Larvae of the syrphid fly Microdon mutabilis are social parasites which live 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 widespread and abundant. Like endoparasitic diseases, social parasites must penetrate formidable defences before ‘infecting’ their hosts. This occurs during the egg stage of M. mutabilis: females are sedentary and oviposit at F. lemani nest entrances, which workers treat as part of their nest, leaving the thin-shelled eggs vulnerable to attack for 10 to 14 days before hatching. We describe experiments which show a strong maternal effect in M. mutabilis. New–laid eggs had > 95% survival when introduced to the individual ant colony that reared each mother fly or to its close neighbours, but survival declined as a sigmoidal logistic function of distance from the mother nest, with F. lemani colonies from 2 and 30 km away killing 80 and > 99% of eggs, respectively, within 24 h. 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. mutabilis 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, transmission and coevolution in parasitic diseases.