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Identification & Distribution

The eggs which the woolly alder aphid host lays on its primary host, silver maple, hatch in spring to give the fundatrices. The first picture below shows the resultant colonies on silver maple (Acer saccharinum). The offspring of the fundatrices are winged, and they migrate to their secondary host, alder, where they form colonies along the branches (see second picture below). Adult apterae of Prociphilus tessellatus are quite large brownish aphids which are covered with a thick layer of white woolly wax. Their antennae, legs, anal plate and subgenital plate are all brown to black, and the wax pore plates are pigmented (cf. Prociphilus baicalensis which has the antennae, legs, anal plate and subgenital plate of aptera all lightly pigmented, and the wax pore plates are pale). The apical segment of the rostrum (R IV+V) is longer than the second hind tarsal segment (HT II), and R IV bears 4-10 accessory hairs (cf. Prociphilus mexicanus where R IV+V is shorter than HT II, and R IV bears 2-3 accessory hairs).

First image above copyright Jim Baker, North Carolina State University, Bugwood.org, Creative Commons Attribution-NonCommercial 3.0 license.
Second image copyright Troy Bartlett, under a Creative Commons Attribution-NoDerivs-NonCommercial 1.0 Generic license.

The colonies of Prociphilus tessellatus on alder grow and persist through the summer (see first picture below). Then, in autumn, alate sexuparae develop (see second picture below). They are also thickly covered in white woolly wax, and have narrow, transverse secondary rhinaria on the antennae, including 1-5 on the base of antennal segment VI (cf. the alate sexupara of Prociphilus mexicanus which has the secondary rhinaria on antennae more oval, and they absent from the base of antennal segment VI).

First image above copyright Lynette Elliott, second image copyright Stan Gilliam, both under a Creative Commons Attribution-NoDerivs-NonCommercial 1.0 Generic license.

The woolly alder aphid (Prociphilus tessellatus) should really be known as the 'woolly silver maple-alder aphid', given it host alternates between silver maple (Acer saccharinum, its primary host) and alder (Alnus, its secondary host). Alate sexuparae (see fourth picture above) fly from alder to silver maple in autumn, where they produce a sexual generation. After mating the oviparae lay eggs which overwinter. Not all the aphids migrate from alder - some do not mate, but overwinter as parthenogenetic apterae under fallen leaves or in loose earth beneath alder trees. The woolly alder aphid is found widely in North America, but has not so far been recorded in Europe or Asia.

 

Biology & Ecology

Ant attendance

Wax secreted by aphids is thought to have a number of possible functions, including protection from predators, preventing contamination with their own honeydew, and for water-proofing. Some aphid species produce little or no wax when the colony is being attended by ants - possibly because, with the added protection from ants, the aphids do not find it necessary to incur the additional metabolic cost of producing wax. Then again, wax may discourage ants from attending - or indeed from eating the aphids.

First image copyright Kirill Ignatyev under a Creative Commons Attribution-NonCommercial 2.0 Generic License.
Second image copyright Judy Gallagher under a Creative Commons Attribution 2.0 Generic License.

However, this does not seem to be the case with woolly alder aphids, which retain their heavy waxing when thus attended (see second picture above). The ant in the picture above is a carpenter ant (Campanotus) - however the colony shown below was attended by a different species of ant, so far unidentified.

Image above copyright Stan Gilliam under a Creative Commons Attribution-NoDerivs-NonCommercial 1.0 Generic license.

Natural enemies

Woolly alder aphids attract a wide range of predators and parasitoids including coccinellid and chrysopid larvae, but also some very unusual predators - caterpillars of the harvester butterfly (Feniseca tarquinius, see picture below).

Image copyright Tom Murray under a Creative Commons Attribution-NoDerivs-NonCommercial 1.0 Generic-License.

The harvester butterfly caterpillar is usually powdered with wax (see picture below) which camouflages it from aphid-attending ants.

Image copyright Tom Murray under a Creative Commons Attribution-NoDerivs-NonCommercial 1.0 Generic-License.

Another specific predator of Prociphilus tessellatus is the larva of the green lacewing (Chrysopa slossonae) which, to protect itself from the ants, also camouflages itself with wax taken from these aphid. Eisner et al. (1994) showed that larvae artificially denuded of wax were seized by the ants and removed from the aphid colonies.

Lohman et al. (2006) explored the semiochemical (=chemical signal) mechanisms by which these predators circumvent attack by otherwise aggressive ants. Ants use specific mixtures of cuticular hydrocarbons (CHCs) as recognition labels, but this simple mechanism is frequently circumvented by nest parasites, which engage in ‘chemical mimicry(tm) of their host ants by producing or acquiring a critical suite of these CHCs. Analysis of the CHCs from the woolly alder aphid, Prociphilus tessellatus, their tending ants, and aphid predators from three insect orders (the larvae of the harvester butterfly Feniseca tarquinius, the green lacewing Chrysopa slossonae, and the hoverfly Syrphus ribesii) showed that while each predatory species had a distinct CHC profile, each profile was chemically more similar to that of the aphids than to either tending ant species. Moreover, the CHCs of each predator species were a subset of the compounds found in the aphids(tm) profile. These results implicate CHCs as a recognition cue used by ants to discriminate honeydew-providing aphids from potential prey and a probable mechanism by which aphid predators circumvent detection by aphids and their tending ants. Although several features of the aphids(tm) CHC profile are shared among the chemically mimetic taxa, variation in the precision of mimicry among the members of this predatory guild demonstrates that a chemical mimic need not replicate every feature of its model.

Some bird species have also been recorded as predating woolly alder aphids, in particular downy woodpeckers. The picture below shows a downy woodpecker eating woolly alder aphids in Columbus, Ohio.

Image reproduced by permission, copyright Marne Titchenell all rights reserved.

Waxy aphids, such as Prociphilus tessellatus, seem to be attractive to bird predators despite their waxy covering - as demonstrated by predation of the beech blight aphid (Grylloprociphilus imbricator) by Tenessee warblers (Oreothlypis peregrina). We have a number of aphid species produce less, or no wax, when being ant-attended - as is to be expected given the additional metabolic cost of producing ant-attractive (=melezitose-rich) honeydew. However, not all birds are likely to be dissuaded by the presence of ants - indeed, in some cases, the ants and aphids are consumed - which perhaps explains why woolly alder aphids retain their heavy waxing even when ant-attended. For more on predation of woolly alder aphids by downy woodpeckers see our page on, the aphid-predating, downy woodpecker.

 

Other aphids on the same host

Primary host

Prociphilus tessellatus has been recorded from one species of Maple: Acer saccharinum, whose commercial importance we outline below.

Secondary hosts

Prociphilus tessellatus has been recorded from 3 species of Alder:

  • Red alder (=Oregon alder, Alnus rubra) a nitrogen-fixing deciduous broadleaf tree native to western North America (USA and Canada). Mainly the west coast of Washington, Oregon, California and British Columbia, but also Idaho, Montana, Yukon and Alaska. It is the wood of choice for smoking salmon, and is becoming a popular hardwood. Alnus rubra is of some commercial importance in land reclamation, occasionally as a rotation crop to discourage the conifer root pathogen Phellinus weirii (Laminated root rot), or as an ornamental - particularly for hedging.
  • ‎Speckled alder (Alnus rugosa = Alnus incana ssp. rugosa) is a fast-growing nitrogen-fixing broadleaf tree native to Northern and Eastern North America: mainly Canada - from Hudson's Bay to Virginia. Naturalized in Central Europe. It is primarily used for land reclamation.
  • Tag alder (=Hazel alder, Smooth alder, Alnus serrulata) is a fast-growing nitrogen-fixing shrub/small tree native mainly in south eastern USA - from Maine to Northern Florida, west to southeastern Oklahoma, Missouri, and Illinois - and along the Mississippi river. It is used as a stabilizer and restorer of riversides or stream banks.

Blackman & Eastop list about 50 species of aphids as feeding on alder worldwide, and provides formal identification keys for aphids on Alnus. Of those, Baker (2015) lists 6 aphid species on common alder (Alnus glutinosa) and/or Italian alder (Alnus cordata) as occurring in Britain: Clethrobius comes, Crypturaphis grassi, Glyphina betulae, Pterocallis alni, Pterocallis maculata and Stomaphis quercus.

  • Despite its relative commercial importance, just 4 aphid species have been recorded on Alnus rubra: Euceraphis gillettei, Oestlundiella flava, Prociphilus tessellatus and Pterocallis alni.
  • 10 aphid species have been recorded on Alnus rugosa: Boernerina variabilis, Calaphis alni, Euceraphis gillettei, Hannabura alnosa, Illinoia alni, Illinoia wilhelminae, Prociphilus tessellatus, Pterocallis alni, Pterocallis alnifoliae and Pterocallis rhombifoliae.
  • 2 species have been recorded on Alnus serrulata: Prociphilus tessellatus and Pterocallis alnifoliae.

 

Damage and control

Silver maple is planted as an ornamental tree because of its rapid growth and ease of propagation. It has also been used as a landscaping and street tree in urban environments, although it has lost favour for this purpose in North America owing to its brittle wood and tendency to produce large numbers of seedlings. The woolly alder aphid is sometimes considered a pest on this species because infested leaves shrivel and drop early, and the honeydew can make sidewalks and cars sticky.

However, the aphids cause little or no permanent damage to the tree, and a better policy is to leave it to the natural enemies to control the populations. This is also recommended by the Home and Garden Information Center, although they do give some insecticide treatment options if required.

Alders are infested by the same woolly aphid, but again numbers are usually kept low by natural enemies. If treatment is required, insecticidal soap or neem oil are recommended.

Acknowledgements

We especially thank Marne Titchenell of Buckeye Yard & Garden onLine, for permission to use her photos of downy woodpeckers feeding on aphids.

We have made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994) and Blackman & Eastop (2006) supplemented with Blackman (1974), Stroyan (1977), Stroyan (1984), Blackman & Eastop (1984), Heie (1980-1995), Dixon & Thieme (2007) and Blackman (2010). We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure provided by Blackman & Eastop (2006).

Useful weblinks

References

  • Eisner, T. et al. (1997). "Wolf-in-sheep's clothing" strategy of a predacious insect larva. Science 199(4330), 790-794. Abstract

  • Lohman, D.H. et al. (2006). Convergence of chemical mimicry in a guild of aphid predators. Ecological Entomology 31, 41-51. Google