1. Whether the potential fecundity of herbivorous insects is realised or not may depend on female behaviour, which in turn may be influenced by host plant acceptability. Female Bupalus piniarius were observed to discriminate against needles growing out the year following defoliation (current-year needles) of its host plant Pinus sylvestris.
2. It was hypothesised that the discriminatory behaviour was due to current-year needles being less secure as a substrate. Field and laboratory experiments were designed to test this hypothesis and to estimate the discrepancy between potential and realised fecundity when females were offered defoliated branches.
3. In a laboratory oviposition experiment, B. piniarius females were exposed to branches bearing either current-year needles only or both mature and current-year needles. Daily oviposition rate, egg batch size, longevity, and mature eggs and fat retained at death were recorded for each female. In field experiments, the rate at which eggs dropped from expanding needles and the capacity of neonate larvae hatching from the dropped eggs to colonise a tree were assessed.
4. Significantly fewer eggs were laid when females were exposed to defoliated branches.
5. Twenty-six and 16% of the eggs laid on current-year needles dropped from the needles in 1998 and 1999 respectively, whereas no eggs dropped from mature needles in 1998 and only one egg (< 1%) dropped in 1999.
6. A very small proportion of larvae hatching on the forest floor (simulated egg drop) was able to recolonise host trees.
7. These results emphasise the importance of oviposition behaviour on realised fecundity when analysing insect population dynamics. In the case of B. piniarius, egg placement, although a minor detail during the normal course of events, became of key importance when defoliation deprived females of their preferred egg attachment site.