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Some factors influencing the populations of the hemlock sawfly, NEODRIPRION TSUGAE Midd. Brown, George Stuart


Investigations were carried out to determine factors which permit the development of outbreaks of the hemlock saw-fly, Neodiprion tsugae Midd., within restricted portions of a wide range. This was done through the observation of populations in the field, supported by laboratory studies designed to show that certain influences are important. Populations were found to be directly proportional to the relative humidity within the tree crown. Epidemic populations occurred only where hemlock made up at least 50 percent of the stand and where the average relative humidity within the crown did not fall below 50 percent for any period of a month during the summer. Relative humidity was governed by stand composition, stand density, age, elevation, and exposure. The relative humidity was inversely proportional to the amount of Douglas fir present in the stand. Western red cedar in amounts up to 30 percent increased the relative humidity where hemlock was present as a co-species. Stand density affected humidity to the extent, that the relative humidity was higher in open stands where crowns were dense. Factors contributing to this appear to be: profuse ground cover which contributed to evaporation: the dense foliage within the tree crown increasing transpiration but retarding air circulation: large crowns extending to the ground cover providing a protective leaf shelter over the entire area, thus preventing the escape of moisture. Maximum populations were recorded on sites of medium quality, poor sites producing trees of insufficient crown density to maintain humidity. Outbreaks have been confined generally to open hemlock regeneration with large, dense crowns along the shoreline or near tidewater and to similar stands inland above the 1500-foot elevation. Relative humidity was shown to be of direct significance to egg-laying, hatching, and movement and survival of small larvae. The sex ratio appeared also to be influenced by population density, the percentage of males varying directly with population. The fecundity varied inversely with population density.

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