October, '12] PATCH: WOOLLY APHID MIGRATION 397 



nymphs of woolly aphid were easily found on both these summer 

 hosts. That these were the progeny of the elm leaf migrants there 

 was evidence enough. One native mountain ash, Pyrus americana, 

 will serve for an illustration. The main trunk of this tree was dead 

 nearly to the ground, but twelve vigorous shoots had grown up meas- 

 uring about five feet each. On June 28 this mountain ash had about 

 150 woolly masses of nymphs grouped on the stem at the leaf axils. 

 These nymphs ranged from very tiny ones to half grown insects, none 

 being mature at that date. One such woolly mass contained 155- 

 individuals of various sizes. (See plate 10, figure 2.) On 

 the ventral surfaces of the leaves of this mountain ash were stationed 

 many elm leaf migrants producing there their broods of nymphs which 

 could be seen, with the hand lens, to be augmenting the woolly masses 

 on the stem. Collections of these migrants thus stationed were made 

 as follows :— July 2, 88 migrants; July 3, 211 migrants; July 5, 92 

 migrants; July 8, 54 migrants; July 9, 80 migrants; July 10, 33 mi- 

 grants; July 11, 14 migrants; July 12, 3 migrants. Only living indi- 

 viduals were collected, dead ones being brushed off and discarded in 

 the counts. Microscopic examination showed them to be identical 

 with winged forms collected in elm leaf curls. Tw^o large elm trees 

 with leaves well stocked with this species stood about a rod distant. 



Correlated with the appearance of the stem colonies of woolly 

 aphides on mountain ash {Pyrus sp.) and water shoots of apple Avhich 

 were definitely ascertained to be the progeny of elm leaf migrants, 

 were woolly bark feeding colonies on the trunks and branches of the 

 elm, Ulmus americana. These were mostly about pruning wounds 

 or protected under the bark, the latter colonies often not visible except 

 on scaling back the bark. That these bark colonies on the elm are 

 also the progeny of elm leaf migrants I have no doubt; but as the 

 chief point of interest this year was centered in watching the elm 

 leaf-apple situation no real attempt was made to get at the elm leaf- 

 elm bark situation by catching the bark colonies in the process of mak- 

 ing. This should not be especially difficult to do and is listed among 

 the plans for 1913. In this connection it should be remembered that 

 Riley's classic work on this species gives a continuous cycle for the 

 elm in which he states of the fourth generation (progeny of the TA-inged 

 generation developing in the leaves): "They are, however, able to 

 sustain themselves on the tender bark of twigs alone, and may be 

 found nearly fully-grown, there exposed to view and enveloped in the 

 white cottony matter, which brushes off at the slightest touch." 



What influences the destination of the elm-leaf migrants and what 

 determines their choice of the summer food plant for their progeny is 

 not known. That they readily accept elm bark under some conditions 



