. Bulletin of entomological research. Entomology. THE USE OF EXPERIMENTAL PLOTS AND FOREST INSECTS. 159 and in 10% they had in vain attempted to enter. The supposition at once presents itself that the primary cause of the attack of the pine-beetle was the defoliation brought about by Bupalus piniarius, which had weakened the trees to such an extent as to render them appetising to the beetles. A closer examinations of the trees on the experimental plot showed, however, that this cannot be the case in this instance, at least not to any great extent. According to the estimates made in 1917, 71 of
. Bulletin of entomological research. Entomology. THE USE OF EXPERIMENTAL PLOTS AND FOREST INSECTS. 159 and in 10% they had in vain attempted to enter. The supposition at once presents itself that the primary cause of the attack of the pine-beetle was the defoliation brought about by Bupalus piniarius, which had weakened the trees to such an extent as to render them appetising to the beetles. A closer examinations of the trees on the experimental plot showed, however, that this cannot be the case in this instance, at least not to any great extent. According to the estimates made in 1917, 71 of the 126 trees of the plot had an average injury, 14 being severely defoliated and the rest having escaped any visible damage. Only one of the 14 severely injured trees had in the spring of 1918 been attacked by the pine-beetle, and that in vain. Moreover, we notice that one of the trees in which the pine-beetle was breeding had not previously been attacked by Bupalus, nor had five other trees into which the beetle had in vain tried to enter been previously injured by the moth. Fig. 2 shows how the attacks of the Fig. 2. Diagram showing the distribution of attacks of Bupalus piniarius (solid line) and the pine beetle (dotted line) on trees of different dimensions. and beetle are distributed in trees of different dimensions, from which it is evident how little the attack of the pine-beetle is connected with that of Bupalus. We notice that only 24% of the trees measuring less than 15 cm. at breast height have suffered from the moth, whereas all trees measuring more than 30 cm. have been, attacked. The pine-beetle, on the other hand has preferred the smaller trees, those exceeding 30 cm. not being attacked at all. From this it is evident that the two insects, the pine-tree looper and the pine- beetle, work on two different lines, one selecting the largest trees, the other preferring the smaller ones. The investigation has also revealed the fact that during the outbreak in 1
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectentomology, bookyear1
