THE WAR ON INSECTS 279 



effectiveness than the extracts just mentioned because 

 of their greater penetrating power. All soaps have 

 some insecticide value, and so washing the leaves of 

 house plants with soapsuds is always good practice 

 provided the suds be not too strong. An ounce of soap 

 to one quart of water is ordinarily quite sufficient to 

 kill green lice and young scales, especially if whale oil 

 soap is used. At twice that strength it kills even the 

 more resistant forms of plant lice, but becomes danger- 

 ous on tender plant foliage. While soapsuds may be 

 freely used on house plants, the suds should not be 

 allowed to accumulate on the surface of the soil for the 

 alkali may easily become detrimental to the plant roots. 

 For greenhouse and garden use, whale oil soap is much 

 improved by an addition of tobacco and such a combi- 

 nation is now obtainable from most seedsmen. 



In the field, whale oil soap is much used against 

 plant lice, and is effective at varying strengths de- 

 pending on the insects to be reached. Against most 

 green lice i pound to 4 or even 6 gallons of water may 

 be satisfactory; against brown or black Aphids twice 

 that strength may be required, and against young scales 

 i pound in 2 gallons is the weakest mixture that can be 

 effectively used. For winter work against armored 

 scales like the San Jose', 2 pounds in i gallon of water 

 are applied. 



In all dealings with plant lice and similar insects 

 that multiply rapidly, prompt action is essential for 

 best results. It is sometimes easy to destroy a slight 

 infestation and to get rid of a few specimens on leaves or 

 stems. If they are allowed to multiply unchecked 

 until the plant begins to show signs of suffering, it will 

 be necessary to make much more thorough treatments, 

 and as many species tend to curl or otherwise to dis- 

 tort the leaves, hiding in the shelter thus caused, it 



