I came across a 'Princeton' American elm (Ulmus americana) planted in a county park in southwest Ohio sporting three pests: Woolly Elm Aphid, (Eriosoma americanum); Elm Cockscomb Gall Aphid (Colopha ulmicola); and European Elm Flea Weevil (EEFW) (Orchestes alni). The woolly and cockscomb gall aphids are native insects that appear in pest records dating back to when American elms were "America's Street Tree." Of course, the flea weevil is a non-native that has shown that it has a taste for a wide range of native as well as non-native elms.
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 (DED) from selections of American elm to non-native elm species to a mix and match of hybrids produced from parents drawn from North America, Europe, and Asia. What impact has this had on elm pests, both new and old?
The National Elm Trial (NET) has provided some helpful answers. The trial was initiated in 2005 with 15 - 18 DED resistant cultivars planted in 17 locations in 16 states. The NET has supported fruitful research with results that can help us anticipate possible key pest problems on elms (see NET Website under "More Information" below).
Researchers at the University of Kentucky, Department of Entomology, used NET elms planted in Lexington, KY, to assess host preferences for an impressive range of potential elm pests (Condra et al. 2010; Potter and Redmond 2013). I will cite some of their results in this BYGL Alert!
The woolly elm aphid is a native pest that causes new leaves to roll along one edge with the tissue becoming swollen to form a gall-like structure within which the aphids feed. Although the damage may appear unsightly, records indicate it is seldom causes enough damage to affect the overall health of the host tree. The aphid alternates between the spring leaf-feeding generation on elm and a summer root-feeding generation on Amelanchier spp.
I had never seen this aphid before in Ohio; however, records clearly demonstrate it was a common pest on American elms. Indeed, the Kentucky researchers reported that this 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.
Woolly Apple Aphid (E. lanigerum) is another native pest that is prominently cited in historic records as a pest of American elms. Spring feeding by this aphid causes shoots to form rosette-like structures comprised of twisted and stunted leaves. As with the woolly elm aphid, the Kentucky researchers demonstrated this aphid is highly selective for American elm; 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.
Like its woolly elm cousin, this aphid can also alternate hosts. In the summer, the aphids migrate to a rosaceous host where they form damaging colonies on both the shoots and roots. Indeed, they are a major pest of apples. Interestingly, various online university resources note that with the loss of American elms, this aphid demonstrated the ability to maintain damaging populations on single rosaceous hosts calling into question the need to alternate with an elm host.
The elm cockscomb gall aphid likewise demonstrated a strong preference for American elms over non-natives as well as hybrids involving Asian or European elms. However again, all cultivars were not equal. Combining two years of data (2011, 2012) 'Jefferson' was most often selected with 'Princeton', 'Valley Forge', 'Lewis and Clark Prairie Expedition' having minor infestations. 'New Harmony' was not infested nor were any of the non-native elms or elm hybrids involving non-native parentage.
European elm flea weevils overwinter as adults and lay eggs into mid-rib veins and major lateral veins of the leaves in the spring. The resulting larvae feed as leaf miners producing "blotch" mines that become noticeable when they turn brown. The adults feed on the leaves at two different times of the year to produce characteristic "shot hole" leaf damage. The overwintered adults produce tiny holes in newly expanding leaves; these holes grow larger as the leaves expand. The new adults that emerge from the leaf mines feed by producing small skeletonized holes; the veins in the holes eventually disintegrate to produce tiny shot holes. New adults are feeding right now.
The weevil was a new Kentucky state record when it was discovered in the NET plantings in 2009. The Kentucky researches measure host preference based on the number of leaf mines. Their results were a bellwether for what we've observed in Ohio and elsewhere: the weevil has a taste for all elms, but there can be significant differences among species and cultivars.
You can access the two scientific papers I've cited by clicking on the hot links in "More Information" below. I've only touched on a very small percentage of the elm pests evaluated in the studies. However, I have found the two studies provide a rich source of information helpful in anticipating possible key pest problems on American elms.
Condra, J.M., C.M. Brady, and D.A. Potter. 2010. Resistance of landscape suitable elms to Japanese beetle, gall aphids, and leaf miners, with notes on life history of Orchestes alni and Agromyza aristata in Kentucky. Arboriculture & Urban Forestry 36:101–109.
Potter, D.A., and C.T. Redmond. 2013. Relative Resistance or Susceptibility of Landscape-suitable Elms (Ulmus spp.) to Multiple Insect Pests. Arboriculture & Urban Forestry 2013. 39(5): 236–243