INSECTS AND DISEASES 103 



under three groups : burrowing insects, biting insects, 

 and sucking insects. No general rule can be given for 

 the treatment of burrowing insects. Each one must 

 be carefully studied by itself to see at what stage of 

 its life history it can be most successfully attacked. 

 Often the eggs are laid on the surface of their food plant, 

 and the young feed for a short time superficially 

 before beginning to burrow. In such cases they may 

 be killed by arsenical sprays like other biting insects. 

 In other cases it is possible to prevent the depositing 

 of eggs by various protective measures. The one 

 almost universal remedy for biting insects is to poison 

 them by spraying their food plants with some arseni- 

 cal compound. Sucking insects have mouth parts 

 arranged for piercing the epidermis of the food plant 

 and sucking out the elaborated sap. They are much 

 more difficult to reach than most biting insects since 

 they cannot be killed by arsenical poisons. It is nec- 

 essary to employ what are known as contact poisons ; 

 that is, some substance that will kill the insects by 

 simply coming in contact with it. The principal 

 contact poisons are kerosene, whale-oil soap, resin mix- 

 ture, tobacco extract, and pyrethrum powder. The 

 first three are much the strongest and most effective, 

 but they have to be used with great care or they will 

 injure the plants. Tobacco extract is only used for 

 very soft-bodied insects like plant lice. Pyre thrum 

 powder is safe and will not injure the most delicate 

 plant. When fresh it is quite effective, but it soon 

 loses its strength, and the commercial article is often 

 unsatisfactory. In greenhouses fumigation with 

 tobacco smoke is more effective than spraying with 



