74 NINETEENTH REPORT STATE ENTOMOLOGIST OF MINNESOTA—I9Q22 
approaches that of water. Each time the bug is frozen a certain 
amount of bound water is liberated, and a small amount of free water 
will prevent much undercooling, for the presence of water causes early 
inoculation and freezing. Then it would appear that the amount of 
undercooling possible in an insect will depend (1) on the amount of 
free water present and (2) on the density of the body fluids, or the 
colloidal content. 
Economic Importance of Perillus 
From time to time enthusiastic reports have come from potato 
growers in Michigan, Ontario, and New York, stating that Perillus 
was becoming so abundant that spraying for the potato beetle seemed 
unnecessary. In Genesee County, New York, the writer had oppor- 
tunity to observe the result in a potato patch where the owner was 
depending on Perillus to control the potato beetle. The first brood 
of potato beetles had undoubtedly done considerable damage to the 
potato vines before Perillus had developed to the point of abundance. 
By the time the second brood of potato beetles had made their appear- 
ance the predatory bugs were quite abundant and apparently destroyed 
the eggs and young larvae of the beetle nearly as fast as they de- 
veloped. However, a considerable number of potato beetle larvae were 
able to reach maturity, and meanwhile these had caused some damage 
to the vines. There was no doubt about Perillus doing good work 
in checking the damage caused by potato beetles, yet the bugs were 
not sufficiently numerous as to cause a scarcity of the beetles. Had 
Perillus been as abundant at the beginning of the season as was the 
case by September 1, very likely this predaceous bug could have been 
relied upon to control the potato beetle. 
Unfortunately there are certain factors in the life history of 
Perillus which work against the increase of the species from one year 
to the next. As brought out under the subject of hibernation, it was 
estimated that not more than 5 per cent of the bugs present in the 
field in September, hibernate successfully. Thus we find each spring 
that Perillus is decimated in numbers, and not until the second gen- 
eration has developed do they appear in numbers which would suggest 
control of the potato beetle. Some years they appear more abundant 
than others, and such seasons apparently follow a mild winter, as that 
of 1920-21 in Minnesota. 
Another factor which enters to prevent the increase of Perillus is 
the fact that the bugs become poisoned by feeding on larvae and adults 
of the potato beetle which have previously fed on sprayed potato 
vines. Very early in the breeding work the writer learned that one 
