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314 W. O. H. Ives
Releases and Recoveries
Bacillus tburingJensis
Berliner
Biological control attempts against the forest tent caterpillar during the past decade have
involved the bacterium B. Ihuringiensis, the microsporidium N. disslriae, and an NPV.
Because some formulations of B. Ihuringiensis have been registered for control of the
forest tent caterpillar, and have therefore been used commercially it is difficult to provide
an accurate record of either the amounts used or the effectiveness of the applications. The
following is a summary of the more readily available information.
The most comprehensive trials of B.I. against the forest tent caterpillar were conducted
in Ontario in 1975,1977, and 1978 (G.M. Howse 1981 personal communication). In 1975,
small-scale aerial spray trials with Thuricide® 168 (Sandoz Inc.) on trembling aspen
stands in southern Ontario showed that the preparation had promise. One application at a
rate of 10 x 10 9 1. U .lha reduced larval populations by 71 %; two applications at the same
rate caused 92% mortality. The treatment was applied too late in the season to save any
foliage. In 1977, evaluations were made of the effectiveness of two applications of
Thuricide® 16B (10 x 10' I. U .lha at each application) in controlling forest tent caterpillar
populations over a total of about 850 ha of trembling aspen and sugar maple, Acer
saccharum Marsh., in three provincial parks in southern Ontario. Resultant population
reduction on trembling aspen ranged from 30 to 79%, and defoliation in the treated areas
ranged from 5 to 46% compared to 82 to 100% in untreated areas. Similar results were
obtained for infestations on sugar maple, where population reductions due to treatment
ranged from 38 to 69%, and defoliation in sprayed areas was about 15%, compared to
100% in an unsprayed area. A further 940 ha were sprayed with Thuricide® 16B in 1977,
but no assessment of its effectiveness was made. In 1978, either one or two applications of
Thuricide® 168, mostly at the rate of7.5 x 10' I. U .lha for each application, were made on
a total of about 515 ha of forest tent caterpillar infestations on trembling aspen, sugar
maple, and red oak, Quercus rubra L., in southern Ontario. Insect populations were
lower than in 1977, so the comparison of defoliation was not a practical way of showing
effectiveness. However, population reduction due to treatment ranged from 79 to 100%
on trembling aspen, from 71 to 100% on red oak, and was 100% in both treated
infestations on sugar maple. In addition, 250 ha of sugar maple were sprayed with one
application of Dipel® WP (Abbott Ltd.) (6.0 x 10' I.U.lha) in late May. Population
reduction due to treatments was 98%.
Data for western Canada are more fragmentary. In Alberta, a hydraulic sprayer was
used in 1974 to treat two 0.2-ha plots of trembling aspen with Dipel® WP (9 x 10'
I.U.lha) and a further two 0.2-ha plots with Thuricide® HPC (Sandoz Inc.) (5 x ur
I.U.lha) (Drouin & Kusch 1975). Control was estimated at 75 and 70% respectively. In
1976, two O.4-ha plots of trembling aspen were treated with Dipel® WP (9 x 10'
I.U./ha). Control was estimated at 75 and 85%, based on the reduction in amount of
defoliation (Drouin & Kusch 1977). In addition, several applications were made by
private operators under contract to various agencies.
In Saskatchewan, Dipel® WP was used to control forest tent caterpillar outbreaks in
Prince Albert and Saskatoon. In Prince Albert, about 325 ha within the city limits were
treated in 1980 when the larvae were in the third and fourth instars, with good results (P.
Kabatoff 1981 personal communication). The chemical insecticide malathion had been
used earlier, when the larvae were in the first and second instars, because it was felt that
there was insufficient foliage at that time for Dipel® WP to be effective. In Saskatoon,
about 800 ha of city parks and about 40 000 avenue trees were treated with Dipel® WP in
1980 (D. Scott 1981 personal communication). About 340 kg of Dipel® WP were used,
and results were generally satisfactory.