The larch casebearer and its host tree: II. Changes in needle physiology of the infested trees

2000

4 Abb., 1 Tab., 42 Lit.

Unselbständiges Werk

14213

200068748

The population dynamics of the larch casebearer (Coleophora laricella Hbn.) in the field and the physiology of larch needles serving as their feeding basis were analysed from 1991 to 1995 using a differential analysis: one insect species feeding at the same time on two needle types (spurshoot and longshoot needles) of two host species (European and Japanese Larch) under different environmental conditions (Solling hills, lowlands). Resistance mechanisms were found which allow a coexistence of the tree and of the insect population at an economically 'bearable' cost for the host trees. With increasing casebearer densities, physiological changes within the intraannual course of the tree's needle physiology were induced. However, they did not prevent the further growth of the casebearer population. High mortality rates of the sensitive needle mining L1-larvae resulted from specific local defence mechanisms of the single needle followed by a premature leaf abscission. A decrease of casebearer densities was initiated when L4-densities in the spring exceeded a tolerance level which seemed to be adjusted to the environmental conditions. The induced physiological changes led to a sudden deterioration of the degestibility of the needles, caused by an increase of procyanidine and tannin contents and a decrease of protein-amino acid contents. The strong disadvantageous effect of these mechanisms on the casebearer population was coinciding with the highest consumption rates of the casebearer larvae.