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A study of some factors influencing the abundance of Adelges cooleyi (Gill.) on Douglas fir Kozak, Antal

Abstract

A study was carried out to determine factors which influence the abundance of Adelges (Chermes) cooleyi Gill, on Douglas fir. This was done through the observation of populations in the field, supported by some laboratory work designed to show that certain influences are important. The work was done during the summer of 1960 in a young stand in Totem Park near Marine Drive and Agronomy Road on the U.B.C. campus, Vancouver. The following items were investigated: 1.) Inter tree differences, 2.) Intra tree variations, 3.) Population changes with time, 4.) Mortality of the insect. Abundance was affected by extrinsic influences on the trees, such as location and exposure and intrinsic factors such as time of bud opening and twig length. Within a tree the abundance of Adelges cooleyi was affected mostly by microclimatic factors, resulting in high abundance of the insect in the peripheral part of the lower crown. The average number of living insects decreased with time rectilinearly in generation 1 (Sexuparae and Progredientes) and logarithmically in generation 2 (Neosistens). A critical period during establishment of generation 2 caused the logarithmic changes. Mortality estimates by direct counts were subject to a large error because many of the dead insects fell off.

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