2l8 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
Contact Insecticides.- — Where an insect feeds by sucking the juices 
from a plant instead of biting it, it is clearly impossible to poison it 
through the stomach. Many of the most important pests of fruit 
trees belong to the sucking type. One only need mention aphis, 
Psylla, red spider, and plant bugs to indicate how important it is to 
possess a means of controlling them. A considerable number of 
such contact insecticides exist. There is room for much work on the 
precise nature of their action, but they all agree in acting on the 
insect either through the skin or through the breathing tubes. 
The question naturally arises as to what are the essentials for an 
effective contact insecticide. The very name supplies the answer. 
Firstly, it must make contact, and, secondly, it must be insect -killing. 
At first sight this appears to be a truism. It is quite possible, however, 
to have a fluid that makes contact, namely wets an insect, without 
being insecticidal, and it is also possible to have a fluid that would be 
insecticidal if only it made contact with the insect. The first condition 
is therefore wetting power. Within the last two or three years the 
importance of wetting power has begun to be realized, but as its 
importance is not so widely recognized as it should be I propose to 
go into the question briefly. As the subject involves rather abstruse 
physical questions, the full discussion here would be out of place, and 
indeed the conditions on which wetting depends have not yet been 
thoroughly investigated. The question has been discussed by Lefroy* 
and by Cooper and NuTTALL.f 
Looked at from a purely practical point of view, however, the pro- 
blem may be stated thus : — Pure water when sprayed on to insects has 
very little wetting power ; that is, instead of spreading evenly over the 
surface it tends to collect into relatively large drops. How, therefore, 
may one alter its nature in such a way that it shall spread evenly over 
the insect sprayed and really make contact ? 
There exist, fortunately, certain substances which, when dissolved in 
water, give to the solution the desired property. Chief among these 
and the longest known is soap. The potassium compound or soft 
soap is the most serviceable and the most commonly used. It is 
cheap and effective up to a certain point, but its value is limited by 
two factors. Firstly, it can only be used in mixtures which do not 
contain any soluble salts of metals or bases except those of sodium, 
potassium, and ammonium, since soluble metallic salts act on the 
potassium stearate of the soap with precipitation of metallic stearates, 
winch not only destroy the wetting properties of the soap, but in 
addition clog up the spraying nozzles. 
Secondly, the wetting power of even strong solutions is not great 
enough to wet such resistant surfaces as that of the waxy coat of the 
woolly apple aphis, or, amongst fungi, the felted mass formed by the 
conidial stage of American gooseberry mildew. It is possible, however, 
* " Insecticides," Journal of Applied Biology, vol. i. Nos. 3 and 4. 
f " The Theory of Wetting," Journal of Agricultural Science, vol. vii; part 2, 
