The odd-looking leaf-twisting handiwork of Woolly Apple Aphids (Eriosoma lanigerum) is becoming evident on newly emerging leaves of American elms (Ulmus americana, ‘Princeton’) in southwest Ohio. ‘Princeton’ is an old cultivar first propagated by Princeton Nursery in New Jersey in 1922. The cultivar was later found to be resistant to the fungal pathogens behind Dutch Elm Disease (DED).
Of course, DED resistance doesn’t automatically convey resistance to other elm pests. This includes the apple aphids as well as another leaf aphid, the Woolly Elm Aphid (E. americanum). Thankfully, neither of these woolly leaf aphids causes appreciable harm to the overall health of their elm host.
The woolly apple and elm aphids are native insects that appear in pest records dating back to when American elms were "America's Street Tree." The significance of the reemergence of these historical woolly pests of American elms is that each has an alternative host. While they may not cause significant harm to healthy, established American elms, they can wreak havoc on their alternate hosts.
Twisted Sisters
The woolly apple aphid can alternate between American elms and members of the rose family (Rosaceae). Of course, as its common name implies, the aphid is a major pest of apples where they form damaging colonies on both the shoots and roots. Interestingly, various online university resources note that with the loss of American elms, this aphid demonstrated the ability to maintain damaging populations on rosaceous hosts.
Spring-feeding damage by woolly apple aphids causes shoots to form rosette-like structures comprised of curled, twisted, and stunted leaves. The affected leaves overlap producing cavities where the aphids live and feed. Although the leaf distortions appear to be random, opening the structure may reveal striking spiral or pinwheel patterns produced by the leaf veins.
The woolly aphids produce copious quantities of honeydew as a by-product of tapping into phloem vessels to extract sap. The honeydew rains down onto lower leaves and stems as well as anything else positioned beneath an infested tree. The sweet sticky goo is commonly colonized by black sooty molds further degrading the aesthetic value of heavily infested trees and their immediate surroundings.
The woolly apple aphid is not considered an outright elm tree killer. However, some historical reports note that heavily infested young trees that are planted on poor sites may become more susceptible to other problems by the combination of leaf damage coupled with early-season sap loss.
The new crop of aphids that develop in the American elm leaf rosettes migrate to a rosaceous host in the summer. However, as noted above, American elms are not required for the aphids to present a continued threat to apples. This has been observed in Belgian apple orchards where there are no American elms present in the region.
Woolly elm aphids cause new leaves to roll along one edge with the tissue becoming swollen to form a gall-like structure which is sometimes referred to as a "pseudo-gall." The woolly aphids reside and feed within the rolled structure.
Although the damage may appear unsightly, records indicate that the woolly elm aphids seldom cause enough damage to affect the overall health of the host tree. The aphid alternates between a spring leaf-feeding generation on American elm and a summer root-feeding generation on Amelanchier spp.
We’re Not Going to Take It
Biological and Chemical Management Options: Populations of these native aphids tend to be effectively regulated naturally by the “3 – Ps”: Predators, Parasitoids, and Pathogens. It’s common to find lacewing and syrphid (hover) fly larvae and lady beetle adults and larvae lurking within the aphid colonies, wolves among sheep. The native parasitoid wasp Aphelinus mali has a targeted taste for woolly apple aphids and has been exported to other countries to suppress this apple pest. In fact, by the time we notice the aphids, the 3-Ps are usually already on the job.
Using insecticides to manage these aphids is problematic because the aphids are located within distorted leaves which places them out of the reach of topical insecticides including insecticidal soaps or horticultural oils. I could find no efficacy data on systemic insecticides but suspect they may provide relief if applications are made with enough lead time for toxic levels to develop within the leaves. The translaminar activity of abamectin (e.g. Avid) may also allow this active ingredient to reach the aphids.
Management by Host Preference: The rise and fall, then rise again of American elm has been remarkable. Along the way, we've picked up a smorgasbord of genetic variations aimed at thwarting Dutch elm disease. These include selections of American elm; selections of non-native elms; and a mix and match of hybrids produced from parents drawn from North America, Europe, and Asia.
Dan Potter and Carl Redmond, working in the University of Kentucky, Department of Entomology, used elms planted in Lexington, KY, as part of the National Elm Trial to assess host preferences for an impressive range of potential elm pests including the woolly and cockscomb aphids. You can access their full 2013 publication titled, "Relative Resistance or Susceptibility of Landscape-suitable Elms (Ulmus spp.) to Multiple Insect Pests," by clicking this hotlink:
https://webdoc.agsci.colostate.edu/bspm/Kentucky2013.pdf
The Kentucky researchers found that the woolly elm aphid is selective for Ulmus americana, but all cultivars were not equal. 'Princeton' was the most susceptible followed by 'Valley Forge', 'Jefferson', and 'New Harmony'. 'Lewis and Clark Prairie Expedition' was not infested. None of the Asian elms or hybrids involving Asian or European heritage were affected.
They also observed that the woolly apple aphid was highly selective for American elm and none of the Asian elms or hybrids involving Asian or European heritage were affected. But again, all American elm cultivars were not equal. 'Princeton' was the most susceptible followed closely by 'Jefferson'. 'Valley Forge' and 'New Harmony' had minor infestations and 'Lewis and Clark Prairie Expedition' was not infested.