60 



another. Those remaining on the plum are presumably re- 

 inforced in the early autumn by a return migration of the 

 descendants of those from grasses or reeds and these produce 

 egg-laying females which lay their eggs at the base of the 

 leaf buds and complete the life cycle. From the above it 

 will be seen that this species appears at a much later date than 

 the Leaf -curling Aphis, not usually being present in abundance 

 until after midsummer. 



Natural Enemies. The remarks under this heading on 

 the Leaf -curling Plum Aphis apply also to the present species. 



Measures of Control.* The best wash for use against the 

 aphis is said to be a paraffin emulsion containing liver of 

 sulphur. 



Some washes fail owing to the fact that they cannot pene- 

 trate the mealy secretion by which the insects are covered. 



III. HOP DAMSON APHIS. 

 (Phorodon humuli, Schrank.) 



This aphis is treated fully in Leaflet No. 88f, and need only 

 be dealt with briefly here. It is found in injurious numbers 

 chiefly on the damson, but it occasionally occurs on other 

 plums. The life-history is briefly as follows : Eggs are land 

 on the damsons in autumn, and, on hatching in early spring, 

 produce aphides which live for 3 to 5 generations on the 

 damsons. Towards the end of May winged forms appear 

 which desert the damsons and fly to hops. There is a return 

 migration to the damsons in autumn. By sucking the juices 

 of the damson trees these aphides may do considerable damage, 

 while they are also the cause of the notorious hop blight. In 

 hop-growing districts, therefore, it is very necessary that the 

 aphis should not be allowed to multiply when on the damson. 

 The same measures of control may be employed as were sug- 

 gested in the case of the Leaf-curfing Plum Aphis-. 



Spray Formulae. 



I. Lime Wash-l 



Quick-lime 10-15 Ib. 



Water 10 gal. 



The best stone quick-lime is to be preferred, and the pro- 

 portion of lime to water should be as high as possible, having 

 regard to the nature of the spraying apparatus available. In 

 making up the wash sufficient water should be poured on to 

 the lime to keep it covered until it is slaked. After slaking, 

 the remainder of the water should be added, the whole well 



* Theobald, Insect Pests of Fruit, p. 385. 

 | Obtainable from the Ministry. 



* Lees, Annals of Applied Biology, I, p. 351. Pether bridge, Jour. Bd. 

 A uric., Vol. XXI, pi 916. 



