Relation of Spruce Budworm Development and Balsam Fir Shoot Growth to Heat Units1

Abstract
Larval devel-opment of the spruce budworm, Choristoneura fumiferana (Clemens), is related to accumulated heat units and closely synchronized with the shoot development of its main host, balsam fir, Abies balsamea (L.) Mill. These relationships are useful in the timing of population sampling and chemical-control operations. The numbers of day-degrees above a threshold of 37[degree]F required for different stages of larval development and shoot growth are tabulated.

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