Reproductive schedule and factors affecting soldier production in the eusocial bamboo aphid Pseudoregma bambucicola (Homoptera, Aphididae)

The reproductive characteristics of the soldier-producing aphid Pseudoregma bambucicola were studied in Kagoshima, Southern Japan, to know the factors affecting soldier production of eusocial aphids. The soldier proportion in aphid colonies was highest from October to November. In some large colonie s, soldiers were observed in all seasons except in July when colony size wa s relatively small. Multiple regression analysis showed that the colony siz e was a principal factor affecting soldier proportion throughout a year. Ot her social or environmental factors such as aphid composition, host plant c onditions and predator abundance were not always significant. Rearing exper iments revealed that large colonies (greater than or equal to 1,000 individ uals) produced soldiers in almost all seasons while small colonies (<1,000) never produced any soldiers. The caste-production schedule of adult female s was examined in the field. When solitary females produced both castes, th ey usually produced normal nymphs first and then soldiers. Females from lar ge colonies tended to produce more soldiers in the earlier period of their lifetime, whereas females from newly established small colonies produced no or only a few soldiers at later times. The average number of soldiers and normal nymphs produced consecutively by a single Female was > 10 and >20, r espectively. Because they have a small number of ovarioles (< 15 on average ), females should alter caste production within the same ovarioles accordin g to changes in environmental conditions. Artificial removal or introductio n of predators and reduction of colony size did not affect soldier producti on over two successive generations, revealing maternal effects on soldier p roduction. Females cannot shift caste production quickly in response to cha nges in predator abundance and colony size. This is probably due to early d evelopmental determination of castes within the mother's body.