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 st udied in the laboratory with and without larval predation by the syrphid Ps eudodorus clavatus (F.). Syrphid larvae fed for a mean of 8 days and the si ze of aphid colonies was significantly reduced compared to controls at day 4 after onset of feeding to day 9 inclusively. Colonies that survived preda tion by syrphid larvae grew to be as large as control colonies. Control col onies matured alatae earlier than did colonies subjected to actual or simul ated predation. Significantly more reproductive apterae emigrated from cont rol colonies than from those in the actual and simulated predation treatmen ts and the emigration occurred significantly earlier. The lack of differenc es between paired T. citricida colonies in the actual and simulated predati on treatments suggests that the only measurable effect of syrphid predation on aphid colonies was density reduction. There was substantial variation i n aphid colony success (=production of migrants) within treatments. Both ap terous and alate nymphs often failed to mature and late-maturing apterae fa iled to reproduce or emigrate. The results indicate that colonies of T. cit ricida that experience predation, whether real or simulated, suffer a delay in the production of alatae and produce fewer apterous migrants than colon ies that escape predation. (C) 2001 Academic Press.