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Identification & Distribution

Adult apterae of Longistigma caryae are pale brownish-grey with 2 longitudinal rows of large black spots (not sclerotised) on each side of the median line of the dorsum, and a transverse series of small black spots along the intersegmental lines. Their dorsum is covered with a variable amount of bluish-white wax powder, and the body, legs and antennae have conspicuously long brown hairs. Antennal segment III is equal in length to segments IV plus V, whilst segment VI is short, about 3 times longer than wide and with the terminal process thumb-shaped. The coxae are dusky, the trochanters and femora, except tips, are reddish-brown, and the tips of the femora, together with tibiae and tarsi, are black except that the middle of the tibia is sometimes reddish-brown. The second hind tarsal segment (HTII) is 1.8-2.3 times the length of the first hind tarsal segment (HTI). The truncate siphuncular cones are black and hairy. The body length of adult Longistigma caryae apterae is 5.1-7.8 mm

First image above by permission, copyright Claude Pilon, all rights reserved.
Second image above copyright Katja Schulz under a Creative Commons Attribution 2.0 Generic license.

Note: In the second image, the balls of woolly white wax by the aphid's anterior are probably Grylloprociphilus imbricator, the beech blight aphid.

Alates of Longistigma caryae have the head and thorax bluish black, and the dorsum pale brownish-grey with similar marking, long hairs, and wax-covering to the aptera. Their wings have an elongate pterostigma extending around the tip of the forewing (see second picture below) and are dusky, especially toward their base. The truncate siphuncular cones are black. Oviparae do not differ in general appearance from viviparous females.

The clarified slide mounts below are of adult viviparous female Longistigma caryae : wingless, and winged.

Clarified mount images above copyright Brendan Wray, AphID, USDA APHIS PPQ, Bugwood.org under a Creative Commons Attribution-Noncommercial 3.0 License.

Longistigma caryae do not host alternate, but are highly polyphagous, being found on the bark of numerous tree species in North America including basswood (Tilia spp.), hickories & pecan (Carya spp.), oaks (Quercus spp.) and sycamores (Platanus spp.). In northern USA and Canada sexual forms develop in autumn, producing alate males and apterous oviparae. The population then overwinters in the egg stage. In southern states of the USA populations are anholocyclic.

 

Biology & Ecology

Feeding Biology

Some work has been done on the feeding biology of Longistigma caryae. Evert et al. (1968) showed that penetration of the bark of Tilia americana by Longistigma caryae is mainly intracellular (=inside the host plant's cells).

Images above copyright Gerald Watson under a Creative Commons license.

Like other aphids, Longistigma caryae secretes a saliva sheath which encloses the path of the stylets, beginning with an external collar of sheath material on the surface of the host's periderm (=corky outer bark). Stylet sheaths within the bark gave positive reactions for callose, suggesting that, in reaction to wounding, punctured parenchyma (=soft succulent) cells secrete callose which diffuses throughout the stylet sheaths. Other, more conspicuous effects of wounding included: proliferation and enlargement of cells of the cortex and dilated rays bordering some stylet sheaths, formation of tylosoids in punctured sieve elements (=elongated, perforated, water conducting, phloem cells), deposition of massive amounts of callose in penetrated sieve elements and in sieve elements bordering penetrated cells, and stimulation of cambial activity and xylem (=vascular woody tissue, conducting water from root to stem) differentiation. Stylet tips located in living sieve elements projected beyond their sheaths which terminated outside the sieve-element walls. It is suggested that such sieve elements can be considered to be functional. None of the living sieve elements containing stylet tips showed any signs of injury which could be attributed to the presence of the stylets. Stylet tips of feeding aphids were found in living sieve elements of both 1965 and 1966 phloem increments clearly indicating that Longistigma caryae can feed on linden (=Tilia) sieve elements more than 1 year of age.

Image above copyright Lynette Elliott under a Creative Commons license.

Population dynamics

Rather little work has been done on the ecology of Longistigma caryae, although marked fluctuations in population size are known to occur. Weed (1891) described how the aphid was "excessively abundant in 1882 on Sycamore trees over a large part of the United States. During the summer and early autumn months nearly every sycamore tree was thickly infested..." Tissot (1944) noted that this aphid appeared to have a period of unusual abundance during the years 1940 to 1942.

Herbert A. 'Joe' Pase of the Texas A&M Forest Service recounts how, during December 2001 and through most of March 2002, an outbreak of this aphid was found covering about the eastern quarter of Texas. They mainly attacked oak trees, especially Quercus virginiana (southern live oak) and Quercus nigra (water oak). Natural enemies, such as coccinellids, played an important role in controlling aphid populations. During winter months when temperatures are cold, predator and parasite insects are not very active, and this may be the reason aphid populations became so large early in 2002.

Weed (1891) reported that the sexual forms appeared late in September, and throughout October. The picture below shows a male Longistigma caryae.

Image above copyright Katja Schulz under a Creative Commons Attribution 2.0 Generic. license

Note the markedly shorter abdomen of the male, above, and his more dusky wings.

Weed (1891) states that the deposition of eggs began early in October, continuing for more than a month. Eggs were deposited on the bark of the twigs and branches in enormous numbers. Wilson (1909) describes the eggs as being elongate-ovoid, orange-brown when freshly laid but changing on exposure to shining black. When newly-laid they are covered with a viscous substance, by which they are securely attached to the bark of the twig. Large numbers are deposited together. Michael J. Raupp of the University of Maryland provides an excellent picture of the giant bark aphid with eggs.

Ant attendance

Longistigma caryae aphids are commonly tended by ants which feed on the abundant honeydew produced by the aphids.

Images above copyright Charley Eiseman under a Creative Commons license.

Note: The yellowish insect with white 'wool' toward bottom left of the second image is probably Phyllaphis grandifoliae, the American woolly beech aphid.

Longistigma caryae seem to be mostly tended by Camponotus species, for example Camponotus chromaiodes (as in the pictures above) and Camponotus pennsylvanicus.

Convergent evolution

Readers in Europe will doubtless have noticed the many similarities between the Nearctic 'giant bark aphid', Longistigma caryae, and the Palearctic 'giant willow aphid', Tuberolachnus salignus - Their dorsal coloration is very similar, both aphids being pale brownish-grey with longitudinal rows of large black spots. Both species feed on the bark of deciduous trees and their numbers tend to peak late in the year, often in December. The two species are both classed in the Lachnini, but are not especially closely related (see Chen et al., 2015), so the similarities may result from convergent evolution: Both species occupy the same or similar niches, albeit usually on different hosts. Their only major difference arises from how they get through the winter. In northern areas Longistigma caryae is holocyclic, with sexual forms and overwinters as eggs, whereas no sexual forms are known for Tuberolachnus, which are assumed to be anholocyclic.

Although Tuberolachnus salignus is native to the Palearctic, as in so many other cases it has been introduced to North America. However, Tuberolachnus is restricted to willow (Salix) and poplar (Populus) species, whereas Longistigma is rarely found on these genera. Hence they are very unlikely to compete.

Fossil evidence

Longistigma caryae is one of the few species of aphids for which we have good fossil remains. Friedrich & Heie (1971) report that a fossil alate of this species was found in tuff (= light porous consolidated volcanic ash) in northwestern Iceland, from the Upper Miocene or Lower Pliocene (= 3.6 to 11.63 million years ago).

Despite the forewing being longitudinally folded, in view of its venation and shape of the pterostigma this specimen was classed unambiguously as a Longistigma - of which Longistigma caryae was the sole known living species. Only parts of the antennae remained, and the rostrum and rhinaria were not visible. Nonetheless given the colour, shape and form of other morphological features, following detailed measurements thereof, the authors saw no significant differences to an extant Longistigma caryae alate. The longer body length, of 7.82 rather than 7.19 mm, was ascribed to the fossil having slightly stretched longitudinally.

However since a number of its other features were similarly larger, this fossil seems to have been a genuinely larger specimen. Blackman notes Longistigma caryae body lengths of 5.1-7.8 mm have been recorded. Even if we assume these two specimens represent two different-sized populations, it would not imply those populations are different species (for example see Cinara smolandiae or Stomaphis wojciechowskii).

In the present time Longistigma caryae lives on several genera of deciduous trees, such as Carya and Fagus, exclusively in the region of the Eastern Deciduous Forests of North America. Iceland is presently classed as Palaearctic, whereas North America and Greenland are Nearctic. The recent aphid fauna of Iceland contains palaearctic, holarctic (Nearctic + Palaearctic), cosmopolitan, and endemic species - and species common with Greenland and arctic Canada. Finding a fossil Longistigma caryae supports paleobotanical evidence indicating that Iceland once belonged in the region of deciduous forests, with a flora more like the modern eastern North American flora than any other modern flora region.

 

Other aphids on the same host

Whilst Lachnid aphids are normally monophagous, Longistigma species are relatively polyphagous. It has been recorded as feeding on several different genera and species of broad-leaved trees including Acer saccharum, Carya (Carya glabra, Carya illinoiensis), Castanea ashei, Cercis canadensis, Fagus grandifolia, Myrica cerifera, Platanus sp, Populus sp, Pyrus (Pyrus communis, Pyrus pyrifolia), Quercus (Quercus laurifolia, Quercus marilandica, Quercus michauxii, Quercus nigra, Quercus palustris, Quercus virginiana), Tilia americana.

 

Damage and control

Denmark (2003) summarizes the economic importance of Longistigma caryae. During the late summer and autumn months, numerous specimens feeding on branches excrete large amounts of honeydew that can form a sticky coating on automobiles, picnic tables, lawn furniture, and plants underneath trees where the aphids are feeding (see picture below).

Image copyright Herbert A. 'Joe' Pase III, Texas A&M Forest Service, Bugwood.org under a Creative Commons Attribution-Noncommercial 3.0 License.

Sooty mould, which is grey-black in color, soon begins to grow on the sugar-rich honeydew. While sooty mould does not directly damage plants, it blocks sunlight and disrupts photosynthesis, contributing to reduce plant vigour. Sooty mould can also damage the finish on cars, chairs, tables or other objects.

Acknowledgements

We are especially grateful to Claude Pilon for his picture of Longistigma caryae (for more of her excellent pictures see).

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

  • Chen, et al. (2015). An aphid lineage maintains a bark-feeding niche while switching to and diversifying on conifers. Cladistics 32(5), 555-572. Full text

  • Denmark, H.A. (2003). Giant Bark Aphid, Longistigma caryae (Harris) (Insecta: Hemiptera: Aphididae). University of Florida IFAS Extension Document EENY-292. Full text

  • Evert, R.F. et al. (1968). Observations on the penetration of Linden branches by the stylets of the aphid Longistigma caryae. American Journal of Botany 55(7), 860-874. Abstract

  • Friedrich, W.L. & Heie, Ole E. (1971). A Fossil Specimen of the North American Hickory Aphid (Longistigma caryae Harris), Found in Tertiary Deposits in Iceland. Insect Systematics & Evolution 2(1), 74-80. Abstract

  • Tissot, A.N. (1944). Additions to the Lachnini of Florida (Homoptera: Aphididae). The Florida Entomologist 27(3), 43-54. Full text

  • Wilson, H.F. (1909). Notes on Lachnus caryae Harris under a new name. Canadian Entomologist 41(11), 385-387. Abstract

  • Weed, C.M. (1891). Fifth contribution to a knowledge of certain little-known Aphidae. Insect Life 3(6), 285-292. Google books