Some strategies for controlling the Douglas-fir tussock moth by regulation of budbreak

Abstract

The young larvae of the Douglas-fir tussock moth (Orgyia pseudotsugata (McDunnough) (DFTM) are dependent on the new foliage of its ho'sts (Doug1as-fir, grand fir, white fir) for their food supply. The phenology of the DFTM and its hosts are synchronized, with egg hatch and dispersal occurring one to three weeks after first budbreak of the hosts. A method of-host management that would upset this synchrony and deny the newly hatched larvae the new foliage necessary for their survival could provide an effective means of control for a DFTM population. The proposed approach is one of host management utilizing plant growth regulators to delay budbreak of the hosts and thus starve the DFTM. This method would minimize effects on non-target organisms. Growth regulators have been identified with the capability of altering phenology of many agronomic and horticultural plants. The study involved application of eight different growth regulators, each at three different concentrations applied during three seasons. Study areas were located in western Oregon and in eastern Oregon in an area where the DFTM had previously been a problem. Both Douglas-fir and grand fir were utilized. Growth regulators were asulam, Cycocel®, DPX-3778, ethophon, Krenite®, maleic hydrazide, NAA, and silvex. The applications were made to simulate aerial deposits. Observations were made on individual trees for date of budbreak, height growth, and foliar damage. A constant record of air temperature was made during the period of budbreak on both study areas. This record was utilized to compare cumulative bud break with accumulated heat units. The effects of the growth regulators on budbreak and height growth were analyzed by various methods of analysis of variance and covariance. On one study area the budbreak history of individual trees was known, and the inherent variation in time of budbreak was removed from the analyses. A silvex treatment in November provided the longest delay, one of 6 days. Some other treatments, notably Erenite® and maleic hydrazide fall treatments, indicated some delay capabilities, but they also caused foliar damage. The majority of the treatments provided no delay, and the day of budbreak analyses indicated a lack of significant delay attributable to any treatment. The treatments had a significant effect on height growth response as expressed by a ratio of growth the year after treatment to growth the year before treatment. Ethophon and silvex stimulated growth, and all other treatments reduced growth. In general, the chemical treatments observed in these experiments did not show potential for de-synchronizing the DFTM and its two important host species. The failure of treatments to cause a major postponement implies that the tendency toward normal phenology is very strong.