10 



week or ten days later. The great thing is to know the 

 habits of tne pest and to apply the spray just at the right 

 time so as to catch the young insects as soon as possible 

 after they have hatched. 



(d) Contact Insecticides destroy insects by actual contact 

 with them. It has been shown that while some insects 

 have jaws and eat solid matter, others have " trunks " 

 with which they suck the sap or juice of plants through * 

 minute hole. These sucking insects cannot be forced to eat 

 a poison spread on the surface of the leaves, since they 

 bore through the poisoned layer and only feed when they 

 have reached the inside of the leaf. In spraying against 

 such pests, a poison which kills by contact on touching the 

 insect must be used. Insects having hard or tough skins 

 are fairly well protected against poisons applied to them, 

 but their defences have a weak spot the breathing hole or 

 spiracle through which poisons can reach the interior. As 

 the spiracles are very small and few in number it is evident 

 that to poison an insect through them the body must be 

 thoroughly wetted, while further, the fluid must be of such 

 a nature that it will run well into minute holes. These two 

 needs give the clue to all contact insecticide spraying. The 

 necessity for the fluid to touch the insect involves a spray 

 applied with force through a coarse nozzle passed up and 

 down each bough in order to reach both sides of every leaf 

 and every hole and corner between buds and in flower 

 trusses, where the pests may be hiding. 



The need for a fluid which runs well involves the use of 

 some substance such as soap which enables the liquid 

 to penetrate the breathing holes (spiracles) of the insect, 

 carrying with it the poison (e.g., nicotine) which may be 

 used. If these two needs are met, good results may con- 

 fidently be expected. This also applies to certain contact 

 insecticides which do not act by poisoning the insect but 

 by smothering it with some sticky substance. Thus red 

 spiders (mites) can be killed by a thin paste made of flour 

 and water which is not poisonous but sticks the pests to 

 the leaves. 



Finally, as in the case of internal poisons, it is most im- 

 portant to time the application of contact insecticides pro- 

 perly, especially not to spray too late. Many insects such 

 as Aphides (Green Fly) cause the leaves they feed upon to 

 curl round and inside these curled leaves the pests are 

 naturally protected against any contact insecticide. To 

 avoid the difficulty it is necessary to spray soon after the 

 pests have appeared and before they have had time to induce 

 the leaves to curl. In judging the period during which 

 useful spraying is possible, some knowledge of the life- 

 history of the pest is clearly invaluable, and such knowledge 

 is pqually necessary to ensure that those spraying are placing 



