In the spring of 1957 populations of a few species of aphids on a pear-tree of 'Yakumo" variety at Nogi, Matsue City, were investigated. The term of the investigations was divided into two periods : the first was from the 25th of April to the 3rd of June, during which leaves of bearing spurs were observed, and the second was from the 6th of June to the 21st of Juue, during which sampling was made from the secondary growths of long shoots.
During the first period only one species, in fact, of aphid, Toxoptera piricola, formed its colonies on leaves of the pear-tree. The colonies had already been formed at the beginning of the investigation, but there was no tendency that the population of T. piricola on the pear-tree grew through the first period, though weather conditions were not unfavourable and there was no natural enemy in fact.
It was shown that the greater part of nymphs in the third and fourth instars in a colony was usually ones with wing-pads, which left the pear-tree for intermediate host plants soon after they came to maturity. This was the reason why the population of T. piricola did not grow through the first period.
Early in June almost all of aphids disappeared from leaves of bearing spurs, whereas new colonies of T. piricola and Aphis citricidus were form in the secondary growths of long shoots. These two species reproduced mingling with each other.
During the second period as well as during the first, the ratio of nymphs in the third and fourth instars with wing-pads to ones without wing-pads in a colony was large in the case of T. piricola. The same ratio in the case of Aphis citricidus was smaller in general than that of T. piricola. It means that the population of A. citricidus had a potency to grow to a considerable size on the pear-tree.
Predators, however, were a decisive check factor during the second period. Larvae of syrphids and coccinellids were abundantly found in aphid colonies, and on the 29th of June it was observed that nearly all aphids of both species on the pear-tree were completely destroyed by predators.