Abstract
The expectation is that the phenology of an herbivore is influenced by abiotic factors and its own population density during development. In this study, we investigated how the pupal density affected the pupal traits and emergence of Thaumetopoea pityocampa moths over a period of 3 years in two clearings in a pine forest. The pupae were larger in years when the pupal density was high and in the clearing exposed to less solar radiation. There was no relationship between the time of pupation and pupal size. Large pupae were positively correlated with an early emergence of adult moths and a longer period of adult emergence. Up to 13.9% of the pupae developed without cocoons, especially in years when they were abundant, but this did not affect the emergence of the moths. Incidence of pupal diapause was density-dependent and only occurred at a low level in dense populations. Overall, our results indicate that gregariousness confers important fitness-related advantages in this species.
Highlights
In some Lepidoptera, such as the pine processionary moth (PPM; Thaumetopoea pityocampa [Denis & Schiffermüller, 1775]), the pupae develop in the soil close to the trees that were inhabited by the larvae
The adults emerge during summer, but some pupae can spend 2 or more years in diapause and the adults generally disperse over short distances (Bonsignore & Manti, 2013; Roques, 2015)
For the first time, we showed that a decrease in the pupal density over successive years at two pupation sites was associated with a decrease in pupal size (Fig. 2 and Table 2)
Summary
In some Lepidoptera, such as the pine processionary moth (PPM; Thaumetopoea pityocampa [Denis & Schiffermüller, 1775]), the pupae develop in the soil close to the trees that were inhabited by the larvae. The PPM is a univoltine species and is widespread in the Mediterranean, including southwest Europe, the Balkans and North Africa (Roques, 2015) This species is one of the most important pests of various types of pines and cedars, and can hinder the growth of infested trees (Jacquet et al, 2012). The larvae are gregarious from early on in their development and spin conspicuous tents in tree tops After completing their feeding, generally during late winter, larvae typically form a procession in which they walk head to tail in a column to reach their pupation sites on the ground. Generally during late winter, larvae typically form a procession in which they walk head to tail in a column to reach their pupation sites on the ground They burrow into the soil to a depth of a few centimeters, spin a silken cocoon in which they pupate. The adults emerge during summer, but some pupae can spend 2 or more years in diapause and the adults generally disperse over short distances (Bonsignore & Manti, 2013; Roques, 2015)
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