Wound reactions of Scots pine (Pinus sylvestris L) to attacks by Tomicus piniperda L. and Ips sexdentatus Boern. (Col., Scolytidae)

Abstract

AbstractBark beetles Tomicus piniperda and Ips sexdentatus were artificially inserted in Scots pines. Attacks were also provoked in other trees by attractants. Resin flow corresponding to each introduction was measured in the case of artificial insertions. In all cases, gallery length, reaction zone length and the total quantity of induced resin contained in the whole reaction were measured 5–6 weeks after insertions or attacks. The attempt of gallery construction (beginning of gallery boring) and the beginning of oviposition were also noted in each case, as well as the presence of associated fungi.Resin flow discouraged T. piniperda, but not I. sexdentatus, to attempt a gallery construction. For the two beetle species, however, it played no role after gallery boring had started. Although tree induced reaction was present during gallery elongation, at least in the case of T. piniperda, it was not clear if it played a role in stopping gallery elongation and in failure of oviposition. Few beetles were killed by resin flow or by tree reaction. The existence of oviposition was not related to attack density, tree characteristics or presence of fungi. Fungi were involved in the development of the induced reaction in response to I. sexdentatus attacks, but not in response to T. piniperda attacks. When expressing reaction zone length or resin quantity as a function of gallery length, galleries where oviposition had begun were clearly separated from those where it had not. For galleries where oviposition did not occur, reaction zone length was always correlated with gallery length for T. piniperda, and not for I. sexdentatus, but in the latter case the correlations existed when considering only galleries without fungi. Accumulation of induced resin was always correlated with gallery length for the two species. Therefore, a tree's induced reaction is not basically a defense reaction. The results corroborate the hypothesis that it is a wound reaction induced by mechanical stress due to gallery boring, and amplified by fungi when they are effective. For T. piniperda, the beetle boring activity seems to be the only way to exhaust tree response capacity.