46 BULLETIN 100, U. S. DEPARTMENT OF AGRICULTURE. 



SUMMARY. 



The life history of walnut aphides in California is briefly as follows: 

 A week or so before the buds open on the trees in the spring the 

 aphidids begin to hatch from the winter eggs. As soon as the young 

 foliage appears the "lice" settle on it, and after feeding for a month 

 or so become adults. These stem mothers are always winged and like 

 plant lice of later generations are capable of migrating to other trees 

 and orchards. As soon as they are fully developed they produce 

 young parthenogenetically. These second-generation young become 

 mature in three weeks and in turn produce young. The individuals 

 of the third and subsequent generations of summer mature in about 

 16 days. On early-leafing varieties there are 10 or 11 viviparous 

 generations in the year while on late varieties there are 8 or 9. The 

 production of the sexual generation is prolonged over four months, 

 these forms first appearing in July. After the sexes (comprised of 

 the winged male and the wingless female) mate, the female repairs to 

 the twigs and limbs of the tree, there to deposit her eggs. Winter is 

 passed in the egg stage only. 



In general the aphidids inhabit the underside of the leaves, but those 

 of the second, third, and fourth generations often attack the nuts, 

 sometimes seriously dwarfing them (see PI. I, fig. 1). Occasionally 

 the "lice" will be found on the upper surface of the leaf. When infes- 

 tation on the leaves and nuts is severe the vitality of the infested tree 

 is impaired. The aphidids excrete a sweet, gummy, transparent sub- 

 stance much sought after by ants, and in this thrives a black sooty 

 fungus. This black fungus often covers the upper sides of the lower 

 leaves and the upper part of the nuts, thereby interfering with the 

 respiratory action of the plant tissues. 



Walnut plant lice have many natural foes, all predatory. These 

 serve to keep the aphidids in check but do not appear in sufficient 

 numbers until after the "lice" have had time to injure the nuts. The 

 most persistent of them is the ashy-gray ladybird beetle (OUa abdom- 

 inalis Say) . 



Aphidids on walnuts can be controlled artificially with sprays. The 

 winter spraying directed against the eggs is the easier to apply, and high 

 trees can be reached by a winter wash with ease, whereas in the spring 

 and summer so thick is the foliage that a thorough application is hard 

 to accomplish satisfactorily. Furthermore, far less material is 

 required when the trees are bare. Lime-sulphur and crude-oil emul- 

 sions are effective, especially the first named. The spray should be 

 directed all over lirnbs and twigs so as to cover every part. If it 

 is necessary to spray in spring or summer, a combination of 2 per cent 

 distillate-oil emulsion and commercial tobacco extract No. 2 (1 to 

 1,500) will prove effective provided it be applied under a pressure of 

 at least 150 pounds and the spray directed on the nuts and underside 

 of the leaves. 



