HATCHIXG OF GIPSY-MOTH EGGS. 7 



Collections made at the nonhatch points up to February 1, 1917, 

 the only month having severe cold that winter, hatched completely, 

 while those obtained March 1 and afterwards were all nonhatch. 

 Those laboratory-bred clusters exposed during the month of Febru- 

 ary were all killed, but those exposed every other month hatched 

 completely. The next winter severe cold came in December, Janu- 

 ary, and February. Collections made up to December 1 gave com- 

 plete hatch, while neither those collected after that date nor the 

 month's exposure clusters for the three cold months hatched. The 

 next winter, 1918-19, was mild, and all clusters for all months 

 hatched completely. The winter of 1919-20 was another very cold 

 one, with the extreme cold coming in January and February, result- 

 ing in hatch records identical with those of the other cold periods 

 for both monthly collections and month's exposure. 



During the entire four winters only two drops greater than 

 — 10° F. were recorded from the point which had never shown non- 

 hatch. The first, in December, 1917, — 15°, resulted in about .30 per 

 cent of the eggs in the monthly collections and month's exposure 

 clusters being killed. The second drop, — 16°, came in February, 

 1920, with the same effect on the eggs. 



The method employed in determining the extent of injury to the 

 eggs in the various experiments may need explanation. In the 

 first place, care was taken to provide plenty? of control clusters for 

 all the experiments and these were treated exactly like those for 

 which they were controls, except that they were not exposed to con- 

 ditions at the, various points. Clusters which did not hatch at all 

 could be listed as complete nonhatch. However, in figuring per- 

 centage of hatch in partially hatched clusters, the number of infer- 

 tile eggs was deducted first, as they could not possibly have hatched. 



A careful comparison of the temperature and nonhatch records 

 has led to the conclusion that —20° F. is about the highest point 

 at which all the eggs in a cluster will be killed and a further drop 

 will make it more certain that all exposed eggs will be killed. There 

 is reason to believe that the resistance of the eggs depends to some 

 extent upon their vitality, as would be only natural. Evidence on 

 this point was secured from the sets of clusters which were only 

 exposed to a single severe drop in temperature. One set exposed 

 to —22° F. was entirelv killed, but a very few eggs in a set exposed 

 to —23° F. hatched. 



There are no records of any eggs hatching after being exposed 

 to a temperature of —25° F. or lower, whether they were subjected 

 to one such exposure in the " single .drop experiments '' or several, 

 as sometimes happened with the " month's exposure " sets. On the 

 other hand, exposures to —15° and —16° F. killed half the eggs in 

 the clusters, but no temperature above —20° F. killed an entire 

 cluster. Various records of exposure to temperatures higher than 

 — 15° F. showed no injury at all. 



The conclusion, therefore, is that the vital point for complete 

 killing lies between —20° and —25° F., with an absolute certainty 

 that all eggs exposed to any further drop will be killed. 



The artificial protection experiments may properly be considered 

 after the monthly collections and month's exposure, for the data 

 secured from the first in a wav serve to substantiate those ob- 



