Mechanisms of evolution of sterile defensive cast in aphids.

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Social biology of Pseudoregma bambucicola and P. koshunensis, both of which have sterile defensive caste (soldier), was studied in Bambusa multiples stands in Kagoshima Pref., and in a rearing room of Nagoya University. The results obtained are as follows: 1) Ratio of soldiers in P. bambucicola colonies became increase in Autumn, when new stems of the host grew, and multiple correlation analysis showed that major factor concerning increase of soldier ratio were colony size and length of bamboo stems, which related to period of colony development. 2) Release experiments and field observations suggested that P. bambucicola soldiers could effectively kill first instar larvae of a predatory syrphid, Eupeodes confrator. Adult females of E. confrator avoided to oviposit near aphid colonies with high soldier ratio, possibly due to active shaking of legs by soldiers when disturbed. 3) Apterous viviparae of P. koshunensis laid some normal larvae first, some soldiers secondly, and in the final stage of their life, laid normal larvae again. Laying normal reproductive caste first and sterile caste secondly is the very reverse of order of laying these castes in all other eusocial insects. It was considered that this discovery may shed a light for further consideration of evolution of defensive castes in insects.