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Analysis of some factors associated with distribution and intensity of attack by cone and seed insects in Douglas fir. Kozak, Antal

Abstract

An analysis was made of the distribution of and interactions between three major species of insects in cones of Douglas fir. This was based on over 3,500 cones from 93 trees in 1961 and 4,000 cones from 97 trees in 1962. High variation in damage was found among trees for each of the three insect species studied (Contarinia oregonensis Foote, Megastigmus spermotrophus Wachtl., and Dioryctria abietella D.&S.) In C. oregonensis this variation was significantly related to the height of the trees and dates when cones became pendent. The percentage of filled seeds and average cone size of the trees were important in M. spermotrophus. The average cone size of the trees and duration of vegetative bud flushing were significantly associated with D. abietella. The within tree-variation of damage by C. oregonensis was highly significant and consistant from tree to tree and year to year. Damage increased significantly from the bottom to the top of live crown. Suggestions are made to help increase the seed yield by chemical control of insects by selection of uninfested or slightly infested trees by sequential sampling, and by selection of cone and seed insect resistant trees for seed orchards. Although no tree was wholly resistant to all three insects the best, with only 4.8 per cent of its seeds damaged, was so much better than average that it may be of considerable economic importance. The great range in apparent resistance of trees to attack by cone and seed insects should be recognized in further studies and attempts be made to determine biologically sound reasons for the observed differences.

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