Cacoecia histrionana Froel. a European relative of the North American spruce budworm, Choristoneura fumiferana Clem., was found in several localities on the north-eastern frontier of Bavaria in 1949, but was not abundant anywhere.
Details are given of its biology based upon observations made during large-scale collecting in the summer of 1949, which yielded approximately 2,100 larvae of C. histrionana.
An important characteristic of the species is the great variation in the stages of larval development present at any given time, which is apparently caused chiefly by the absence of diapause as a synchronising factor.
Approximately 4°C. was found to be the lower temperature limit for movement and feeding of larvae ready for overwintering.
Three samples of larvae taken in favourable localities showed that 0-15 to 0-70 larvae were present per metre length of spruce forest edge from ground level to a height of 2 metres. An estimate of the minimum population is made for a limited and well searched area.
From 1803 C. histrionana larvae collected in the field, 15 species of primary parasites and four of hyperparasites were bred. Combining the result of three samples taken at different times the minimum reduction of the population after hibernation is calculated to be 66-8 per cent., the parasitism of pupae at least 13-5 per cent. Egg parasitism occurred.
Nine of the 20 parasites, known hitherto from C. histrionana, have been bred from other hosts of the same animal community.
The most important larval parasite was Meteorus ruficeps Nees which completes 2-3 generations in one generation of C. histrionana. A proportion of the population also overwinters in other Microlepidoptera, concentrating on C. histrionana in summer. Details of its life-history are given.
Occasionally some larvae were found that had probably died from disease but disease, as a mortality factor, was of little consequence in the observation area in 1949.
Mortality caused by parasites proved to be very important in affecting changes in the population density. This result does not agree with Eidmann's theory that the scarcity of C. histrionana is due to climatic control.