Influence of intra‐tree variation in time of budburst of white spruce on herbivory and the behaviour and survivorship of Zeiraphera canadensis

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Abstract
Field studies were carried out to determine the effects of intra‐tree variation in the time of budburst of white spruce, Picea glauca Moench (Voss.), on the behaviour and survivorship of, and herbivory by, the spruce bud moth, Zeiraphera canadensis Mutt. & Free. There was significant variation in the time of budburst among whorls, shoots and buds. Budburst was acropetal, with buds in the interior of the lower crown bursting first and terminal buds on terminal shoots in the upper crown bursting last. Bud moths laid the greatest proportion of their eggs in the middle of the crown and egg hatch was usually best synchronized to budburst in this region. Many eggs hatched before terminal buds on terminal shoots in the upper crown had burst and thus intra‐tree variation in budburst decreased the probability that first‐instar larvae would colonize the most important plant parts for growth. However, many later instars dispersed upwards and outwards in the crown and colonized the late bursting buds in the upper crown. Such dispersal reduces the effectiveness of intra‐plant variation in budburst to reduce herbivory and permits Z.canadensis to eat young nutritious buds for a longer period of time. Intra‐tree variance in the date of budburst was greater than that between trees but there were no consistent differences between the intra‐tree variance of trees in half‐sib families with high or low susceptibility to Z.canadensis. Differences between trees in herbivory, bud moth density and survivorship were not related to the amount of intra‐tree variation in budburst.