Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Woolly elm aphidOn this page: Identification & Distribution Biology & Ecology Life cycle Natural enemies Other aphids on the same host Damage & Control
Identification & Distribution
The developing colonies of Eriosoma americanum on American elm (Ulmus americana) feed on the edges of young elm leaves. This causes them to roll downwards and form a pseudogall around the colony (see first picture below). The developing alatae in these rolled leaf galls are orange-pink and densely covered with grey wax (see second picture below).
Both images above copyright Claude Pilon, all rights reserved.
Emigrant alatae (=spring migrants) emerging from galls in June are dark green to blue-black with some wax wool, and rather dark wing veins. The terminal process of antennal segment VI is less than 0.5 times the length of the base of that segment. Antennal segment III is more than 0.8 times the head-width as measured across the eyes (cf. Eriosoma mimicum, which has segment III less than 0.8 times the head-width). Antennal segment III is a little shorter than antennal segments (IV+V+VI) (cf. Eriosoma crataegi, Eriosoma lanigerum & Eriosoma pyricola, which all have segment III as long as or longer than segments IV+V+VI). Antennal segment V has secondary rhinaria (cf. Eriosoma grossulariae and Eriosoma ulmi, which have no rhinaria on segment V). These alatae have rather conspicuous raised siphuncular pores, which have partially chitinized rims and are ringed by hairs. The body length of alatae is 2.0-2.5 mm. The alatae migrate to form colonies of pale pink, woolly apterae on the roots of their secondary hosts, especially Amelanchier laevis (see second picture below).
Eriosoma americanum host alternates between American elm (Ulmus americana) and the roots of saskatoon (also known as serviceberry, Amelanchier species). In August to October, the return migrants (sexuparae) to elm produce dwarf sexuales which lay overwintering eggs on bark. Woolly elm aphid occurs throughout the natural range of Ulmus americana in eastern North America, and in the west on planted elms.
Biology & Ecology
The life cycle of Eriosoma americanum is described by Patch (1915) and Palmer (1952). The woolly elm aphid overwinters in as eggs in cracks and crevices on the bark of American elms (Ulmus americana). In early May, just as the leaves on elm begin to unfold, these aphids emerge from their eggs and begin feeding on the underside of the developing leaves. The feeding action of the aphid causes the elm leaves to curl, so providing a protected shelter for the aphids. In late May this founding-generation of aphids mature and start giving birth to a generation of young which also feeds within the leaf curl. In mid to late June these aphids develop wings and leave the protection of the leaf curl in search of their secondary host, saskatoon / serviceberry. Once the secondary host has been located, the winged aphids move to the underside of a leaf and each aphid gives birth to approximately 15 young. These young descend to the roots where they feed as a colony on the root. Through the rest of the summer the aphids multiply and produce large colonies on the roots. The soil around the colony takes on a blue-purple coloration due to the waxy secretions by the aphids. Starting in late August and continuing throughout September the young root aphids develop into winged aphids. These leave the soil and migrate back to American elms where they give birth to a minute wingless sexual stage. This is the only time in the life cycle of the woolly elm aphid that male forms are present. These aphids search for protected sites in the cracks and crevices of the bark, where they mate and the females each deposit a single overwintering egg.
In North America, some predators have been observed feeding on Eriosoma americanum. For example, Wheeler & Jubb (1979) recorded the coccinellid Scymnus brulleri as a predator of the woolly elm aphid, and the predaceous mirid Saileria irrorata is given as a predator in Forest Pest Insects in North America. Larvae of the harvester butterfly (Feniseca tarquinius) may also be found amongst colonies of Eriosoma americanum much as the case with the wax-covered beech blight aphid (Grylloprociphilus imbricator) and the woolly alder aphid (Prociphilus tesselatus).
Other aphids on the same host
Eriosoma americanum only occurs on 1 species of elm, Ulmus americana.
Blackman & Eastop list 14 species of aphid as feeding on American white elm (Ulmus americana) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 7 as occurring in Britain (Show British list).
Blackman & Eastop only list 1 species of Amelanchier (Amelanchier laevis) as host to Eriosoma americanum, but other sources also give Amelanchier alnifolia.
Blackman & Eastop list 5 species of aphid as feeding on saskatoon / serviceberry (Amelanchier laevis) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 1 as occurring in Britain (Show British list).
Damage and control
The woolly elm aphid causes damage by removing sap from their host. On American elm, the aphid causes the leaves to swell and curl downward along the leaf edge. This results in numerous unsightly leaves that remain on the tree throughout the year, but it causes no permanent damage to the tree. As the aphid population increases within the curl, the aphid excrete a sweet, sticky substance called honeydew. This honeydew can be a nuisance especially when it covers walks, cars and other objects under the trees. On saskatoon/serviceberry, the root systems beyond the point of the aphid infestation are damaged or destroyed, resulting in reduced vigour and fruit production of established plants. On seedlings, severe aphid infestations can result in the death of young plants. A useful guide to important insects and diseases attacking saskatoon/serviceberry roots is provided by Dr Duke Elsner, Michigan State University Extension.
Image above copyright Whitney Cranshaw under a Creative Commons Attribution 3.0 License.
Chemical insecticides could be used to control woolly elm aphid infestations on American elm, but would be unlikely to have any effect on pest levels on the secondary (crop) host. For control of the pest on the roots of saskatoon/serviceberry, post-berry harvest is the best time for application as no residues will be left on the berries. The roots are protected for twelve months, preventing infestations the following spring. One insecticide used is orthene, which is applied to the soil surrounding young non-fruit-bearing plants. The use of pesticides on fruit-bearing saskatoon/serviceberry is generally prohibited. The utilization of biological agents to control woolly elm aphid is still relatively underdeveloped, but there is some evidence that fungi and nematodes have the potential to mitigate infestations in both the crown and roots of Amelanchier (Fry, 2002).