DEFOLIATION OF THE SCOTS PINE AND PERFORMANCE OF DIPRIONID SAWFLIES

(1) We tested for the presence of delayed inducible resistance of the Scots pine against diprionid sawflies by rearing larvae of Neodiprion sertifer Geoffr. (early season species) and Diprion pini L. (late season species) on mature foliage of undefoliated and previously defoliated trees. (2) No reduction in larval growth or pupal masses was found, whether the experimental trees were defoliated 1, 2 or 3 times or whether defoliation occurred in early or late season, Instead, there was a weak tendency for improved larval growth, especially after several successive late-season defoliations. (3) The nitrogen content of current-year needles was increased and their dry matter content reduced after 3 years' successive defoliations. (4) These observations are consistent with the hypotheses that (a) the mature foliage of the Scots pine lacks an effective delayed inducible defence against diprionid sawflies, and (b) the defoliation of Scots pine leads to a relative carbon deficiency which may constrain the accumulation of carbon-based substances in the foliage.