The Role of the Monoterpene Composition in Pinus spp. Needles, in Host Selection by the Processionary Caterpillar, Thaumetopoea pityocampa

This paper presents preliminary results on attempts to extract and characterize the volatile secondary metabolites contained in needles of different Pinus species and to ascertain the role played by these substances on the behavior of Thaumetopoea pityocampa (Denis et Schiffermuller) 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 from P. pinea needles, the least favored species of pine processionary caterpillar (PPC) females, although it is present in only very low amounts in other Pinus species. An increase was observed in limonene production by P, 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 and P. 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 on P. 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.