ESTIMATION OF EGG POPULATIONS OF THE LARCH SAWFLY, PRISTIPHORA ERICHSONII (HTG.)

Abstract

The larch sawfly, Pristiphora erichsonii (Htg.), lays its eggs in the new terminal shoots of tamarack, Larix laricina (Du Roi) K. Koch. The oviposition injury usually causes the shoots to curl. During 1952 a sampling project was conducted in the Whiteshell Forest Reserve, Manitoba, to determine the feasibility of sampling tamarack trees to obtain estimates of the egg population of the larch sawfly. Additional data on the frequency distribution of the number of larch sawfly eggs per shoot were collected in 1953 and 1954 from several areas in Manitoba and Saskatchewan. The number of eggs per curled tip varied between plots and between trees on one plot, but no factors contributing to variation could be found. The frequency distribution of the number of eggs per curled shoot was found to be a modified logarithmic normal distribution. The number of curled tips per branch and the number of branches per crown level varied between crown levels and between tree types. Stratification of the sample increased the efficiency of sampling, reducing the standard error of the mean by about 15% and the required sample size by about 30%. The large variation in the estimated number of curled tips per tree indicates that a large sample of trees is required to obtain accurate estimates. As a compromise between accuracy and practicability it is recommended that six-branch samples be taken from each of at least 15 trees, using stratified sampling with proportional allocation. Simple random sampling, taking two branches from the mid-crown of at least 25 trees, is suggested to provide a population index of sufficient accuracy for survey purposes.