After several years of relative obscurity, woolly apple aphids are becoming more numerous in Illinois. This aphid has an interesting life cycle in that it alternates between rose family and elm hosts. It is normally noticed on its rose family hosts of apple, hawthorn, mountain ash, and pyracantha, where they form white, woolly clusters in twig crotches during the summer. Individuals will also be feeding on the roots of the same host and can commonly be seen migrating on the trunk in late summer to and from the twigs and roots. Continued feeding can cause woody galls to form on the twigs and roots. In the fall, winged aphids fly to elm to lay overwintering eggs, although nymphs also overwinter on the roots of the rose family host. In the spring, the eggs hatch on elm, with the resulting aphids causing twisted, deformed clusters of leaves at the branch tips on elm. In late spring, winged aphids fly from elm to a rose family host.
With the population being spread between the twigs and roots of the rose family host, complete insecticidal control is rarely achieved, particularly when new infestations arrive from elm in late spring. A variety of contact insecticides are effective against those aboveground on the rose family host, but it is difficult to achieve control on those living on the roots. The elm hosts may be hard to find or on neighboring properties where there is no access. Thankfully, in Illinois it is rare to have damaging numbers of this insect, so control is rarely needed. Normally, the white, cottony masses in the twig crotches of the rose family host are the ones that are noticed, but they are usually not numerous enough to cause serious damage.