The Eucalyptus weevil, Gonipterus scutellatus Gyllenhal, is the most serious pest of established Eucalyptus globulus plantations in southwestern Australia where it may have been introduced from its endemic origin in southeastern Australia. This species has been introduced to many countries but has been brought under successful biological control by the introduction of the egg parasitoid, Anaphes nitens Girault, from southeastern Australia. I investigated whether the elevated pest status of G. scutellatus in southwestern Australia is a result of inadequate biological control by quantifying parasitism rates of the pest. Parasitism rates of G. scutellatus egg masses by A. nitens were very low between late winter and mid spring when most oviposition occurs, and a high percentage of parasitised egg masses at these times were incompletely parasitised. Parasitism rates from late spring onwards approached 100%, but fewer egg masses were laid at these times. Low parasitism rates up to mid spring are likely to be due to low numbers of A. nitens at these times because hosts are not available for half the year. In southwestern Australia G. scutellatus has one principal oviposition period in early spring and a second lesser oviposition period in early summer. This short reproductive period appears to be caused by a scarcity of suitable new flushing foliage after spring that is essential for G. scutellatus reproduction. A. nitens did not emerge from eggs of the closely related weevil Oxyops despite eggs of this species being available at a time when G. scutellatus hosts were largely unavailable. Taxonomic difficulties with Gonipterus and possibly Anaphes may also explain the breakdown of biological control in southwestern Australia, and taxonomic revision of these insect groups must be a future research priority.