THE OCCURRENCE OF 2 TYPES OF ENTOMOPATHOGENIC BACILLI IN NATURAL-POPULATIONS OF THE SPRUCE-BUDWORM, CHORISTONEURA-FUMIFERANA

This paper reports the occurrence of two types of pathogenic bacilli i n cadavers of spruce budworm, Choristoneura a fumiferana, larvae and p upae on balsam fir, Abies balsamea (L.) Mill., foliage sampled in seve ral locations in New Brunswick, Canada. One type forms spores and prot ein crystals at sporulation typical of Bacillus thuringiensis (B.t.), while the other forms spores but no crystals. The noncrystal-producing bacillus is likely Bacillus cereus rather than an acrystalliferous va riety of B.t.. The bacilli infect budworm during most of its life cycl e, from hibernating second-instar larvae to pupae, with varying rates of infection, but are never a major cause of mortality. The bacilli al so infect other lepidopterous species occurring on balsam fir foliage. Circumstantial evidence indicates that both bacilli occur naturally o n the foliage, despite the history of aerial application of B.t.-based insecticides near, if not directly over, out study plots. A subtle im pact of these bacilli on the dynamics of spruce budworm populations is discussed.