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248 J. C. Cunningham and G. M. Howse
Background
Field Trials
B. Viruses: Application and Assessment
J.C. CUNNINGHAM and G.M. HOWSE
Five different types of viruses have been found to infect larvae of the spruce budworm,
Chorisloneura fumiferana (Clem.). Four of them share one feature in common, that is,
virus particles are contained within proteinaceous inclusion bodies. These infectious
particles are released when the inclusion bodies are ingested by larvae and the inclusion
body protein dissolves in their alkaline gut juices. The four viruses comprise a nuclear
polyhedrosis virus (NPV), a granulosis virus (GV), a cytoplasmic polyhedrosis virus
(CPV), and an entomopoxvirus (EPV). Between 1971 and 1980, all four viruses have
been field tested either alone or in various combinations. The fifth virus, which has been
found to infect spruce budworm, is cricket paralysis virus. It has no inclusion body, has
a very wide host range, has little potential as a biological control agent, and will not be
discussed further. Collapse of a spruce budworm population due to a naturally occurring
virus epizootic has never been observed. Both NPV and CPV are occasionally detected
in eastern spruce budworm populations in Ontario. but generally less than 1 % of larvae
are infected.
The first field trials of spruce budworm viruses were conducted in 1959 and 1960 by
applying NPV and GV on single small trees (Stairs & Bird 1962). In 1969, different
dosages of NPV were applied on single trees at 2- to 3-day intervals throughout the larval
period (Bird & McPhee 1970). In 1970, an EPV was found in 2-year cycle spruce
budworm, C. biennis Freeman (Bird el al. 1971). In the laboratory very low dosages of
this EPV (in comparison to other viruses) caused high larval mortality and, although it
was slow acting, there was high hope that it would prove to be an effective biological
control agent. By 1970, the use of artificial diet had greatly improved methods of rearing
spruce bud worm (McMorran 1965), and sufficient larvae could be reared in order to
produce viruses for aerial spray trials (Grisdale 1970).
The first aerial spray trial was conducted in 1971 and trials have been carried out each
season since then. Sixty plots were treated with viruses or combinations of chemical and
viral insecticides in Ontario between 1971 and 1980. Such factors as dosage, timing of
application, tank mixtures, spray equipment, impact on different species of host trees,
and persistence of viruses from one year to the next have been evaluated. The following
viruses and combinations of viruses were tested during this period: NPV, EPV, GV,
EPV plus NPV plus CPV, and NPV plus CPV. In addition, NPV and EPV plus NPV have
been applied following chemical insecticide sprays.
Assessment of the impact of treatments
The same methods used to evaluate chemical insecticide treatments in Ontario were
used to assess virus spray trials. In addition to carrying out larval population studies
and defoliation estimates, attempts were made to determine the levels of infection in
the spruce bud worm population, to assess the effect on successful pupal emergence and,
on some plots. to continue these studies for one or more years after the initial application.