Monoterpene composition of Pinus sylvestris varieties resistant and susceptible to Dioryctria zimmermani

The proportion of monoterpenes and enantiomeric ratios of chiral monoterpenes were determined in two susceptible and two resistant varieties of Scotch pine, Pinus sylvestris, by capillary GC analysis of headspace volatiles emitted by 9-year-old trees. Half the trees sampled in each variety had been wounded by Dioryctria zimmermani, while the other half were unwounded. The presence of wounds had no significant (P < 0.05) effect on the proportion of monoterpenes or the ratio of enantiomers of each of the optically resolved monoterpenes in the four varieties. In contrast, monoterpene composition varied significantly (P < 0.05) among varieties. However, the results do not entirely support the putative role of car-3-ene and terpinolene as an oviposition stimulant for D. zimmermani in resistant varieties of Scotch pine. Although the resistant Lake Superior Blue virtually lacked these two monoterpenes, the resistant Belgian had a relatively high concentration of car-3-ene and terpinolene. Both resistant varieties, however, emitted relatively high levels of limonene, a monoterpene associated with increased oviposition in three other Dioryctria species. There was no evidence linking enantiomeric composition of chirally resolved monoterpenes with varietal resistance. The relative importance of monoterpenes as resistance factors and oviposition stimulants is discussed.