286 



The author did not encounter any animal parasites of .4. vapor- 

 (iriorum, either of the nymphs or the adult, though they seem to be 

 occasionally devoured by spiders and mites. Fungi sometimes kill 

 both adults and nymphs. The transparency of the larvae is probably 

 a factor in eluding predaceous enemies, as it renders them practicajly 

 invisible ; the adults escape by dropping and flight. 



A bibliography of nine references (185G-1913) is appended. 



Lees (A. H.) Winter Cover Washes. — Ann. App. Biol, London, i, 

 no8. 3 & 4, January 1915, pp. 351-364. 



Winter cover-washes, or more properly late spring cover-washes, 

 were first tried on a large scale by Mr. Howard Chapman of Kent. 

 He found by experience that a lime- wash, applied as late as possible 

 before the buds burst in spring, produced a very decided lessening of 

 Psylla attack and consequent increase of crop. Cover-washes have 

 also other subsidiary, but very real, advantages : (1) They can be 

 applied when labour is easily obtainable, while a summer wash has to 

 be put on at the end of April, nearly two months later, when labour 

 is urgently needed for other operations ; (2) they tend to keep the 

 bark clean ; (3) they add a small amount of lime or chalk to the soil. 

 The two hypotheses as to the action of the wash on the insects are, 

 that it is directly destructive owing to its causticity, or that it acts 

 as a seal and prevents hatching. The former is untenable, as the 

 causticity is far too feeble to have any chemical action on the chitin 

 of the eggs worth considering. The second is more probable, and it is 

 apparent that to make such a wash most efficient certain conditions 

 must be fulfilled — viz., it must give a thick covering ; it must resist. 

 all external conditions causing lessening of the coat when once on the 

 tree, i.e., it should not flake when dry or wash away when wetted 

 by rain ; it should be applied as late in the spring as possible ; the 

 materials should be reasonably cheap and easy to obtain and mix. 

 A number of field and laboratory experiments were made with various 

 materials calculated to give the required results. A most important 

 matter is the thorough slaking of the lime ; a 40-gallon barrel of 

 lime- wash made in the evening will be warm the next morning, and 

 unless the hydration be completed before application, it continues on 

 the tree and the wash flakes and cracks off. It seems to be best for 

 use when about 48 hours old. The experiments were undertaken to 

 find a cover-spray which is thicker than ordinary lime-wash, resists 

 weather conditions and is cheap and easily made. Of these, a formula 

 containing whiting, glue, starch, potassium dichromate and water 

 fulfils the first two conditions, but its cost works out at '2d. a gallon, 

 and it is not sufficiently easy to make, as so much hot water is required. 

 If lime alone is used it must be used when fresh, and should be allowed, 

 if possible, to stand for 12 hours before application. To do this on a 

 large scale one-third of the total quantity of water may be added first. 

 When it is to be applied, the other two-thirds may be added. This 

 does away with the necessity of having a large number of tubs. The 

 beneficial action of lime-wash on Psylla eggs is due to the mechanical, 

 sealing action rather than to anv chemical effect. 



