THE phorid Megaselia hdterata Wood has been a widespread pest of mushrooms since 1953, when it caused severe damage to commercial crops m southern England. Since that time it has been commonly known as the " Worthing Phorid " or " Mushroom Fly ". Unlike M. nigra Meigen, which attacks the sporophore and was the species referred to in the older literature dealing with pests of mushrooms cultivated outdoors. M. hdterata infests the mycelium. Although present in small numbers throughout the year on mushroom farms, it is only common and potentially destructive between June and October. To permit uninterrupted investigations on the biology and control of this pest a continuous rearing technique is necessary. Moreton and John (1955) reported rearing this species in 3in. X lin. tubes containing a small quantity of spawned compost. We had diflficulty in obtaining eggs under these conditions, and investigated the factors limiting success.Discovery of the parasitic eelworm Bradynema sp. within the body cavity of many adult flies (Hussey and Wyatt, 1958) revealed that the reproductive potential of infested females was often reduced from about 50 to only two or three eggs. The eelworm enters its host during the first or second larval instar by boring through the body wall. The eggs of M. hdterata in the compost are unaffected by the free-living stages of the eelworm, so eggs were collected by exposing spawned compost on heavily infested farms for a few days and then extracted by agitating the compost in water above a sieve within a Baermann funnel. M. halterata males infested with Bradynema are often impotent, and as virgin females may lay about 30 infertile eggs these are commonly extracted from compost. Successful cultures depend, therefore, on the recognition of fertile eggs which are translucent, whereas infertile ones are cloudy and almost opaque.The proportion of M. halterata infested by Bradynema varies from place to place and season to season, but 80 per cent infestation has commonly been found. Colonies initiated with wild flies in small culture vessels are almost invariably infested. Within the small volumes of compost used, infestation of later generations is rapid and increasingly complete, so that such colonies inevitably die out after a few generations. It is possible that Moreton and John collected their material from a population of M. hdterata unaffected by this eelworm and therefore avoided the difficulties now often encountered. Indeed, evidence from recent outbreaks suggests that the absence of the eelworm in 1953 may well have accounted for the severity of the early attacks.The degree of mycelial colonization of compost has also been shown to influence its attractiveness to female M. halterata for oviposition (Hussey, 1961). Mycelium must be actively growing to achieve optimal results and when the compost has been permeated with mycelium the females will not oviposit. This may be a reaction to the lack of developing hyphae on which the young larvae feed, or it may be a response to the lower moisture conte...
