PATHOGENICITY OF NOSEMA FUMIFERANAE (THOMSON) (MICROSPORIDA) IN SPRUCE BUDWORM, CHORISTONEURA FUMIFERANA (CLEMENS), AND IMPLICATIONS OF DIAPAUSE CONDITIONS

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Abstract
A standardized bioassay procedure was used to determine median lethal doses (LD50) of the microsporidium, Nosema fumiferanae (Thom.), on newly molted fourth- and fifth-instar eastern spruce budworm larvae (Choristoneura fumiferana (Clem.)). The LD50 for fifth-instar larva was 1.23 × 106 ± 2.82 × 105 spores. The fourth-instar LD50 was 2.23 × 104 ± 4.30 × 103 spores per larva for populations experiencing prolonged post-diapause cold storage or an elevated temperature during diapause and 2.00 × 105 ± 6.66 × 104 spores per larva for populations not experiencing stressful conditions during and after diapause. Median lethal times (LT50) ranged from 6 to 19 days, depending on instar and dose level. Sublethal responses of fourth- and fifth-instar larvae inoculated with serial dilutions of spores were estimated by significant linear models. These regressions were negative for pupal weight and adult longevity and positive for development time (duration of instar VI). Inoculations of newly molted sixth-instar larvae produced similar models, although development time was not significantly affected. Insects reared following stress during and after diapause had consistently longer developmental times. The importance of prolonged developmental time on disease expression and insect susceptibility is discussed.