Solar radiation can affect the performance of insect herbivores directly by increasing body temperature, or indirectly through alteration of either host plant quality or natural enemy activity.
To test for the direct effect of solar radiation on larval performance, young Pinus sylvestris trees growing on the island of Gotland (Sweden) were assigned to one of four shading treatments for the whole duration of the first larval instar of the northern pine processionary moth Thaumetopoea pinivora.
There was a strong, linear relationship between shading and the temperature of the first-instar colonies of T. pinivora, resulting in higher growth of the larvae exposed to full sunlight, but there were no effects on developmental rate or larval mortality. Putative negative effects of UV radiation on the larvae are not consistent with higher growth in full sunlight, but it is possible that UV effects might have modulated the response.
Thaumetopoea pinivora has a strong preference for light and open pine stands, i.e. habitats with frequent intense incoming solar radiation. The data in the present study suggest that the opportunity for young larvae to bask in the sun during cold spring weather is an important determinant of the spatial distribution of T. pinivora.