Shift in life-history strategy from reproduction to defense with colony age in the galling aphidHemipodaphis persimilis producing defensive first-instar larvae |
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Authors: | S Akimoto |
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Institution: | (1) Systematic Entomology, Faculty of Agriculture, Hokkaido University, 060 Sapporo, Japan |
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Abstract: | Summary Colony defense in some aphids is performed by sterile soldiers but in others by monomorphic larvae of a specific instar stage.
This paper, focusing on a galling aphidHemipodaphis persimilis with monomorphic defensive first instars, examined the mechanism by which the proportion of defenders is regulated in the
colonies. Demographic analyses showed that the ratios of first instars (defenders) were kept constantly high (58% on average)
from mid June to late September. High proportions of first instars could be explained by consistently high birth rates (birth
rate hypothesis) or by a prolonged duration of the first instar stage (instar span hypothesis).
With the progress of colony age, the mature-embryo content of apterous adults, used as an index of the birth rate, decreased
and the proportions of advanced instars increased. These results did not support the birth rate hypothesis. By contrast, calculation
of a newly proposed index, the molting rate, showed that the duration of the first-instar stage was short in incipient galls
but became longer with colony age. The duration of other instar stages was always kept short. These results corroborate the
instar span hypothesis and suggest that the prolongation of the first-instar stage is an adaptive mechanism by which the defender
ratio is kept high in mature colonies where the birth rate is declining. The frequency of aggressive behavior in first instars
increased from incipient to mature galls. Seasonal changes in the instar span and aggressiveness of first instars suggest
that inH. persimilis colonies there is a strategic shift from the reproductive to defensive phase with colony age. |
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Keywords: | colony defense demography aphids galls soldiers |
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