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1. During range expansions of phytophagous insects, secondary or novel hosts may allow colonisation of areas without primary hosts. Because plant species often differ in their relative attractiveness and suitability for insects, insect preference for, and performance on, these hosts can determine recruitment potential in the current and future expansion areas.2. This study explores the relative roles of female preference and larval performance in an important pine defoliator, Thaumetopoea pityocampa (Denis & Schiffermüller) (Lepidoptera, Notodontidae), which colonises three Pinus species at its current range margin in the Italian Alps: P. nigra (primary host), P. sylvestris (secondary host), and P. mugo (novel host).3. Host use patterns in multiple insect populations were studied through choice and no-choice oviposition experiments in cages, field surveys of mixed stands, and laboratory and field monitoring of larval growth and mortality. It was predicted that a specific lifehistory trait -time limitation of short-lived females to lay a single batch of eggs -would act as a component of female performance, and lead to similar rates of host acceptance in no-choice settings.4. In the choice experiment, P. nigra was accepted the most frequently while P. sylvestris was accepted the least frequently, confirming nest density patterns in the field. Contrary to prediction, females remained discriminating in no-choice settings in spite of time limitation. In contrast, relative growth rate (RGR) and mortality of larvae did not differ significantly among the three hosts, highlighting a discrepancy between female preference and larval performance.5. Recruitment potential of T. pityocampa in future expansion into stands of P. sylvestris and P. mugo is evaluated by combining host quality, conservatism in oviposition behaviour, habitat suitability, and the opportunity for local adaptation. Entomology, 31, 481-490 * Number of larvae found in each of the remaining colonies included in parentheses. † Mean ± SE of relative growth rate (mg mg −1 day −1 ). 488 Michael Stastny et al.

1 The winter pine processionary moth complex includes some major defoliating insects of Pinus and Cedrus forests in southern Europe and the Mediterranean Basin, where they also cause health problems to humans and animals. 2 The complex includes at least two species that were separated recently based on molecular and morphological evidence: Thaumetopoea pityocampa in the west and Thaumetopoea wilkinsoni in the east of the Mediterranean Basin. 3 Individuals from two populations, selected as representative of Th. pityocampa and Th. wilkinsoni, were used to test whether hybridization is possible under controlled conditions. 4 The hybrid offspring showed intermediate morphological and performance traits, whereas heterosis for pupal weight was detected in one of the hybrid lines. The genetic analysis confirmed the crosses. 5 Both species have large phenological plasticity and may come into contact at the edge of their range, where they could hybridize. 6 Based on the evidence accumulated so far, it is recommended that the current species designations are maintained, although a deeper study of the trait variability is required, especially in the contact zones.

Adaptation of parasitoids to their phytophagous host is often mediated by environmental conditions and by the food plant of the phytophagous host. Therefore, the host food plant can indirectly affect the survival and fitness of parasitoids that also attack quiescent host stages, such as eggs, in which the resources available to the immature parasitoid stages are limited. Our aim was to investigate how two egg parasitoid species of the pine processionary moth, Thaumetopoea pityocampa (Denis & Schiffermüller), respond to variations in egg traits at the extremes of a west-to-east geographic gradient in northern Italy. We considered one specialist [Baryscapus servadeii (Domenichini)] and one generalist [Ooencyrtus pityocampae (Mercet)] parasitoid, which reproduce mainly by thelytokous parthenogenesis and are common throughout the whole range of this pest. The size and shell structure of the pine processionary moth eggs were studied under light microscopy and tested experimentally under controlled conditions. We can conclude that 1) the pine processionary moth egg shell thickness is inversely proportional to the parasitism performance; 2) the larger eggs from the pine processionary moth eastern population produce parasitoid females of a larger size, which have greater realized fecundity; 3) the generalist parasitoid performs successfully with either the "home" or "away" (i.e., from both extremes of the geographic gradient) pine processionary moth eggs, which is not the case for the specialist parasitoid. The implications of these responses in the regulation of phytophagous populations are numerous and should be considered in population dynamics studies and pest management programs.

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