation strategy is highly adapted with their habitats.