INSECT ENEMIES OF WESTERN FORESTS 45 



B. Trees appearing in fairly good health but leaves or stems badly 

 stunted, galled, or swollen; sometimes with queer protuber- 

 ances gall makers, page 52. 



1. Cone-shaped galls on terminal twigs of spruce 



spruce gall bark lice, page 47. 



2. Swollen twigs of white pine, covered with white incrusta- 



tions woolly pine louse, page 48. 



3. Galls at base of pine needles causing premature shed- 



ding inne needle mite, page 52, 



APHIDS OR PLANT LICE 



Aphids are small, delicate, soft-bodied insects with pear-shaped or 

 globular bodies and long legs. They range from almost colorless 

 translucent to greenish or almost black. As a rule they are without 

 protective covering and often occur in dense colonies on leaves or 

 tender terminals of trees, where they feed by sucking the juices. 



The aphids exude quantities of honeydew, which drips over the 

 leaves and onto the ground beneath. This is a favorite food of 

 ants, who cultivate and tend the aphids for it, and for this reason 

 the aphids are often referred to as "ant cows." The honeydew also 

 becomes a fertile medium for the growth of a black smut that covers 

 the leaves, causing the trees to appear as if they had been sprayed 

 with crude oil. Shade and ornamental trees are rendered particularly 

 unsightly, besides being weakened by the aphid feeding; and forest 

 trees are sometimes so weakened that after a season or two they 

 die from the injury. 



Aphids are remarkable because of their peculiar manner of de- 

 velopment and the difference in the mode of reproduction of sepa- 

 rate generations of the same species. They reproduce both sexually 

 and also without mating, and both winged and wingless forms 

 occur. The number of generations of aphids may vary from one or 

 two to several in a single season, with more or less overlapping. 



On shade and ornamental trees the aphids can be controlled by 

 spraying the insects, when they are first observed, with a mixture 

 of 4 or 5 pounds of fish-oil soap in 20 gallons of water, or with one- 

 half pint of nicotine sulphate in 50 gallons of water in which 2 

 pounds of soap has been dissolved. Crude-oil emulsion and com- 

 mercial lime-sulphur solutions are used as dormant sprays to kill 

 the eggs. They are applied in the spring, about the time the buds 

 begin to swell. 



The spruce aphid {Aphis abietina Walk.) is by far the most 

 destructive member of this group of sap-sucking insects that attack 

 forest trees in the West. In recent years it has killed millions of 

 feet of Sitka spruce along the tidelands of the Oregon and Wash- 

 ington coast (fig. 20) and the Columbia Kiver, as well as having 

 caused considerable damage to this conifer on the better inland sites. 

 The wingless aphids occur early in the summer on the needles and 

 tender growth of Sitka spruce. These insects are dull green and 

 range in size from very minute young insects to full-grown winged 

 aphids about three-sixteenths of an inch in length. Apparently this 

 insect has an alternate host, disappearing from the Sitka spruce in 

 midsummer, only to reappear again the next spring. No practical 

 remedy has been suggested under forest conditions, but on shade 

 and ornamental trees the pest can be controlled by spraying with 



