132 Observations on the Habits o/Oscinella frit 



application was not effective. Nitrogenous manures are not likely to 

 repay their cost when used on average land in England, while phos- 

 phorus and potassium alone have no value in limiting damage due to 

 frit-fly. 



While collecting these data the degrees of infection were estimated 

 (1) near the hedge and (2) about the centre of plot B (Fig. 1), an average 

 of 328 plants being examined in each case. The percentages of plants 

 attacked and killed near the hedge were 73-5 and 6-7 and in the centre 

 of the plot 61'8 and 1'3 respectively. 



Baranov(:i) states that the flies attack chiefly on the boundary 

 strips round fields to a width of 9 feet, the central areas being seldom 

 damaged. From the figures given above, it will be seen that this is 

 probably not usually the case in this country. Although the damage in 

 the central areas is distinctly less than on the boundary strips it is still 

 very considerable. 



Parasites. 



Certain Hymenoptera, not previously recorded as being parasitic on 

 the frit-fly, have been reared from puparia collected for experimental 

 purposes^. There is, of course, a slight element of doubt as to the exact 

 identity of the host in such a genus as Oscinella, but from the hundreds 

 of puparia collected from oat plants, no other species than O.frit emerged. 

 It is practically certain therefore that O.frit was the host of these parasites 

 either directly or indirectly. 



In the two seasons 1919 and 1920 parasitism was not at all pronounced 

 and also comparatively iew Ichneumonoidea were collected while sweeping. 



Out of one batch of fifteen puparia collected on 16. 7. 19 at random, 

 from oat stems in the field, eight parasites emerged, but on no other 

 occasion was a similar degree of infection noticed, and this batch was 

 probably abnormal. 



The few parasites (or hyperparasites) which have been reared are 

 distributed among the parasitic Hymenopterous families as follows: 

 Proctotrypidae, one specimen unidentified to date; Braconidae, 

 Chasmodon apterus, Nees (emerged Sept. 1919); Cynipidae, Psichaera 

 (Forst.) spp. (emerged Sept. 1919); two different species of this subgenus 

 have been reared, and they are of interest because other members of 

 the genus Eucoila are known to be parasitic on Diptera; Apkidius 



^ I am indebted to Mr G. C. Lyle and the Rev. J. Waterston for the identification of 

 these insects, as far as is possible at present. The latter gentleman hopes to jjublish a paper 

 shortly on the more interesting species, which are not yet completely identified. 



