The effect of nitrogen fertilization of white spruce (Piceaglauca) on the yellow-headed spruce sawfly (Pikonemaalaskansis)

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Abstract
Fertilization with 224 and 448 kg Nha−1 applied to white spruce, Piceaglauca (Moench) Voss, as ammonium nitrate significantly increased concentrations of foliar N. Trees with 1.10 to 1.29% foliar N on July 3 had the highest average number of yellow-headed spruce sawfly, Pikonemaalaskansis (Rohwer), larvae per tree (final population) and the lowest percent mortality. This corresponded to trees receiving 224 kg N ha−1. Although not significantly, the final population increased, and the percent mortality decreased with increasing N concentrations up to 1.29% (July 3) (N-deficient trees). As foliar N concentrations, at the time of feeding, increased above 1.29% (N-sufficient trees), final population decreased and percent mortality increased. Therefore, indiscrimate use of fertilizers may increase insect numbers.