1. On the basis of his study of patch time allocation by Venturia cane
scens, a larval parasitoid of phycitid flour moths (Lepidoptera: Pyral
idae), J. K. Waage proposed a decision mechanism for patch exploitatio
n. This putative mechanism would be efficent in habitats, such as gran
aries, consisting of patches with heterogeneous host densities. Howeve
r, the distribution of hosts in mummified fallen fruits, a common natu
ral substrate, differs from their distribution in granary stores, tend
ing to be rather uniform with patches containing a few, mostly one, ho
st larva. This discrepancy led us to re-examine Waage's mechanism. 2.
We investigated V. canescens's decision mechanism in small patches con
taining low host densities. Following the previous study, we tested th
e relation between the probability per unit time to abandon a patch an
d 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. 3. A major component of Waage's mo
del is an increase in the tendency to remain in a patch after an ovipo
sition. In habitats where hosts are uniformly distributed, and particu
larly 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 re
sidence time. Ovipositions, however, decrease the amount of time subse
quently spent by the parasitoid on the patch. 5. Based on these result
s we formulate a post-hoc 'count-down' model for the decision rule for
patch leaving in V. canescens in habitats with uniform host distribut
ions. 6. There is no evidence for or against the possibility that this
parasitoid may be capable of facultative changes in its patch exploit
ation rule as a function of host distribution. 7. Although increases i
n patch time after oviposition have been found in several other parasi
toid species, re-examination of Waage's experiments shows that his res
ults do not unambiguously support the existence of such a mechanism in
Venturia.