1. ‘Ecological fitting’ is the process whereby the suites of traits an organism carries from previous evolutionary relationships are used to enable colonisation of novel environments or resources.
2. The concept has much explanatory power in studies of novel host associations, particularly when data suggest a deviation from optimal foraging theory, but is often overlooked in studies of herbivore host selection behaviour in favour of evolutionary hypotheses.
3. In the present study, the concept was used to explain the unusual host selection behaviour of the New Zealand endemic oligophagous butterfly Lycaena salustius Fabricius, the larvae of which feed on endemic Polygonaceae species and the introduced and closely related Fagopyrum esculentum Moench.
4. In field cage oviposition choice assays involving only endemic plants, females preferred to oviposit on the rare Muehlenbeckia astonii Petrie. However, the novel host F. esculentum was overwhelmingly preferred in an additional greenhouse assay. In larval no-choice performance assays, fitness indicators were variable for the novel host.
5. This imperfect relationship between oviposition preference and larval performance is discussed as a possible example of ecological fitting and highlights the potential use of the concept as an explanatory tool in novel host selection behaviour studies.