. Current topics in forest entomology : selected papers from the XVth International Congress of Entomology, Washington, , 1976. Forest insects Congresses. Table 1 (continued) Living jack pine Egg clusters Eggs peK trees per unit area Plot Year per tree cluster ( metres sq.) 06 1972 027 080 + 04 841 058. 687 + 02 250 030 240 + 02 696 06 1973 025 760 04 198 063. 592 T 01 965 029 480 T 03 012 06 1974 013 760 + 01 907 087. 420 T 02 350 027 000 07 1970 000 060 T 00 034 092 480 + 04 177 07 1971 000 000 +" 00 000 090 600 07 1972 000 000 T 00 000 086 000 + 03 498 07 1973 000 000 T 00 00
. Current topics in forest entomology : selected papers from the XVth International Congress of Entomology, Washington, , 1976. Forest insects Congresses. Table 1 (continued) Living jack pine Egg clusters Eggs peK trees per unit area Plot Year per tree cluster ( metres sq.) 06 1972 027 080 + 04 841 058. 687 + 02 250 030 240 + 02 696 06 1973 025 760 04 198 063. 592 T 01 965 029 480 T 03 012 06 1974 013 760 + 01 907 087. 420 T 02 350 027 000 07 1970 000 060 T 00 034 092 480 + 04 177 07 1971 000 000 +" 00 000 090 600 07 1972 000 000 T 00 000 086 000 + 03 498 07 1973 000 000 T 00 000 090 600 T 03 878 07 1974 000 000 T 00 000 081 500 With standard error. No sample of less than 25 egg clusters included. For missing entries, the mean for the series () is used. Samples with no variance estimates are extrapolations from Plonski's (1960) site index curves for jack pine. reached a peak in 1965, but declined in 1966 owing to accidental drift of insec- ticide from an operation in a neighbour- ing stand. The populations rebounded to pre-spray levels in 1967, but events were complicated in the early 1970s by a massive outbreak of a pitch nodule maker, Petrova spp. (McLeod and Tostowaryk 1971) which caused extensive top killing and deformation in the tree crowns. Because Petrova is a sap-feeder, it was able to compete successfully with the sawfly for food. Survival of sawfly larvae declined dramatically in the Petrova outbreak years, and by 1974 sawfly populations were at a very low level. Plot 7 — Monitoring in this 51-year-old stand was carried out from 1970 to 1974. Only in 1 year of the 5 were sawflies recorded. Casual observations in this stand from 1964 to 1971 would also indicate that sawfly populations were very low during that Figure 3.—Standard error of mean as function of mean for N. swainei eggs/sq. metre as calculated by propagation of error (see text). Only complete data sets of table 1 are used. The sharp discontinuities in the distri
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