Prevalence, transmission and mortality associated with Nosema fumiferanae infections in field populations of spruce budworm Choristoneura fumiferana

1The prevalence, intensity and transmission of Nosema fumiferanae (Thomson) (Microsporidae) infections and potential impacts on the survival of field populations of spruce budworm Choristoneura fumiferana (Clem.) were examined in three plots in New Brunswick, Canada, from 1983 to 1992. 2The highest prevalence of N. fumiferanae infection in post-hibernation second-instar larvae occurred in the plot where prevalence in female pupae was the highest in the previous generation, suggesting higher rates of vertical transmission. There was little change in the prevalence of N. fumiferanae infections between the second and sixth instars in the later generations. In the two other plots, N. fumiferanae prevalence increased by approximately 25% from the second to sixth larval stadia. Coincident with the changes in N. fumiferanae prevalence were substantial declines in the populations of spruce budworms, making it difficult to determine rates of horizontal transfer of the disease. 3In all plots and in all years, there were progressive increases in the intensity of N. fumiferanae infections (spore loads/individual) from the second to sixth instars and pupae. 4Annual spruce budworm mortality associated with N. fumiferanae was ≤15% of all mortality in reared specimens and was positively correlated with but generally less than 30% of annual N. fumiferanae prevalence.