392 THE MONTHLY BULLETIN. 



o£ the outer tissues inviting decomposition and is a condition often encountered on 

 heavy soils. 



As concerns the woolly aphis the critical period in a pear tree's existence is that 

 preceding the establishment o£ its root system in the orchard, and of this period the 

 most critical time is the summer and fall following the planting of the tree in the 

 orchard. On heavy soils there is still danger for three more years, yet every suc- 

 ceeding year finds the roots of the tree more firmly established and the danger from 

 aphis diminishes with time. 



Seedlings disinfected before planting in spring are liable to bear heavy infestations 

 at any place on their roots the succeeding fall, due either to having been planted in 

 the nursery close to infested trees or to having received spring migrants from cork 

 elms, a phenomenon which is discussed below. The spread of the aphis in nurs- 

 eries is frequently rapid. The second year in the nursery may find abundant aphis 

 on the trees now grafted. Nursery trees supplied with plenty of water and cultiva- 

 tion are rarely badly injured. When the trees are planted out in the orchard in the 

 third or fourth year of their life they receive as a rule considerably less moisture 

 than they were given the years previous in the nursery and are hard pressed to 

 withstand a heavy infestation of aphis, especially on hard soils. This is the critical 

 time in the existence of the tree and emphasizes the need for careful disinfection of 

 stock before planting. Even if the trees are planted clean there is danger of 

 infestation in June and July through the agency of the spring migrant form if 

 cork elms occur in the neighborhood. The extent of this danger can not be meas- 

 ured accurately, but it is not great enough to justify foregoing the disinfection 

 of infested nursery stock even if this is destined to be planted in close proximity to 

 cork elms regularly infested with woolly aphis. 



The French stock which has been used heretofore in the great majority of Cali- 

 fornia pear orchards is very susceptible to aphis injury. The Kieffer, Japanese 

 pear and yuince stock are much more resistant, although the insect will thrive upon 

 the two first as successfully as upon the French root. The relative immunity 

 enjoyed by the Japanese stock has of late years led to its adoption in place of the 

 French stock by many nurserymen. Wisker (4) reports on the resistance qualities 

 of this root as found in the Loma Rica Nursery at Grass Valley, California. 



Biology. 



The life cycle of the pear woolly aphis is complicated, by reason of the fact that 

 it has an alternate hose, the cork elm (Ulnms suberosa, Doud). The aphis remains 

 on the pear all the year in succeeding generations of wingless forms, but large num- 

 bers of those individuals developing between July and November mature as winged 

 insects, forsake the pear roots, issue above ground and fly to elms, to deposit on 

 the trunks about eight young, male and female. These take no food, molt four skins 

 in about ten days and then the sexes mate, after which the female deposits in a 

 crack of the bark or under a bud scale a single egg. The mature sexual insects are 

 very small and bare. The male is dark green with a lilac mottling, the female orange 

 or crimson. The egg is reddish. If healthy this egg hatches the following spring, 

 generally in April, and the resultant aphis proceeds to settle on a young elm leaf. 

 Its continued puncture in the tissue causes an abnormal growth of the leaf in the 

 form of a thick-walled spiral gall, at first tightly closed. This gall grows larger 

 for about two months, latterly becoming large and bag-shaped, wrinkled and ribbed 

 on the outside and often includes the whole leaf. Mature galls vary in size and 

 may measure as much as 4* inches maximum diameter. When daily exposed to 

 sunshine they assume bright yellow and rosy hues. Galls that are hidden from 

 sunshine remain green. In the fall all the galls turn brown before the rest of the 

 leaves and most of them remain hanging on the trees throughout winter. This 

 phenomenon is perhaps due to the fact that the peduncle of a gall-bearing leaf is 

 greatly thickened and coupled with its abnormal weight presumably enables the 

 leaf to withstand winds. 



The aphis born from the egg lives its whole life in the gall and is the parent 

 of a large progeny, most of which develop wings and forsake the galls in June 

 and July. These are the spring migrants and they fly to pear trees, where they 

 locate sometimes on the foliage, but more usually about the crown. They deposit 

 about twenty-two young on a trunk and these strive to make their way down to 

 the roots. The progeny of the spring migrant do not differ in structure from those 

 of the wingless root form. They remain wingless and have the same habits as all 

 the wingless root aphids. Hundreds of migrants are produced in the larger galls, 

 but the developing nymphs have many natural enemies. Although the galls are 



