Patch departure mechanisms and optimal host exploitation in an insect parasitoid

1, Patch-leaving decisions are of utmost importance in determining parasito id foraging 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. 2. Previous studies have shown that, depending on the species, ovipositions either increase (an incremental mechanism) or decrease (a decremental mech anism) the patch residence times of parasitoids. Reports in the literature conflict on which mechanism is used by Venturia canescens, a parasitoid of pyralid moth larvae. 3, We hypothesize that, as a consequence of saturation in the capacity of t he parasitoid to discriminate between host densities at high host numbers, V. canescens uses a decremental mechanism at low host numbers and an increm ental one at high host numbers. We call this a 'switching mechanism'. 4. Our experiments show that even if discrimination capacity saturates, V. canescens uses a decremental mechanism over a wide range of host densities. 5. The distribution of hosts in different fruits species under field condit ions suggests a switching mechanism would not evolve in natural situations. 6. A model of patch departure in V. canescens is constructed and tested usi ng an independent set of experiments. The model suggests that the patch lea ving mechanism in V. canescens is a stochastic decremental one. As might be expected from Weber's Law, the initial leaving tendency is a convex decrea sing function of kairomone concentration. The leaving tendency increases ex ponentially with the time spent in the patch without ovipositing. Ovipositi ons cause a sudden increase in leaving tendency. 7. Simulations suggest that a decremental mechanism would be out-competed b y either one indifferent to ovipositions or an incremental one, only when t ravel times are much larger than those that are likely to occur in the fiel d.