Extreme host specificity by Microdon mutabilis (Diptera : Syrphidae), a social parasite of ants

Larvae of the syrphid fly Microdan mutabilis are social parasites which liv e up to two years, feeding on ant brood in nests of Formica lemani ants. We questioned why M mutabilis is extremely localized when its host is widespr ead and abundant. Like endoparasitic diseases, social parasites must penetr ate formidable defences before 'infecting' their hosts. This occurs during the egg stage of M. mutabilis: females are sedentary and oviposit at ii: le mani nest entrances, which workers treat as part of their nest, leaving the thin-shelled eggs vulnerable to attack for 10-14 days before hatching. We describe experiments which show a strong maternal effect in M. mutabilis. N ew-laid eggs had > 95% survival when introduced to the individual ant colon y that reared each mother fly or to its close neighbours, but survival decl ined as a sigmoidal logistic function of distance from the mother nest, wit h F. lemani colonies from 2 and 30 km away killing 80 and > 99% of eggs, re spectively, within 24h. Attacks on eggs also increased in proportion to the delay before introduction to laboratory nests. We suggest that they may be coated with a mimetic chemical disguise that lasts for three to four days after oviposition. The results indicate extreme local adaptation by an M. m utabilis population not simply to one species of host, but to an individual host population and possibly to local strains or family groups within an F . lemani population. This conclusion is discussed in terms of virulence, tr ansmission and coevolution in parasitic diseases.