Observations on some Parasites of Oscinella frit Linn

Abstract

The stem infestation of Oscinella frit in England is attacked by at least two species of hymenopterous parasites of considerable importance. The species are Halticoptera fuscicornis Walk. (Chalcidoidea) and Rhoptromeris eucera Htg. (Cynipoidea). It is also parasitised to a much lesser degree by Loxotropa tritoma Thoms. (Proctotrypoidea). These three species are described and figured in detail.The three species named above have been reared by the writer from host puparia obtained in the Harpenden district; they have also been reared in the Leeds and Oxford districts by other observers. They attack the host in the larval stage of the latter and emerge from the puparia. All three species are characteristic parasites of cyclorrhaphous Diptera.In the Harpenden district a parasitism of 27 per cent. was observed in 1926 and of 37 per cent. in 1927, Rhoptromeris eucera being the dominant parasite.Evidence is brought forward which appears to indicate that the parasites, collectively, become more abundant as the season advances, with the result that Oscinella frit affecting late sown oats suffers markedly heavier parasitisation than when it attacks oats drilled earlier in the season.The time of maximum emergence of Oscinella frit in out-of-door rearing cages during the years 1926 and 1927 coincided very closely with its period of greatest abundance in the field, as recorded by Cunliffe.