May IS, 1915 



Wilt of Gipsy-Moth Caterpillars 



119 



The solid line represents the plot of the deaths from the wilt and the 

 broken line deaths due to another cause. The number of daily deaths 

 is inserted in the squares. When caterpillars died of the wilt disease 

 on a certain day and others succumbed to other causes on the same day, 

 a fraction is used, the numerator representing the number of deaths 

 from the wilt and the denominator the other deaths. 



The interpretation of the curve follows: The experiment was begun 

 on June 18; on June 27 one caterpillar died of the "other cause" at 79° 

 F. ; on July i, at 84°, another caterpillar died from the same cause; on 

 July 4, at 83°, one died of the wilt disease and two died of the "other 

 cause;" on July 5, at 85°, two died of the "other cause;" on July 6, 

 at 90°, four died of the wilt; on July 7, at 77°, one died of the wilt; on 

 July II, at 73°, one died of the "other cause;" on July 14, at 76°, one died 

 of the "other cause;" on July 27, the day the experi- 

 ment ended, at 80°, one died of the "other cause." 



Summing up the results of this experiment, six 

 deaths were caused by wilt, nine deaths were due 

 to the "other cause," one caterpillar escaped dur- 

 ing the course of the experiment, and four female 

 moths emerged. Just as soon as a caterpillar 

 died, it was examined for polyhedra, and no death 

 was recorded as having been caused by wilt unless 

 these bodies were in evidence. It is certain that 

 the deaths recorded by the broken line were not 

 due to wilt, because the animals were not flaccid 

 and did not disintegrate on handling; on the con- 

 trary, their skin was very elastic and tough, and 

 smears never revealed polyhedral bodies. Many 

 round crystals with radiating lines (PI. XIV, fig. 1 1 , 

 1 2) , however, were found. These, the writer believes, 

 are a sign of some metabolic disturbance, and the 

 deaths may have been caused by excessive heat coupled with dryness. 

 Laboratory conditions are poor at best, and the food plants in the trays 

 dried out rapidly, although replaced very often by fresh foliage. Thus, it 

 seems that these deaths from another cause are simply a laboratory occur- 

 rence, for nothing similar in the field has ever been discovered. The first 

 series of experiments was performed in an excessively hot room below 

 the roof, and, while not a single control insect died of wilt, nearly all the 

 caterpillars succumbed to this general physiological disturbance. 



Attempts were made to isolate bacteria from caterpillars which died 

 of wilt in the infection experiments, but in nearly all cases the tubes so 

 inoculated remained sterile. This procedure always gave a good check 

 against the platings made from the infectious material before it was used. 



Figure 3 shows the mortahty of the 20 controls which accompanied the 

 20 caterpillars fed with the Berkefeld filtrate. Of these, 19 died on 



Fig. 3. — Curve showing the 

 mortality among the 20 

 control gipsy-moth cater- 

 pillars. Compare with 

 figures. 



