1. Gibbs, J. N., and Brasier, C. M., Nature, 241, 381 (1973).
2. Little, E. L., Misc. Publs. US Dep. Agric., 1146 (1971).
3. Pomerleau, R., Bi-mon. Prog. Rep. Forest Ent. Path. Branch, 20, 1 (1964).
4. Finnegan, R. J., and Sippell, W. L., Bi-mon. Prog. Rep. Forest Ent. Path. Branch, 20, 3 (1964).
5. Gibbs, J. N., and Howell, R. S., Forest Rec., Lond., 82 (1972).