Several species of syrphids are common predators on the balsam woolly aphid, Adelges aiceae (Ratz.). in eastern Canada. Larvae are found on infested trees throughout the summer in a certain sequence of species. Occasionally adult syrphids are collected while ovipositing.

Attempts have been made to rear the larvae in the laboratory on small pieces of infested bark but the difficulty of keeping the bark moist and preventing the growth of mould and of transferring the larvae to fresh bark made this method unsatisfactory. The prey died before rearing was complete. Rearing the larvae in the field appeared to be the solution to this problem and a suitable cage was designed which was simple to make, install, and examine (Fig. 1).