Where to lay an egg mass? Multiscale oviposition-site selection in the pine processionary moth

The pine processionary moth ( Thaumetopoea pityocampa ) is an important defoliator of circum-Mediterranean pine forests. Females are capital breeders and lay their eggs in a single egg mass, making oviposition site selection potentially important for offspring survival. We examined host-tree selection and within-tree egg mass distribution in three pine stands in southeastern Spain spanning an elevational range of 900–1820 m a.s.l. We recorded the number and position of egg masses and analysed their relationships with tree isolation, tree height, late-afternoon solar direction and local topography. In the primary model, more isolated trees had a higher probability of being selected, whereas taller trees received more egg masses once selected. Egg masses were preferentially located on the side of the tree facing the late-afternoon sun at all three sites, and solar alignment was stronger for downslope positions. The downslope–upslope contrast in the probability of sun-facing placement was greatest at the upper site. Vertical placement also differed among sites: egg masses occurred predominantly in lower canopy positions at the lower site, were more evenly distributed at intermediate, and occurred mainly in intermediate and upper positions at the upper one. Overall, these results indicate that oviposition-site selection operates across multiple spatial scales, from host-tree selection to within-tree orientation and vertical distribution.