. Current topics in forest entomology : selected papers from the XVth International Congress of Entomology, Washington, , 1976. Forest insects Congresses. 1. the number of sawfly egg clusters per tree from 75 to 100 randomly selected living whole trees per sample, 2. the number of eggs per cluster obtained from approximately 50 randomly selected egg clusters from (1), 3. the number of living jack pine trees per chain-square (20 x 20 metres) block mea- sured annually in each of 25 blocks per plot. S-„ = standard error of B xB C = number of living jack pine trees per un. 120 Figure 2.—Time b
. Current topics in forest entomology : selected papers from the XVth International Congress of Entomology, Washington, , 1976. Forest insects Congresses. 1. the number of sawfly egg clusters per tree from 75 to 100 randomly selected living whole trees per sample, 2. the number of eggs per cluster obtained from approximately 50 randomly selected egg clusters from (1), 3. the number of living jack pine trees per chain-square (20 x 20 metres) block mea- sured annually in each of 25 blocks per plot. S-„ = standard error of B xB C = number of living jack pine trees per un. 120 Figure 2.—Time bounding of the Swaine jack pine sawfly life system. Jack pine forests are pure and even aged and usually regener- ate after fire. Sawfly populations are usually low but may suddenly erupt and persist at higher levels for years, espe- cially as forests reach maturity. Periods between outbreaks are frequent (about 10 years). The means and standard errors of each sample are shown (table 1), as well as the combined errors (fig. 3). Calculated by propagation of error, the combined error estimate is: SxC)] + [B 2 . (SxA * SxC)] (SxA • SxB)] where A = number of sawfly egg clusters per tree S_, = standard error of A x A. B = number of sawfly eggs per cluster xC area ( sq. metres) = standard error of C A brief history of the population trends and events in each of the plots follows (fig. 4): Plots 1 and 2 — Aged 51 and 70 years respec- tively in 1974, these two areas were in outbreak condition in 1964, and in 1965 were treated with the insecticide Phos- phamidon at 4 oz. per acre ( kg/ha). Populations of sawflies dropped to extremely low levels in 1966, the year following the one-shot application of insecticide, and have remained at low levels ever since. Plot 3 — Seemingly out of phase with the majority of Neodiprion cwainei infesta- tions in the mid- to late 1960s (McLeod 1970), the infestation in this 60-year- old stand reached extremely high levels and collapsed
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