Notes on the Biology of Dexia rustica F., a Dipterous Parasite of Melolontha melolontha L.

SUMMARY. There are in Great Britain four important chafer species, of which the larvæ, known as white grubs, feed on the roots of plants, young trees and grass, causing serious damage. The four species are Melolontha malolontha L., Amphimallus solstitialis L., Phyllopertha horticola L. and Serica brunnea L. Large numbers of white grubs of all species were received from various localities, mainly forest nurseries, and were examined for the presence of insect parasites. Only one parasite species was found, the Tachinid Dexia, rustics. F., the larva of which burrows into the host, grub. No parasitism of Serica brunnea was found, and attempts to induce parasitism artificially wert unsuccessful. Parasitism of Phyllopertha horlicola was rare in the field, but readily brought about in the laboratory. Amphimallus solstitialis was not very numerous, but parasitized individuals were common. With one exception the presence of Dexia rustica in:ti1 area was always associated with the presence of Melolontha melolontha, and its distribution in Britain appears to be restricted to latitudes south of the Mersey Estuary, although Melolontha is found farther north. Other subjects included in the analyses of the field data are the seasonal variations of parasitism and the numerical distributions of the parasite larvae among the host grubs. The latter invariably show combinations of low rates of parasitism with high degrees of superparasitism. In particular, attention is drawn to the effect of a flight year of the hosts on the parasite population. In regions whew there is overlapping of the host generations there are always suitable hosts at the season of parasite oviposition. When the generations do not overlap, the host population in the autumn of the‐flight, year consists mainly of very small first,‐stage grubs which are either undetectable or unsuitable for parasitism by Dexia. The percentage parasitism of such grubs is never high, and it is doubtful if many of the parasites in them reach maturity. If any large grubs are present, the percentage parasitism in them is relatively high. By reason of the parasite losses in the host‐flight year the degree of parasitism reached is not, as high as that in places where the host generations overlap. An account is given of the laboratory methods of rearing the adult parasites and of inoculating healthy grubs with the new generation of parasite larvæ. Parasitized grubs were shipped to Mauritius for trial in the attempted control of Phytalus smithi. The life‐history of Dexia rustica. is described in some detail. At relevant points in the story of the life‐cycle accounts are included of experiments which throw light on the parasite's instincts and behaviour. Sets of data obtained from recorded observations on field material are also given. The general conclusion is that in the areas studied a further increase of parasitism by Dexia rustica is unlikely, because a high proportion of the host, grubs, 70 to 75 per cent., is inaccessible to the parasites. It is pointed out that this is an advantage to the parasite, as it prevents the elimination of the host species and, therefore, of the parasite itself. In addition, the effects of the heavy superparasitism are shown to be less detrimental to Dexia than one might imagine. From the economic standpoint Dexia rustica plays a limited, though useful, part, as an agent in the natural control of Melolontha melolontha.