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, Rg. 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 (Olla 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 limbs 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. 
