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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. SUMMARY (1) We investigate properties of a model of predator distribution in relation to prey abundance, when the predation process is subject to:(i) non-negligible handling time and mutual interference; (ii) predator learning; (iii) intake rate maximization by individual predators. The model is a development of that of Bernstein, Kacelnik & Krebs (1988).(2) The independent variables are between-patch travel cost and structure of the environment. The outcome of the model is described in relation to the predictions of the ideal free distribution (IFD). We consider both the numerical distribution of predators and the mortality rate imposed on the prey population.(3) When travel cost is small, prey depletion is slow and interference is moderate, predators conform to the predictions of the IFD and prey mortality is densitydependent.(4) As travel cost is increased, rate-maximizing predators become more sedentary and the population settles at distributions far from the IFD. In common with all other disturbances of the predation process that impair the correlation between prey and predator densities, this causes mortality to approach density independence and later negative density dependence.(5) In semi-continuous environments where prey density is correlated between neighbouring patches, the slower the spatial rate of variation in prey density (the coarser the environmental grain), the poorer is the adjustment to the IFD. This effect is due to the predators' need for learning: when the environmental sample experienced within the reach of each individual predator is unrepresentative of the global average prey density (as it happens when the environmental grain is very coarse relative to migration range of predators), the predators cannot learn the ? Present address:

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Summary 0[ Patch!leaving decisions are of utmost importance in determining parasitoid for! aging success[ Parasitoids are known to use both marks left by hosts "chemical or otherwise# and ovipositions to assess host availability and to decide when to leave a host patch[ 1[ Previous studies have shown that\ depending on the species\ ovipositions either increase "an incremental mechanism# or decrease "a decremental mechanism# the patch residence times of parasitoids[ Reports in the literature con~ict on which mechanism is used by Venturia canescens\ a parasitoid of pyralid moth larvae[ 2[ We hypothesize that\ as a consequence of saturation in the capacity of the parasitoid to discriminate between host densities at high host numbers\ V[ canescens uses a decremental mechanism at low host numbers and an incremental one at high host numbers[ We call this a {switching mechanism|[ 3[ Our experiments show that even if discrimination capacity saturates\ V[ canescens uses a decremental mechanism over a wide range of host densities[ 4[ The distribution of hosts in di}erent fruits species under _eld conditions suggests a switching mechanism would not evolve in natural situations[ 5[ A model of patch departure in V[ canescens is constructed and tested using an independent set of experiments[ The model suggests that the patch leaving mechanism in V[ canescens is a stochastic decremental one[ As might be expected from Weber|s Law\ the initial leaving tendency is a convex decreasing function of kairomone con! centration[ The leaving tendency increases exponentially with the time spent in the patch without ovipositing[ Ovipositions cause a sudden increase in leaving tendency[ 6[ Simulations suggest that a decremental mechanism would be out!competed by either one indi}erent to ovipositions or an incremental one\ only when travel times are much larger than those that are likely to occur in the _eld[ Key!words] count!down mechanism\ optimal foraging\ patch leaving decisions\ pro! portional hazards model\ Venturia canescens[ Journal of Animal Ecology "0888# 57\ 334Ð348

Summary 1.Although parasitoids are used widely as a biological models for understanding the evolution of animal behaviour, most studies have been constrained to the laboratory. The dearth of field studies has been compounded by the almost complete ignorance of the physiological parameters involved in foraging and dispersal, in particular of the energetic constraints imposed by resource limitation. 2. We estimated the dynamics of carbohydrates and lipids reserves of Venturia canescen s (Gravenhorst) females by releasing individuals of known nutritional status in a natural environment and recapturing them using host-containing traps. The recapture rate was around 30%. These results were compared with the reserves of caged animals kept under different experimental conditions (freshly emerged, starved to death, fed ad libitum and partially starved). Wild animals were also sampled in order to estimate the resource levels of the local population. 3. The results show that: (i) wasps are able to maintain a nearly constant level of energy over an extended foraging period; (ii) V. canescens takes sugars in the field; and (iii) the lipid reserves accumulated during the larval life may be limiting as lipogenesis does not take place in adults even under conditions of high sugar availability. 4. These results demonstrate that wasps can forage for hosts and food and disperse in this habitat for hours and days without running into a severe risk of energy limitation.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. On the basis of his study of patch time allocation by Venturia canescens, a larval parasitoid of phycitid flour moths (Lepidoptera: Pyralidae), J. K. Waage proposed a decision mechanism for patch exploitation. This putative mechanism would be efficient in habitats, such as granaries, consisting of patches with heterogeneous host densities. However, the distribution of hosts in mummified fallen fruits, a common natural substrate, differs from their distribution in granary stores, tending to be rather uniform with patches containing a few, mostly one, host larva. This discrepancy led us to re-examine Waage's mechanism. 2. We investigated V. canescens's decision mechanism in small patches containing low host densities. Following the previous study, we tested the relation between the probability per unit time to abandon a patch and the following variables: the concentration of the contact kairomone produced by host larvae, the time elapsed since the first patch entry and the occurrence of ovipositions. A major component of Waage's model is an increase in the tendency to remain in a patch after an oviposition. In habitats where hosts are uniformly distributed, andparticularly when patches contain a single host, this behaviour would not be adaptive. 4. Our results confirm that V. canescens spends more time on patches with higher concentrations of contact kairomone and that the probability per unit time of leaving the patch increases with patch residence time. Ovipositions, however, decrease the amount of time subsequently spent by the parasitoid on the patch. 5. Based on these results we formulate apost-hoc 'count-down' model for the decision rule for patch leaving in V. canescens in habitats with uniform host distributions. 6. There is no evidence for or against the possibility that this parasitoid may be capable of facultative changes in its patch exploitation rule as a function of host distribution. 7. Although increases in patch time after oviposition have been found in several other parasitoid species, re-examination of Waage's experiments shows that his results do not unambiguously support the existence of such a mechanism in Venturia. the rate at which hosts are found exceeds the rate at which new hosts develop. The time spent in each patch Parasitoids searching for patchily distributed hosts increases the number of offspring realized there, but experience diminishing returns within patches when reduces the search time that can be spent on other patches. In consequence, the mechanisms determining

Theory predicts that asexual reproduction has a competitive advantage over sexual reproduction because of the twofold cost of producing males. Few systems are suitable for directly testing this prediction. In the solitary parasitoid wasp Venturia canescens both arrhenotokously (sexual) and thelytokously (asexual) reproducing individuals occur sympatrically. We sampled 922 wasps from 22 localities along the coast of south‐eastern France. Thelytokous wasps were less abundant (23%) than arrhenotokous wasps and were almost always found in sympatry with arrhenotokous ones. An analysis of genetic relatedness using amplified fragment length polymorphism (AFLP) markers showed the existence of a widespread thelytokous clone. In addition, a few thelytokous individuals were found to be closely related to arrhenotokous ones and vice versa. These data suggest the occurrence of occasional gene flow between both reproductive modes and/or recurrent origin of thelytokous clones from coexisting arrhenotokous populations in the area. The results are discussed in the context of the paradox of sex.