1907 ENTOMOLOGICAL SOCIETY. 77 



tion is that those who have any doubts about the rain having injured the 

 spray should examine the trees a day or two later and see whether it has been 

 washed off. If it has, the bare twigs or branches should be given another 

 coat of the mixture. I sprayed a few trees purposely during a moderately 

 heavy shower to see the effect. A day or two afterwards I examined them 

 and found, as I expected, that there was very little of the spray on them. 

 Som-e other trees, howevej, had been sprayed an hour before the shower be- 

 gan and had had time to become dry; these were not visibly affected by the 

 rain and sleet. It stands to reason, moreover, that one should not spray 

 immediately after a shower but should wait until the trees are dry. Before 

 leaving this subject it is perhaps well to remove the misconception that at 

 least some mixtures of lime-sulphur, once they have become thoroughly dry, 

 are scarcely at all affected by rain. On one occasion this spring I investi- 

 gated this point during a fairly heavy rain. Some of the trees had been 

 sprayed with Port Colborne lime, others with Beachville, others with 

 Guelph. In some cases the wash had been boiled 45 minutes, in others 60, 

 in others 90, and in others as long as 3 hours. In every one of these cases 

 lime was quite visible in the large drops on the underside of the twigs and 

 branches. So that the length of time the wash remains on (one season com- 

 pared with another) depends to a large extent upon the amount of rain that 

 has fallen. In a very wet spring it is quite probable, therefore, that the 

 efficiency of the lime-sulphur, while not entirely destroyed, would be 

 greatly lessened. 



How THE Spray Kills. 



It is very difficult to say just how the spray kills. Chemists have found 

 that as a result of the chemical reactions that are constantly taking place on 

 the tree, minute particles of sulphur are being constantly set free. Sulphur 

 is known to be not only a good fungicide, but also an insecticide and it is 

 possible that many of the insects like the San -lose scale, which have a thick 

 covering to protect them, are killed by inhaling the sulphur which would 

 naturally permeate everywhere, even beneath the scale. At any rate spray- 

 ed branches examined bv me this year showed that by June 8th the San 

 Jose scale insects were nearly all dead beneath their coA^ering. In many 

 cases the direct caustic nature of the wash seems to be the cause of death, 

 e.g., of freshly hatched aphids. In others, such as the Oyster-shell scale, 

 the wash seems to harden around the scale and prevent the young from hatch- 

 ing or from emerging after hatching. Under the same scale coverings of 

 this insect I have found some of the eggs unhatched and others hatched 

 but the young larvae lying dead as if unable to escape. Of course it is very 

 probable that some sort of caustic action also helps in destroying this scale. 

 I have noticed that the scales are nearly always more brittle and so more easi- 

 ly removed on the sprayed trees than on the unsprayed ; so that the eggs may 

 to some extent be exposed to the weather and thereby destroyed. The same 

 thing may hold true of the San Jose scale insects. I have not found, how- 

 ever, that a very large percentage of deaths is due to the scales being loos- 

 ened and falling from the trees. Not more than three per cent., I think, of 

 the Oyster-shell scale were destroyed in this manner. Later on in the sea- 

 son, however, the old dead scales on the sprayed trees began to peel off and 

 leave the bark fresh and clean. Probably one of the most effective ways 

 in which the wash acts as an insecticide is in keeping the bark covered and 

 either making it difficult for the young scale insects to find a suitable and at- 

 tractive place to settle down and insert their sucking tubes, or else making 

 it impossible for them to insert them at all. 



