264 THE PRACTICAL APPLICATION OF ECOLOGY 



cultural conditions and the prevailing weather admit of such a 

 procedure. Evidence gathered from many sources shows that 

 late sown spring crops suffer most of the attacks of this insect. 

 The frit-fly is so well known as a pest of oats, and also of other 

 cereal crops, that a detailed account of its biology is unnecessary. 

 In a few words, it may be said that, under English conditions, the 

 flies of the first generation oviposit on the leaves or stems of spring 

 oats and on various grasses ; they attain their greatest abundance 

 about May 26th.i The larvae bore into the shoots, causing the 

 death of the central leaves and growing point. Flies of the second 

 generation oviposit in largest numbers about July 15th ^ on the 

 ears of oats, and the larvae feed upon the spikelets and young 

 grain. Oviposition by the third generation of flies occurs during 

 August and September or later, the eggs being laid on the leaves 

 of winter cereals and various grasses ; the flies are most numerous 

 about August 22nd. ^ Winter is passed in the larval condition at 

 the bases of the shoots, which are ultimately destroyed, and the 

 over-wintering larvae give rise to the first generation of fl-ies of the 

 next year. 



Oat plants in the shoot and panicle stages have definite phases 

 of growth within which they are most susceptible to attack. 

 The shoot is most susceptible when in the stage bearing two or 

 three leaves ; once it has attained the four-leaf stage, liability to 

 infestation declines rapidly (Cunliffe, Fryer and Gibson, 1925; 

 Cunliffe, 1928). The grain is most susceptible about the time of 

 fertilisation, and the period of greatest liability terminates before 

 the grain attains half its normal size (Fryer and Colhn, 1924). 



With varieties of oats at present under cultivation the provision 

 of an adequate growing period, to ensure that the plants reach the 

 four-leaf stage sufficiently early, depends upon the date of sowing. 

 It is only practicable with spring oats by eliminating sowings 

 later than the end of February, or in some years the middle of 

 March. Adverse soils or climatic conditions may easily entail a 

 set-back in plant growth, and this necessitates a safety margin, 



1 These dates appear to indicate a three-cycle rhythm in the annual life of 

 the insect, and were observed to hold good during six years' observations 

 {vide Cunliffe, 1929). 



