68 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 47 



hairs, is the main leaf eater of walnut trees and, for some un- 

 known reason, is attracted to isolated trees. It is very fond not 

 only of walnut but of butternut, hickory, oak, honey locust, and 

 willow; sometimes it feeds on apple and other fruit trees. Each 

 caterpillar develops through a series of molts from one of many 

 eggs laid in masses on leaves of the host tree. An egg mass pro- 

 duces a colony of caterpillars that may destroy the foliage on a 

 whole limb before moving to fresh leaves. The colonies migrate 

 downward on a limb or tree trunk, molt, and leave a clump of 

 skins the size of a man's fist. When grown and about 2 inches 

 long, the caterpillars leave the infested tree and dig into the soil. 

 There they pass the winter in the pupal stage. The adults, 

 brownish moths, emerge in early summer. The female flies into 

 one of the preferred kinds of trees, where she deposits eggs. 



Control Measure 1 or 2 (end of circular) just as soon as 

 feeding can be detected. Bands of sticky material around tree 

 trunks are of no benefit. 



Black Walnut Curculio, Conotrachelus retentus (Say) . — This 

 reddish-brown weevil feeds on tender shoots and foliage of wal- 

 nut for a while in the spring. The adult female makes, in the 

 young walnuts, crescent-shaped cuts in which she deposits eggs. 

 Feeding of the larvae that hatch causes the nuts to drop when 

 only about half grown. 



Control Measure 1 (end of circular) applied to new foliage. 

 Fortunately this walnut pest is attacked by several parasites, 

 which help keep it under control. 



WILLOW 



Willow Aphid, Clavigerus smithiae (Monell), fig. 48. — This 

 rather large, dusky aphid may infest twigs and leaves of willows 

 during the summer. Unlike many kinds of aphids, this one is 

 restless, easily disturbed, and rapid in its movements. Both 

 winged and wingless forms may be present on an infested tree. 

 By withdrawing juices from leaves, this aphid may cause the 

 leaves to wilt and drop. 



Control Measure 3 (end of circular). 



Bagworm, fig. 2. — See under Arborvitae. Control Measure 

 1, 3, 11, or 21 (end of circular). 



Borers. — See under Poplar. Control Measure U (end of cir- 

 cular). 



