1. An outline of the life history of Cephus pygmaeus Linnaeus is given.2. The parasites of Cephus are listed and are discussed briefly in relation to the particular parasite Collyria calcitrator Gravenhorst.3. It is shown that if the host larvae collected from wheat-stubble in autumn are used for the investigation of the extent of parasitism by Collyria, allowance must be made for the fact that the parasite Pleurotropis benefica Gahan is intrinsically inferior to Collyria, and that its cocoons must represent hosts left unparasitized by Collyria.4. In summer the adults of Collyria emerge, on the whole, earlier than those of the host Cephus. This suggests that at the beginning of the season of oviposition there will be a scarcity of hosts in comparison with the numbers of parasites wishing to oviposit. The relative activity of host and parasite during the egg-laying period is worked out on hypotheses of the average length of life of the individuals, and is shown diagrammatically.5. The Cephus larvae travel down the stem and hibernate at the base. When the wheat is cut, a proportion of the larvae and their parasites are removed with the straw. The way in which this may prevent the stubble material from giving a true indication of the degree of parasitism in the population as a whole is discussed.6. On the basis that 3910 hosts receive 3600 parasites, various hypothetical distributions are worked out arithmetically in order to find which type of discrimination between healthy and parasitized hosts gives an arrangement of parasites most in agreement with what has been found for Collyria in Cephus. The most satisfactory result is that obtained when it is assumed that the probability of an already parasitized host being re-attacked varies with the proportion of parasitized hosts. It is also shown that a shortage of hosts at a time of parasite activity may cause a high degree of superparasitism, thus giving a false impression of the parasite's power of discrimination.7. Suggestions for further field work are given.
The wheat crops in certain districts of England are attacked every year by the wheat-stem sawfly, which in its turn acts as host to various species of parasitic insects. The aim of the present investigation is to try to discover from an analysis of records if any sort of simple numerical relations exist within the complex including the wheat field, the sawflies which infest the wheat-stalks, and the parasites which attack the sawflies. The results of this work lead to a discussion of the peculiar difficulties involved in the application of mathematics to the study of animals and plants in their natural environments.
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 ...
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