The role of the monoterpene composition inPinus spp. needles, in host selection by the pine processionary caterpillar,Thaumetopoea pityocampa

This paper presents preliminary results on attempts to extract and characterize the volatile secondary metabolites contained in needles of differentPinus species and to ascertain the role played by these substances on the behavior ofThaumetopoea pityocampa (Denis et Schiffermüller) females, which show a marked preference, during the oviposition period, for some indigenous and exotic species of host plants existing in mixed formation. Limonene is the most abundant monoterpene extracted fromP. pinea needles, the least favored species of pine processionary caterpillar (PPC) females, although it is present in only very low amounts in otherPinus species. An increase was observed in limonene production byP. pinea at the start of the flight period of the PPC adult, and subsequently at the beginning of the females’ oviposition period. Assays carried out in two pine stands in central Italy showed that limonene, emulsified with water and sprayed on foliage of four different pine species plants,P. sylvestris, P. nigra, P. pinaster andP. radiata, during the PPC oviposition period, provided a satisfactory degree of protection. In fact, the number of egg clusters collected from treated plants was often lower than the number of egg clusters collected from control plants and was comparable to the number of egg clusters laid onP. pinea plants. In particular (R)-(+)-limonene, although not produced in nature by pines, was the most effective deterrent. However, the effect of (S)-(-)-limonene, the enantiomer biosynthesized by pines, was also adequate.