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

This paper presents preliminary results on attempts to extract and characte rize the volatile secondary metabolites contained in needles of different P inus species and to ascertain the role played by these substances on the be havior of Thaumetopoea pityocampa (Denis et Schiffermuller) females, which show a marked preference, during the oviposition period, for some indigenou s and exotic species of host plants existing in mixed formation. Limonene i s the most abundant monoterpene extracted from P. pinea needles, the least favored species of pine processionary caterpillar (PPC) females, although i t is present in only very low amounts in other Pinus species. An increase w as observed in limonene production by P, pinea at the start of the flight p eriod of the PPC adult, and subsequently at the beginning of the females' o viposition period. Assays carried out in two pine stands in central Italy s howed that limonene, emulsified with water and sprayed on foliage of four d ifferent pine species plants, P. sylvestris, P. nigra, P. pinaster and P. r adiata, during the PPC oviposition period, provided a satisfactory degree o f protection. In fact, the number of egg clusters collected from treated pl ants 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 p lants. In particular (R)-(+)-limonene, although not produced in nature by p ines, was the most effective deterrent. However, the effect of (S)-(-)-limo nene, the enantiomer biosynthesized by pines, was also adequate.