THRIPS INJURING THE FOLIAGE 203 
is recommended. The potato vines themselves, also, should be 
burned. 
The Hop-aphis (Phorodon humuli Schr.) 
Hops are subject to infestation by a green, soft-bodied louse, one eight- 
eenth to one twelfth of aninchinlength. The male plants in hopyards 
always are infested first, and from these the lice spread two or three 
weeks later to the female or fruiting plants. Their attack causes the 
leaves to turn yellow, and seriously reduces the yield. 
Recent studies show that the winter may be passed in an egg stage 
somewhere near or on the hop-vines, although it is known that the 
same species winters also on plum trees. There are many generations 
in the course of a summer. 
Thorough spraying with a contact insecticide, such as tobacco ex- 
tract or 5 per cent kerosene emulsion, will kill the aphids. Remedial 
treatments should be begun while the lice are still isolated on the male 
plants, before spread has started. 
The Greenhouse Thrips ( Heliothrips hemorrhoidalis Bouché) 
The foliage of plants in greenhouses sometimes is injured by a very 
small, sucking insect properly known as a thrips. Evidence of the 
work is seen first in numerous whitish spots, where the juices of 
the leaf have been sucked out. These spots show in the beginning 
more plainly on the lower surface. As attack goes on, the spots 
spread, forming blotches, dead areas appear around the edges of 
the leaf, the foliage wilts, and finally drops off. The surface of the 
leaf is covered with small drops of reddish fluid, which frequently 
turns black. 
The adult insect is one fifteenth of an inch in length, dark bodied, and 
though it has wings is not much disposed to fly. The young are lighter 
colored, and have no wings. They suck the leaf juices just as do the 
adults. Three or four weeks are required for a generation. 
Usually this pest is controlled successfully by fumigation with 
tobacco. 
