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C. AppliclItion of microsporidia and fungi. 261
The natural levels of microsporidian infection in spruce budworm populations tend to
increase with the age of the infestation. Examination of spruce budworm larvae collected in
the Uxbridge forest of southern Ontario over a 6-year period indicated that the infection
rate from N.fllmiferanae increased from 36% in 1973 to 69% in 1978 (Wilson 1977b). The
Uxbridge forest was in fact an old infestation; severe defoliation was recorded in 1952
and has persisted at fluctuating levels ever since.
Fungi
Steady progress was made during the 1970s in documenting the effects of fungi in natural
epizootics and developing various species for use in biological control, with the result
that Hirslllelia thompsoni Fisher, the first fungus to be registered for mite control in
North America, is now available under the trade name Mycar® (Abbott Laboratories)
and is also used against citrus rust mites.
The occurrence of entomophthoraceous fungi on spruce budworm has been documented
throughout much of the insect's range in the last 10 years. Outbreaks of fungal disease
occurred in 1974 and 1979 in northern Ontario; in the former year the outbreak was of
Zoophlhora radicans (Brefeld) A. Bakto (= Entomophthora sphaerosperma Frasenius)
(Harvey & Burke 1974), in the latter it was of the same fungus together with E. egressa
MacLeod (Tyrrell, unpublished). In Newfoundland, the same two fungi were first
recorded from the spruce budworm in 1972. The disease was widespread in western
Newfoundland in 1973, and subsequently spread to the whole of the Island (Otvos &
Moody 1978). Mortality varied from 10-40%, depending on weather conditions, and
occasionally reached much higher values in localized areas. Both fungal species, together
with an unidentified Conidiobo/w species, were responsible for spuce budworm mortality
in northern Maine, where disease prevalence varied between 4 and 7% during 1975-77
(Vandenberg & Soper 1978).
In northern Ontario, mortality was higher in white spruce than in balsam fir in 1974,
when fungal incidence was high (Harvey & Burke 1974), but no difference was observed
between tree species in 1980, when fungal incidence was much lower (TyrreU, unpublished). In
Maine, fungal prevalence was found to be both density-dependent and significantly
higher in the lower crown area of the tree, and appeared to be enhanced by periods of
cool, wet weather (Vandenberg & Soper 1978). The link between moist conditions and
• increased incidence of fungus has also been observed in Newfoundland (Otvos &
Moody 1978).
Fungal infection occurs in the later stages of larval development. Incidence of Z.
radicans peaked between the fifth and sixth instars, E. egressa between the sixth instar
and pre-pupa, and Conidiobolus sp. between the pre-pupal and pupal stages, respectively,
in Maine (Vandenberg & Soper 1978); while in northern Ontario peak fungal infection
coincided with the peak development of sixth-instar larvae (Tyrrell unpublished).
Earlier instars however, are not resistant to the disease, and can readily be infected with
Z. radicans under laboratory conditions (Kenneth 1978, Perry unpublished).
Both Z. radicans and E. egressa can be isolated and grown in pure culture, and a
number of different isolates of both species are now available. Z. radicans grows readily
on a variety of solid and liquid media, and the mycelial stage can also be grown in small
fermentors. E. egressa grows slowly on complex media such as coagulated egg yolk, but
can be grown as a protoplast in osmotically stabilized media.
Z. radicans can sporulate on artificial media to give both conidia and resting spores.
The effects of environmental conditions, including temperature, photophase, and nutrient,
on conidial germination have been determined (van Roermund, personal communication).
Maturation of laboratory-produced resting spores under controlled conditions has been
shown to be a key factor in potentiating their subsequent germination (Perry & Tyrrell
in press), and the effects of environmental conditions on their germination are currently