Forecasting Outbreaks Of The Douglas Fir Tussock Moth From Lower Crown Cocoon Samples

Excerpt from Forecasting Outbreaks of the Douglas-Fir Tussock Moth From Lower Crown Cocoon Samples A sample plot was 50 randomly selected host trees having lower branches within reach from the ground with the trees distributed over 2 to 5 acres to ha). Plots were selected for the study and sampled for cocoons only if they were expected to have measurable larval populations the next summer. Cocoons were sampled in the fall after moth emergence and oviposition. First and second instars were sampled on the same plots the following June. Densities were estimated in each plot by the frequency of occurrence of tussock moth (that is, cocoons in the fall or larvae in the summer) on the lower branches of 50 host trees (mason 1977, 1979, With this method for sampling cocoons, the underside of three 18-inch (45-cm) branch tips in the lower crown was visually examined on each tree for the presence or absence of cocoons. The proportion, -p, of infested sample trees on the plot was then converted to density of cocoons/1000 square inches m2) of branch area, X, by the theoretical distribution of the Poisson series (mason 1977) where. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.

A sequential sampling plan is described for identifying tussock moth populations which, at a maximum expected rate of increase, could be within 1 year of outbreak status. The plan uses a new technique of sampling early larvae by the nondestructive examination of lower crown foliage. Larvae at inconspicuous low densities are classified into relatively low or high categories by their frequency of occurrence on foliage samples. The sampling plan is applied independently on individual plots to classify the density of each plot. It is an appropriate method for quickly screening suboutbreak populations in ground evaluation surveys.

Procedures for monitoring larval populations of the Douglas-fir tussock moth and the western spruce budworm are recommended based on many years experience in sampling these species in eastern Oregon and Washington. It is shown that statistically reliable estimates of larval density can be made for a population by sampling host trees in a series of permanent plots in a geographical monitoring unit. The most practical method is to estimate simultaneously densities on a plot of both insect species by the nondestructive sampling of foliage on lower crown branches of host trees. This can be done either by counting all larvae on sample branches or by estimating the frequency of occurrence of a selected threshold number of larvae in samples. Statistics are given on the expected within- and between-plot variances and the number of sample plots needed in different sized monitoring units. In large monitoring units, plot densities of tussock moth and budworm larvae usually are not normally distributed, but they can be normalized by logarithmic transformation to predict the probability of subpopulations of any given density occurring somewhere in the unit. It is urged that sampling methods be consistent and that monitoring be done annually to accumulate continuous databases that reflect the behavior of defoliator populations over a long period.