12. 



eg^s may not be killed, rendering- it necessary to make an additional 

 treatment after a sufficient period has elapsed to allow all the eggs to 

 hatch and the young to escape. 



The black scale is especially adapted to control by gassing on account 

 of its being, in the main, single-brooded. Applied late in October 

 or early in November after all the young scales have hatched, badly 

 infested orchards have been completely cleaned by a single treatment. 

 Gassing in midsummer for this insect will be inefi'ective, because a 

 large percentage of the old females at this period cover and protect 

 unhatched eggs. 



Gassing consists in inclosing a tree at night with a tent and filling 

 the latter with the poisonous fumes generated b}' treating refined 

 potassium cyanide (98 per cent strength) with commercial sulphuric 



Fig. 6. — Tents, tackle, and cheminals loaded for transportation. 



acid {Q6 per cent) and water. The treatment should continue from 

 thirty to forty minutes, the longer time being preferable. The work 

 is done at night to avoid the scalding which follows day applications, 

 at least in bright sunlight. 



The proportions of the chemicals as now employed in California are 

 considerably in excess of the amounts recommended a few years since. 

 The gas treatment was first chiefly used against the black scale, and at 

 a season of the year when these scales were all in a 3"0ung stage and 

 easily killed. The efl^'ort is now made not only to kill the black scale, 

 but also the red scale, and to do more effective work with both than 

 formerly. The proportion of chemicals commonly employed in Los 

 Angeles, Orange, and some other counties in southern California arc 

 indicated in the following table, published by the horticultural com- 

 missioners of Riverside County, Cal. : 



172 



