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contained 17 trees and control plots 16 trees. The average tree diameter at breast height (dbh) was 29.5 cm. One plot in each
pair was randomly selected for fertilization. Analysis of results was on a paired plot basis. In October, 27.2 kg of urea (47%
N) was applied by hand to the fertilized plot in each of the 10 pairs. This simulated an application rate of 350 kg N ha -_.
Insect and Tree Measurements
Effects of the treatment were evaluated by measurements of tree buds, foliage, radial increment, and insect biomass
on treated and untreated plots. Samples for foliage measurements and insect weights were collected periodically from tree
crowns with pole pruners according to methods recommended by Schmid et al. (1982). Radial increment was evaluated at
the end of the study from increment cores taken from two trees per plot. All sample trees were selected from the center of
plots to minimize edge effect. The schedule for measurements and collections follows:
Shoot and Insect Insect Radial
Date foliage larvae pupae increment
Fall 1989 X
May 1990 X
Fall 1990 X X
Fall 1991 X
May 1992 X
Fall 1992 X X
Larvae were sampled only once in 1990, using methods developed by Schmid et al. (1982). Sampling also was attempted in
1992, but only a few larvae were found, indicating a virtual population collapse in the study area. Larvae were collected for
weighing from 2 midcrown, 45-cm branch samples on each of 3 trees per plot. Larvae were preserved in 70% ethanol and
oven dried at 45 °C to a constant weight. Mean dry weight per individual larva was calculated by dividing dry mass by
density. Mean fresh weights of pupae were determined from a collection made from the soil of approximately 5 pupae per
plot.
Branch samples were collected each August or September from 1989 to 1992. Two 45-cm mid-crown branches per
tree, from 3 trees per plot were cut:with pole pruners. In the laboratory, new buds were excised and current needles were
stripped from the branches. These were oven dried at 45°C to a constant weight and expressed as foliage dry weight per
centimeter of twig length. Mean foliage weight was summarized by plot and year. Buds were represented as weight per bud.
In August 1992, at the end of the growing season, 2 cores per tree were collected at dbh on 2 trees per plot to measure radial
increment. Cores were mounted, sanded, and measured to the nearest 0.01 mm on an incremental measuring instrument
interfaced with a desk top computer, as described in Wickman et al. (1992).
Insect frass was collected from canvas panels placed under trees on 5 treated and 5 untreated plots at the peak of
larval feeding in June 1990. Frass was air dried and analyzed for total and available levels of N, P, K, and S at the University
of Arizona, Tucson, in 1993. Dried foliage from 1989 and 1990 samples also was analyzed for N, P, K, and S and available
nutrients at the same laboratory in 1993. Dried 1989 needles were analyzed for total reducing sugars and phenolics by Mr.
B.A. Birr, North Central Forest Experiment Station, East Lansing, MI, in 1994.
Statistical Analysis
Data were analyzed using paired t-tests on the randomly selected paired plots for each year of measurement. Differences
were judged to be significant when the probability was