Interrelationships between the Eusocial Aphid, Pseudoregma bambucicola, and Its Syrphid Predator, Eupeodes confrater

The survival rate of larvae of the syrphid Eupeodes confrater, released into colonies of the eusocial bamboo aphid, Pseudoregma bambucicola, was highest in colonies with low numbers of soldiers, although not significantly so (r=-0.51, n=13). Although not significant, all correlations between the numbers of syrphid larvae seen in aphid colonies and the number or percentage of soldiers in these colonies were negative (r=-0.16 to -0.18, n=50 to 66). These facts suggest that sterile soldiers are an effective defence against syrphid predators. A significantly higher percentage of small aphid colonies (<1000 individuals) were free from syrphid attacks than large colonies (>1000 individuals). In September, when P. bambucicola began to produce soldiers, there was no significant correlation between the percentage of soldiers in a colony and the number of eggs laid by the syrphid on spiders' threads close to the colonies, while, in November, there was significant negative corelation (r=-0.34, n=50, P<0.05), suggesting that E. confrater avoids aphid colonies with a high percentage of soldiers.