Estimating the Past Annual Activity of the Fir Budworm Choristoneura murinana (Hb.) (Lep., Tortricidae) through Retrospective Analysis of the Foliage and Growth Losses of the Trees

1988

S. 357-381

5 Abb., 5 Tab., zahlr. Lit. Ang.

Unselbständiges Werk

200014569

The budworm Choristoneura muriana has recently proved to severely damage silver fir (Abies alba Mill.) stands in the south of France. Understanding this situation, perhaps only new apparently, and assessing its implications would need to have informations on the past activity of the pest. Three criteria from trees likely to supply such data were tested: the needles loss of the shoots, the annual length increment of the branches and the annual radial increment of the trees. Observations were made in the department of Ardeche, in two plots of a mature silver fir stand infested by the budworm. Twenty dominant-codominant trees were selected in each plot. Branch samples were collected both from the top third and from the middle third of their crowns. Needle loss and defoliation estimates were based on rating of the shoots under consideration in percentage loss or defoliation classes. Radial growth was measured at breat height. One starting supposition was that estimating the respective needle loss percentages for the different age classes of shoots, being only taken into account those shoots apparently not yet affected by the natural needle fall,would give values approximately equal to those of the annual rates of budworm defoliation for the corresponding years. The use of such a method, we callretrospective estimation of the defoliation, was theoretically made possible by the conjunction of the following factors: (i) the fact budworm larvae feed on the current-year needles and do not noticeably damage the old foliage even during outbreaks (fig. 1); (ii) the high lift span, sometimes reaching at least 15 years, of the silver fir needles; and (iii) the monocyclism of silver fir, which allows us to accurately date the shoots. Two retrospective estimations of the budworm defoliations were successively made during winter 1985-1986 then in June 1987. The respective values they provided were compared between them, with those obtained the previous years by estimating current budworm defoliation a little after larval feeding period was completed, and also with old field observations(fig. 2). From these comparisons it appeared that the retrospectively estimate d defoliation values gave an accurate picture of the variations in budworm damage intensity during the 10 previous years (fig. 3) and were not very different,at least for the 6 more recent years, from the actual budworm defoliation valu es. It is a result of great interest for its possible applications and which most certainly makes C. muriana a rather exceptional case among forest defoliators. Both the natural needle fall and the action of other defoliation-causing factors are the cause of a certain error but the risk it should be important seemsslight. Whether or not budworm defoliation influences terminal growth of branc