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79 
(6) Artificial Methods of Control. 
These are for the most part concerned with the application of 
chemical agents known as insecticides—agents which are used with the 
object of directly killing noxious insects. They may be divided into 
three classes: 
1. Stomach poisons. 
2. Contact poisons. 
3. Deterrants. 
1. Stomach poisons are applied to the leaves of plants with the 
intention of directly poisoning the insects feeding thereon. Of such 
substances Bordeaux mixture and various compounds of arsenic are in 
general use. The best known arsenical preparations are copper aceto- 
arsenite (“ Paris Green ”), lead arsenate, copper arsenite (“ Scheele’s 
Green ”), arsenite of lime and arsenite of soda. These substances are 
highly destructive to insect life, and have the advantage of being 
efficient when used in extremely small proportions. According to 
Maxwell-Lefroy and Finlow (1913, p. 270) a plant sprayed with lead 
arsenate, for instance, using 1 part by weight to 1000 of water is 
poisonous to many insects; at a strength of 1 in 500 (1 lb. to 50 gallons 
of water) it renders plants poisonous to all leaf-eating insects. Copper 
aceto-arsenite is still more virulent, and can be used in even smaller 
proportions. Such compounds have the objection of being poisonous 
to cattle should a certain standard strength be exceeded. Theobald 
(1909, p. 515) states that fruit trees should not be sprayed with them 
when in flower as bees are readily poisoned, and they should be applied 
at least one month before the fruit is gathered for consumption. Among 
other stomach poisons, lime-sulphur, both alone and combined with 
reduced strengths of arsenate of lead, has proved successful in certain 
instances in America (Scott and Siegler, 1913). Maxwell-Lefroy and 
Finlow {loc. cit.) have conducted experiments with lead cln-omate in 
India, and they claim it to be a better stomach poison than lead arsenate 
for that country. It has the advantage of being devoid of arsenical 
compounds, does not affect the plants, is not easily washed off from the 
leaves, and its yellow colour renders sprayed plants very easy of re¬ 
cognition. It remains, however, to be ascertained how far it will prove 
useful in this country. 
An ideal insecticide, acting as a stomach poison, should be insoluble 
to the action of rain, dew, or other atmospheric conditions, cheap and 
effective at weak strengths, and non-iujurious to the plants. 
