MISSOURI BOTANICAL GARDEN BULLETIN 39 



very important factor in preventing the rapid spread of this 

 pest. For this reason everything should be done to encourage 

 the growth of the parasites. Of course, if only a small 

 number of the bags is collected, the beneficial parasites which 

 are destroyed might have ultimately been more effective 

 than the elimination of comparatively few bagworm eggs. It 

 is consequently advisable to keep the hand-picked bags in 

 some sort of a receptacle instead of burning them. If a barrel 

 is used for this purpose, it should be covered with a wire net- 

 ting which w T ill confine the caterpillars as they hatch out, but 

 will permit the numerous parasites to escape and be ready to 

 assist in the control of the bagworms the following year. In 

 general, then, the collecting of the bags is effective only where 

 a comparative^ few trees arc infected, and is not likely to 

 be of much benefit when the pest is so widespread as it was 

 in St. Louis during the past season. A striking example of 

 the futility of this method is given by the late Professor C. 

 V. Riley. He stated that for two consecutive months he 

 worked attempting to keep a single cedar tree, not more than 

 six feet high, free from caterpillars. Almost every day he 

 found fresh specimens which had been overlooked at the 

 previous visit, and he estimated that this prodigious number 

 of caterpillars was the progeny of not more than two females. 

 Spraying. — Wherever appliances, capable of reaching all 

 parts of a tree, whatever its size, are available, there is 

 no question but that the use of some arsenieal spray offers 

 the only adequate means of combating the bagworm. If care- 

 fully carried out, spraying will result in the destruction of all 

 the bag-manufacturing caterpillars so that in the following 

 winter there will be no bags to collect. Paris green has been 

 used with good effect for this purpose, but arsenate of lead, 

 because of its greater adhesivem'ss which prevents its being 

 easily washed off by. rains, is the poison generally employed. 

 Arsenate of lead is usually sold in the form of either a pow- 

 der or paste, although it may likewise be obtained as a 

 cream. The powder, while suitable for dry spraying, is the 

 worst form to use for wet spraying. This is beeause the dry 

 pow r der, on remixing with the water, loses much of its fine- 

 ness, being relatively coarse and quick-settling. Consequently 

 the paste is the form most generally used at the present time. 

 The pastes vary widely in the fineness of their particles as 

 well as in the character of their chemical properties. It is 

 very desirable that a good grade of arsenate of lead be ob- 

 tained, for if arsenate of soda be present, it will invariably 

 scorch the leaves. When mixed with the right quantity of 

 water, it should require a considerable time to settle, since 



