13 



Flacherie, while all caterpillars which were kept apart for con- 

 trol remained healthy. From these results I was therefore 

 led to believe that the artificially produced Flacherie might be 

 utilized as a valuable aid in the destruction of gypsy moth 

 caterpillars. If the disease appears in nature during normal 

 weather conditions, the caterpillars are in most cases almost 

 full grown when attacked; but the farther the weather condi- 

 tions of the year differ from the normal, the earlier in their 

 life will the disease affect them. The fact that I succeeded in 

 rendering the caterpillars susceptible to the wilt before the 

 third molt suggested that this may be of importance in the 

 practical use of the disease, since by artificially inducing 

 Flacherie, relief might be had weeks sooner than happens in 

 nature. 



Better to understand the conclusions drawn from the last 

 experiment, which was performed on trees on which a consid- 

 erable number of gypsy moth caterpillars occurred naturally, 

 the following account is given: A group of oak and another of 

 willow trees were infested with about 5,000 caterpillars each. 

 Shortly before their fourth molt, upon each of these two groups 

 of trees 100 sick and 50 dead caterpillars were destributed. 

 The disease spread with amazing rapidity, and even on the 

 following day many freshly killed caterpillars could be counted. 

 By the time of pupation about 4,000 caterpillars on each group 

 of trees had succumbed to the disease. Two conditions which 

 did not enter into my previous experiments united to cause this 

 unusual result. In the first place, the two groups of trees had 

 been badly infested and injured by gypsy moth caterpillars 

 the previous season, as was evidenced by the unhealthiness of 

 the leaves during the summer of 1909, and the caterpillars had 

 probably become predisposed to the disease on account of the 

 resulting decrease in the value of their food. As a second 

 very important factor may be mentioned the dry weather, 

 which by its desiccating effect upon the leaves served to render 

 the food for the caterpillars still less nutritious. Hence, from 

 the beginning there was a decreased vitality in the tissues, and 

 digestion was early disturbed. In short, the caterpillars were 

 already very susceptible to the disease at the time I introduced it 



