EFFECTS OF SPRAYING. 139 



caterpillars survived. Yet a difference in the effect of the 

 spraying on the caterpillars was noticed. Occasionally in 

 badly infested places so many of the caterpillars died within 

 two or three days after the spraying that the people living 

 on the infested estates were obliged for the sake of cleanliness 

 to sweep up the dead from the walks and burn them. The 

 unevenness in the effect of the spraying at once aroused 

 suspicion that it might be caused by improper mixing or 

 application of the insecticide. Although some inefficiency 

 on the part of inexperienced sprayers was expected, a great 

 difference in the effect of the insecticide was observed even 

 when the poison was most thoroughly and carefully mixed 

 and uniformly applied. 



Every effort was made during the spraying season to de- 

 termine why the results of spraying were not uniform and 

 satisfactory. The feeding caterpillars were watched day and 

 night by many observers. The spraying was most carefully 

 superintended and the conclusion finally arrived at was that 

 under ordinary conditions spraying with Paris green for the 

 gypsy moth was ineffective and unsatisfactory.* 



* In a few cases, where spraying was done just after a storm, an unusual mortality 

 among the caterpillars was noticed during the next few days. At first it was thought 

 that this result was due to the poison being more effective at such times, on account 

 of its better retention on the foliage. It was known, also, that the caterpillars sought 

 shelter and ate but little during rains, but that they fed ravenously during the warm, 

 clear weather that frequently follows a storm. It was believed that the unusual 

 amount of poisoned food taken at such a time, when the system was weakened by 

 fasting, might have been sufficient to cause a greater mortality than usual. It was 

 soon noted that the poison appeared to produce the same effect occasionally in dry 

 weather or after light rains. This seemed to disprove the first hypothesis advanced, 

 and raised the question whether the larvae, which sometimes succumbed in such 

 numbers, were not first weakened by disease or other causes. 



In some cases the accuracy of the observations was doubted. It is difficult to make 

 accurate observations on the results of such work done in the field. For instance, 

 it is almost impossible to determine the proportion of larvae killed by spraying. 

 When they disappear immediately after spraying, there is no certainty as to whether 

 the spraying alone is responsible for their disappearance, or whether they have not 

 been carried off by birds or predaceous insects, or have not crawled to other localities 

 or to more congenial food. In some places, where the dead larvae fell to the ground 

 in considerable numbers, most of them were immediately carried away by ants or 

 other scavengers, which left few behind to tell of the results of the spraying. Cor- 

 rect conclusions cannot be drawn where there is such liability of error, unless obser- 

 vations recorded by many careful individuals and extending over a considerable 

 period of time agree in the main. As no extended spraying with Paris green was 

 done after 1891, the opportunity passed for an exhaustive study of the effect of 

 meteorological or other conditions on the results of spraying. The cause of the 

 apparent differences in the results of carefully conducted spraying with Paris green 

 for the gypsy moth is a question which deserves further investigation. 



