Jon Leferink, Larry Matwee, Keith Knowles
Manitoba Conservation, Forestry Branch

SERG Project #1999/03


Executive Summary

Since 1997, tebufenozide (Mimic®) has been used by Manitoba Conservation for the management of Western Spruce Budworm (SBW) to protect commercial timber stands. The operationally used dosage rate of 70g a.i./ha has proven successful for low to medium levels of infestation (up to 50 larvae per branch). However, at high population densities, too many larvae survive this treatment and as a result the treatment does not provide the desired protection.

Previous studies indicated that applications of higher dosages of tebufenozide will eliminate a greater proportion of SBW larval populations. In this field trial the application of variable dosage rates of tebufenozide was evaluated for their efficacy in protecting high value timber from a range in SBW larval population densities by reducing the population density to around 6 larvae per branch. In the spring of 1999, dosage rates of 70, 105 and 140 g a.i./ha of tebufenozide were aerially applied to selected areas with low, medium and high larval SBW population densities.

Larval densities were monitored at pre-application, 10 days post-application and in the spring of 2000. In addition, egg mass counts were conducted on samples taken during the winter of 1999-2000 and defoliation rates were assessed.

Treatments resulted in reduction of SBW larvae to below 10 larvae per branch for all three larval population density category areas. Surveys in August of 1999 indicated significantly lower (P=0.03) egg masses in the treated versus the untreated areas. However, results were inconsistent with the treatment levels applied. This was likely due to moths flying into the treatment areas from adjacent areas during oviposition. Larval washes in November of that year showed significantly fewer (P<0.01) surviving larvae per sample (21.0) in the treatment areas than in the untreated control (63.1). Surveys of the same plots in the spring of 2000 revealed significantly lower larval numbers in the treated plots than in the untreated control. Spring 2000 surveys showed that second generation larval population densities were just above the target density of 6 larvae per branch for all treatments. Densities ranged from 7.1 to 10.3 larvae per branch in the 105 g a.i./ha and 140 g a.i./ha treatments, respectively, but the untreated control averaged 21.0 larvae per branch.