a b s t r a c tThe pine processionary moth (PPM, Thaumetopoea pityocampa) is the main defoliator of pines in the Mediterranean area, necessitating constant surveillance and regular pest management. A sound understanding of the spatial distribution of infested trees, both within and between stands, is required to increase management efficiency. We hypothesized that both host tree density at stand scale and tree apparency at individual tree scale were responsible for between-and within-stand patterns of PPM infestation. We tested these hypotheses on a sample of 171 maritime pine stands in the Landes de Gascogne, the largest plantation forest in Europe. We showed that PPM infestation (percentage of infested trees) decreased significantly with stand density, and was therefore greater in older than in younger stands. The probability of a pine tree being attacked increased significantly with tree height and proximity to the edge of the stand. Mortality rates of exposed sentinel egg batches did not differ with distance from the stand edge. We discuss three likely explanations for higher infestation of taller trees at stand edges: better survival of larvae on sun-exposed trees, and random interception vs. active host selection by gravid females. Our findings suggest that stand management could be adapted in order to decrease the risk of damage by the pine processionary moth, and that predictive tools for infestation dynamics can be based on forest growth models.

1 Little is known about the relative effect of different mortality factors triggering the population dynamics of outbreaking Lepidoptera (e.g. climate, natural enemies, habitat diversity), especially at the egg stage. 2 The presence of broadleaved hedgerows bordering pine stands has been shown to reduce infestation by Thaumetopoea pityocampa (Denis & Schiffermüller, Lepidoptera). This effect of habitat diversity has been proposed to result from the disruption of visual and chemical cues that the female moth uses to locate its host.In the present study, we further hypothesized that egg mortality was higher behind broadleaved hedgerows because of higher predation or parasitism, or as a result of harsher microclimatic conditions. 3 A total of 120 egg masses obtained from laboratory rearing were exposed under three exclusion treatments (large predators, predators and parasitoids, no exclusion).Overall egg mortality was 35%. Predation was the main cause of mortality (22%). Mortality as a result of abiotic factors (abortion) was only 12%. Almost no parasitoids were observed. 4 Overall mortality did not differ between pines bordered (or not) by deciduous hedgerows, nor did mortality differ as a result of predation or abortion. 5 Despite an important contribution of predation to mortality in T. pityocampa eggs, the effect of deciduous hedgerows on tree infestation by this pest is more likely to result from disruption of female moth behaviour than from differential mortality.

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