Beetles on Willow
The vernacular name refers to a genus of deciduous trees and shrubs that are instantly recognised by just about everybody but are otherwise a very diverse group of about 400 species which are mostly distributed in temperate regions of the Northern Hemisphere although several species, which were introduced into southern Australia to help prevent erosion along rivers, have become extensively established and are regarded as invasive weeds. Among the most atypical and impressive of species is the Arctic Willow (Salix arctica Pall.), this species occurs in higher latitudes across Northern Canada and Eastern Asia, it reaches far above the tree line in mountain areas and occurs across the northern coast of Greenland, it is long-lived and very slow-growing, reaching at most 50 cm in height but usually much less, it has transverse leaves covered with silvery pubescence to reduce water loss and the species is dioecious, the female having red catkins while those of the male are white. There are other ‘creeping’ species’ adapted to harsh environments but the general idea of willows is that of large and graceful trees with long pendulous branches that grow beside rivers and lakes, and this sort of thing is the subject of this article. Willows are included in the genus Salix L., some narrow-leaved species are called Osiers, these have long been used for hedging and basket-weaving and so have some commercial utility, and some broad-leaved species are called sallows, these often occur in wetland situations alongside willows and are of interest here as they often host the same beetles. The vast majority of species are deciduous and all but one are dioecious, having male and female catkins on separate plants, most are fast-growing and despite the common perception, many grow in a wide range of situations, but beyond this many display some extraordinary characteristics. The Common Osier, or Basket Willow (S. viminalis L.), like many species, has a wide Palaearctic distribution and is generally common in the UK, its long flexible branches are used for weaving and making living plant structures, charcoal made from its wood is used by artists and it is increasingly being used in ‘energy forestry’, it is also useful under certain circumstance in water purification as it is a hyperaccumulator of metals such as lead, cadmium, chromium and mercury as well as various organic solvents. Willows grow quickly and are extremely resilient, fallen
Willow regenerating from fallen branches. River Colne, Watford.
branches and twigs will grow readily in water or damp soil and damaged trees readily heel and compensate by producing lavish growth. The roots can survive long after trees have been felled, and where they come into contact with the air they usually produce shoots, when branches come into contact with soil they often produce roots and damaged arial parts of trees may also grow roots into accumulated debris in rot holes etc. The bark is very sappy and this is loaded with salicylic acid, which is a plant hormone that has many medical uses and is a key metabolite of aspirin (acetylsalicylic acid), and rumour has it that cattle and horses will consume willow foliage when feeling under the weather. There really is no point in discussing the different species of willow because this is a specialist subject and in any case many identification guides are available for those interested, those of interest to the coleopterist are Crack-willow (S. fragilis L.), White Willow (S. alba L. (and, for those interested, the variety Caerulea which is used in making cricket bats)), Almond Willow (S. triandra L.), Purple Willow or Purple Osier (S. purpurea L.), Common Osier, Goat Willow (S. caprea L.), Grey Willow (S. cinerea L.) and Eared Willow (S. aurita L.). There are about 35 species in the UK and the range of varieties and hybrids and whatever they’re called is absolutely baffling, but to most beetlers a willow is a willow, or a sallow or an osier. There is no doubt that some interesting results might be obtained by carefully noting which beetles occur on which willows and there are a few species that may well be monophagous or oligophagous but most willow-feeding weevils and leaf beetles are polyphagous on Salix species and many also occur on other trees, especially various poplars.
Willows are really good trees for sampling beetles because they usually grow in wet or damp habitats and thrive in open areas exposed to the sun, this means they provide a cool and humid habitat on the ground below which is ideal for carabids and staphs and any number of other beetles, and this is especially true where they grow beside or among reed beds, they also host a very diverse range of phytophagous beetles e.g. even at the most northern limits of its distribution the Arctic Willow hosts the tundra weevil Isochnus arcticus (Korotyaev, 1976), and because in any stretch of waterside willow there will usually be plenty of damaged and decaying wood, they also host a diverse range of wood-boring species. Two factors contrive to make willows good hosts for a wide range of fungi; the wood is brittle and so in older trees there will often be large branches that have split and toppled but remain attached to the trunk, and they usually grow in wet or damp conditions, this means that a very diverse range of beetles associated with fungi may be found by careful searching. Beyond this the foliage does not produce the dense shade that many deciduous trees do and so a good understory of vegetation is usually present, and at the right time of year this means umbel flowers and other damp tolerant species that attract beetles. Leaf litter may accumulate around trees growing in parks and gardens etc., but in marginal habitats it often gets swept away by winter flooding and this means that in many situations beetles will overwinter in tussocks or under loose bark rather than among litter or in the soil. Searching for beetles among stands of willows can therefore be very productive throughout the year, and because there are many mycophagous and saproxylic species it can be very productive searching trunks and fallen timber on warm spring and summer nights. Sweeping by night can also be productive because the foliage tends to hang within easy reach and is delightfully easy to sweep, it is also a good way of finding hopping weevils and leaf beetles as they usually sit still in the net and are thus easy to tube.
Most of this discussion will be concerned with phytophagous beetles but it should be stressed that willows are interesting in many other ways and so first we take a short diversion. Ground beetles occur wherever willows grow, in drier situations these may be generally common species such as Calathus melanocephalus (Linnaeus, 1758) or Harpalus affinis (Schrank, 1781) on the ground below, or Dromius quadrimaculatus (Linnaeus, 1758) or Ocys harpaloides (Audinet-Serville, 1821) on the trunks and branches, but in wetland situations there is a very diverse fauna to be found by searching below willows. The best time to do this is at night because many of the species are naturally active at this time, there are exceptions such as Elaphrus Fabricius, 1775 or Blethisa multipunctata (Linnaeus, 1758), but in general things really get active after dark. Among the species that occur in large aggregations are Paranchus albipes (Fabricius, 1796), Agonum fuliginosum (Panzer, 1809), A. thoreyi Dejean, 1829 and various Bembidion Latreille, 1802 but there will always be other species among them and so samples should be taken, some species are obvious e.g. Carabus granulatus Linnaeus, 1758 or Agonum marginatum (Linnaeus, 1758), but many are very similar to common species and so need to be searched for very carefully e.g. very local Agonum such as A. versutum Sturm, 1824, A. viduum (Panzer, 1796) and A. scitulum Dejean, 1828 are easily overlooked for common species in the field. Among the interesting carabids often found on willow bark are Syntomus obscuroguttatus (Duftschmid, 1812), S. truncatellus (Linnaeus, 1761), Microlestes minutulus (Goeze, 1777), Calodromius spilotus (Illiger, 1798) and Philorhizus sigma (Rossi, 1790) but many others may be found. Rove beetles are usually diverse and very common in this habitat but, again they will need to be examined very carefully e.g. among the many Stenus there are two generally common ‘spotted’ species, S. bimaculatus Gyllenhal, 1810 and S. comma LeConte, 1863, but these are closely similar to the less common S. guttula Müller, P.W.J., 1821 and S. biguttatus (Linnaeus, 1758), similarly S. cicindeloides (Schaller, 1783) and S. solutus Erichson, 1840 both occur in such situations and are easily confused. Many other Stenus e.g. S. boops Ljungh, 1810 and S. flavipes Stephens, 1833 are common in this habitat but rarer species are easily overlooked e.g. S. nitens Stephens, 1833 is common among reed litter shaded by willows at a local Watford pond and we found the rare S. fornicatus Stephens, 1833 under bark on a partly submerged willow branch in a Watford park. There are a few larger staphs to be found in this habitat e.g. Ocypus brunnipes (Fabricius, 1781) and various Philonthus Stephens, 1829, Quedius Stephens, 1829, Lathrobium Gravenhorst, 1802 and Paederus Fabricius, 1775 but the greatest diversity is among the smaller and more obscure stuff such as the many species of Carpelimus Leach, 1819 and numerous Aleocharinae.
Aromia moschata (Linnaeus, 1758)
Searching under willow bark and among decaying wood will produce many of the usual suspects such as histerids, throscids, elaterids, ptinids, erotylids, monotomids (Rhizophagus are often common in this situation through the winter), silvanids, cerylonids, latrids and tenebrionids etc, but close inspection will often reveal ptilids and corylophids that thrive among damp wood. Among the larger beetles to be found are the lucanids Dorcus paralellopipedus (Linnaeus, 1758) and Sinodendron cylindricum Linnaeus, 1758, the longhorns Aromia moschata (Linnaeus, 1758), Lamia textor (Linnaeus, 1758) and Oberea oculata (Linnaeus, 1758) and species of the elaterid genus Melanotus Eschscholtz, 1829. A complete list of the beetles that may be found among decaying willow wood and bark would be very long and over time would be continually expanded e.g. the corylophid, Arthrolips obscurus (Sahlberg, C.R., 1833), a recent arrival to the UK, has been found on burnt gorse in the New Forest but on the continent is more generally associated with fungoid wood and has been found in rot holes on willows. Another recent addition to the British list is the ptilid, Baranowskiella ehnstromi Sörensson, 1907, which is the smallest known European beetle and associated with the fungus Phellinopsis conchata (Pers.) Y.C. Dai which is widespread in the UK and grows on Salix caprea. Willow bark is deeply fissured and often forms loose layers and so provides good shelter for many overwintering beetles such as ladybirds, leaf-beetles and weevils, this often becomes loose and so samples are easily taken for extraction at any time.
For most beetle collectors or recorders the attraction of Salix is the phytophagous species that it hosts, these are rarely monophagous, or even restricted to willows or to Salix in general, but are easiest to obtain by sweeping willows. For example rhynchitids are not really associated with willows but our three species of Temnocerus Thunberg, 1815 may occasionally appear when sweeping the foliage. The same applies to apionids where the majority of our species are associated with herbaceous foliage but Melanapion minimum (Herbst, 1797) is almost always found on Salix. A whole range of other species such as the
anthribids Anthribus fasciatus Forster, 1770, A. nebulosus Forster, 1770, Pseudeuparius sepicola (Fabricius, 1792) and Tropideres albirostris (Schaller, 1783) might occur on willow foliage because they develop either in decaying wood or among fungi and teneral adults may disperse to the foliage to feed or assemble, the same sort of thing applies to species associated with ivy that may be growing on trunks and stems. Among the ‘nice’ species that might occur in the sweep net are Agrilus laticornis (Illiger, 1803), A. viridis (Linnaeus, 178) and Trachys minutus (Linnaeus, 178) (Buprestidae), Calambus bipustulatus (Linnaeus, 1767) and Paraphotistus nigricornis (Panzer, 1799) (Elateridae), Xestobium rufovillosum (De Geer, 1774) (Ptinidae), Scymnus limbatus Stephens, 1832 and Chilocorus renipustulatus (Scriba, 1791) (Coccinellidae), Anisoxya fuscula (Illiger, 1798), Abdera flexuosa (Paykull, 1799) and Melandrya caraboides (Linnaeus, 1760) (Melandryidae), Prionychus ater (Fabricius, 1775) (Tenebrionidae) and the longhorns Gracilia minuta (Fabricius, 1781) and Saperda carcharias (Linnaeus, 178) (Cerambycidae). Others could be listed but the point is that sweeping or beating willows for phytophagous species can be very rewarding beyond what might be expected. The following leaf-beetles are either generally associated with willows or they may occasionally be found by beating or sweeping foliage. Zeugophora flavicollis (Marsham, 1802) and Z. subspinosa (Fabricius, 1781) (Megalopodidae, larvae mine leaves), Cryptocephalus labiatus (Linnaeus, 1760), C. punctiger Paykull, 1799, C. pusillus Fabricius, 1777, C. decemmaculatus (Linnaeus, 178), C. sexpunctatus (Linnaeus, 1758), Labidostomis tridentata (Linnaeus, 1758), Smaragdina salicina (Scopoli, 1763), Chrysomela populi Linnaeus, 1758, C. saliceti (Weise, 1884), C. tremula Fabricius, 1787, Plagiodera versicolora (Laicharting, 1781), Phratora vulgatissima (Linnaeus, 1758), P. laticollis (Suffrian, 1851), P. polaris (Schneider, J.S., 1886) (on dwarf willow only), P. vitellinae (Linnaeus, 1758), Gonioctena pallida (Linnaeus, 1758), G. viminalis (Linnaeus, 1758), G. decemnotata (Marsham, 1802), Luperus flavipes (Linnaeus, 1767), L. longicornis (Fabricius, 1781), Galerucella lineola (Fabricius, 1781), Lochmaea caprea (Linnaeus, 1758), Crepidodera aurata (Marsham, 1802), C. aurea (Fourcroy, 1785), C. fulvicornis (Fabricius, 1792) and C. plutus (Latreille, 1804) (Chrysomelidae). Weevils are also well-represented and though there are
Crack Willow, Salix fragilis L.
Welwyn Garden City, Herts
fewer species there will usually be plenty of adventive species on the foliage, the following are more generally associated with willows. Dorytomus hirtipennis Bedel, 1884, D. majalis (Paykull, 1792), D. melanocephalus (Paykull, 1792), D. salicinus (Gyllenahl, 1827), D. taeniatus (Fabricius, 1781), D. rufatus (Bedel, 1888), Cryptorhynchus lapathi (Linnaeus, 1758), Archarius salicivorus (Paykull, 1792), Acalyptus carpini (Fabricius, 1792), Ellescus bipunctatus (Linnaeus, 1758), Cossonus linearis (Fabricius, 1775), C. parallelepipedus (Herbst, 1795), Trypophloeus binodulus (Ratzeburg, 1837), Tachyerges decoratus (Germar, 1821), T. pseudostigma (Tempére, 1982), T. salicis (Linnaeus, 1758), T. stigma (Germar, 1821), Isochnus foliorum (Müller, O.F., 1764), I. sequensi (Stierlin, 1894) and Rhampus pulicarius (Herbst, 1795).