Impact of Syrphid Predation on Production of Migrants in Colonies of the Brown Citrus Aphid, Toxoptera citricida (Homoptera: Aphididae)

Colonies of the brown citrus aphid, Toxoptera citricida (Kirkaldy), were studied in the laboratory with and without larval predation by the syrphid Pseudodorus clavatus (F.). Syrphid larvae fed for a mean of 8 days and the size of aphid colonies was significantly reduced compared to controls at day 4 after onset of feeding to day 9 inclusively. Colonies that survived predation by syrphid larvae grew to be as large as control colonies. Control colonies matured alatae earlier than did colonies subjected to actual or simulated predation. Significantly more reproductive apterae emigrated from control colonies than from those in the actual and simulated predation treatments and the emigration occurred significantly earlier. The lack of differences between paired T. citricida colonies in the actual and simulated predation treatments suggests that the only measurable effect of syrphid predation on aphid colonies was density reduction. There was substantial variation in aphid colony success (=production of migrants) within treatments. Both apterous and alate nymphs often failed to mature and late-maturing apterae failed to reproduce or emigrate. The results indicate that colonies of T. citricida that experience predation, whether real or simulated, suffer a delay in the production of alatae and produce fewer apterous migrants than colonies that escape predation.