Effect of a Rugulosin-producing Endophyte in Picea glauca on Choristoneura fumiferana

Disease-free second instar Choristoneura fumiferana (eastern spruce budworm) were placed on trees infected with a rugulosin-producing needle endophyte in two experiments. They were allowed to grow to sixth instar when survivors were collected. First, by using 3-yr old trees, a comparison was made of budworm growth on infected and uninfected trees. A second experiment used 4-yr old trees to study the effect of rugulosin content in the needle on growth. This permitted an examination of a dose response in relation to growth, and allowed us to eliminate the potential for differences in environment or foliar chemistry affecting the results. At sixth instar, budworms feeding on infected trees that contained rugulosin were smaller than those on uninfected trees. At needle concentrations above the dietary low observed effect level of rugulosin for C. fumerana >0.5 μg g⁻¹, a dose response was seen. For the first time, this demonstrates an inverse effect in outdoor nursery experiments between budworm weight and rugulosin concentration.