Beetles on dock plants
Docks are very familiar plants; they include several broadly similar species but the subject of this article is the Broad-leaved Dock, Rumex obtusifolius L. This is a common and widespread herbaceous perennial that occurs throughout the UK in a very wide range of habitats, it is not limited by soil type (although it may be less common and vigorous on very acidic soils) or climate and can tolerate wide variations of moisture and nutrient levels, it is present year-round and very quick to colonize disturbed ground and so is very likely to be easily available to any novice coleopterist wishing to add to a reference collection. Broad-leaved Dock is easily recognized by its large oval or slightly elongate leaves which have wavy edges and reach 30cm when fully-grown, and the dark brown or red stems supporting older leaves. Dispersal is by seed but seedlings generally do not flower in the first year of growth, during this time they establish a very tough and vigorous branched tap-root that penetrates down to a metre into the soil and stores ample reserves of food to produce new foliage even following repeated and prolonged grazing or damage, these seedlings may become dormant but will often continue growing through the winter and then produce mature foliage and stems during the following spring. Growth from established plants begins with leaves emerging from a short underground stem which grows from the tap root, this foliage matures and becomes dense before a distinct stem begins to
Broad-leaved dock, Rumex obtusifolius
grow, stems ultimately grow up to a metre high and support leaves alternately along their length, a characteristic of the family is the broad membrane (technically an ocrea, formed by the fusion of two stipules) which forms a sheath around the petiole where it joins the stem, and each stem will produce an inflorescence consisting of numerous groups of racemes, each of which contains many small green flowers that darken and turn red as they mature. Flowers are produced from June through to the autumn and they produce a continuous supply of dry dark brown seeds; a single plant being capable of producing 60000 seeds over a single season, these are very resilient as they contain antimicrobial chemicals which inhibit decay and they can remain viable in undisturbed soils for more than fifty years. Docks continue to produce stems and lavish foliage through to the autumn, much of the lower foliage will wilt and form layers around the plant during the summer and this will provide a refuge for many ground beetles etc during the warmest and driest parts of the year Most of the mature foliage dies back during the winter and forms a mat under which younger leaves are protected from the worst of the weather, but growth persists in all but the coldest spells and small rosettes of leaves produced in the autumn and winter often persist exposed among grass etc into the following spring. Rapid growth begins and may be well-underway by February in milder years, it is invasive and dense, often with clumps of numerous leaves growing close to each other, and they tend to exclude adjacent plants, the stems soon become tough and erect and new leaves quickly expand, soon reaching up to 40cm in length. Mature plants often exceed a metre in height and produce lavish growth at the expense of nearby plants, this can cause problems in horticultural and arable situations as the plants are very resistant to physical damage e.g. they may persist in lawns for years despite regular mowing, and for this reason it is classed as an ‘injurious weed’ in the UK and is considered to be an invasive pest in many areas of the world outside its native range e.g. in North America where it was first recorded in the 1840s.
Water dock, Rumex hydrolapathum
Common Hybrid Dock
This hybrid is also common everywhere and will often be found beside or even among the Broad-Leaved Dock, and in many areas it forms a high proportion of the dock population as it is more vigorous than either parent species, conversely it is much less fertile and so parent populations tend to be resilient. Hybrids can be very confusing but Curled Dock is easily identified by its slender, lanceolate and wavy leaves which are shorter, up to 40 cm when fully-grown, and which taper to a point, and its tall, densely-packed flower spikes.
Water Dock (Rumex hydrolapathum Huds.) may also be confused for Broad-Leaved Dock as it is common throughout the UK and grows just as large but it usually grows beside water and will often be found growing in shallow pond and lake margins, it may be distinguished by looking at the underside of its long lanceolate and slender leaves where the veins radiate at right angles from the mid-rib.
Species found on docks
Docks host a wide range of insects, most notably moths and butterflies (Lepidoptera), bugs and a range of aphids (Hemiptera), a sawfly or two (Hymenoptera), a few flies (Diptera) and a few beetles. Because the matted leaves below dock plants provide shelter and a more-or-less permanently damp habitat they should occasionally be lifted and examined as ground beetles, rove beetles, weevils and other beetles often hide there during the day. Similarly a range of beetles will be found by sweeping dock leaves, aphidiophagous ladybirds such as the 2-spot, 10-spot and 14-spot are common, as are some species of longhorn and cardinal beetle which like to bask on the leaves, some rove beetles such as Tachyporus are often found on docks and searching at night may produce common carabids such as Paradromius linearis (Olivier, 1795), Curtonotus aulicus (Panzer, 1796) or various Amara Bonelli, 1810 or Harpalus Latreille, 1802 etc. A few beetles such as the vine weevil, Otiorhynchus sulcatus (Fabricius, 1775), are so widely polyphagous that it would be odd not to find them on dock roots, and the closely related O. ligustici (Linnaeus, 1758), while not so widely polyphagous, has been recorded from the roots of R. obtusifolia. But there are a few beetles that are associated with docks in general, even if some of these are more widely polyphagous e.g. the chrysomelids Chaetocnema concinna (Marsham, 1802), Mantura rustica (Linnaeus, 1767) and Galerucella sagittariae (Gyllenhal, 1813) will often be found on a range of docks as well as plants from other families. The weevil Pelenomus quadrituberculatus (Fabricius, 1787) will sometimes be found on Curled Dock but not other species of Rumex despite it being associated with other Polygonaceae, and the weevil Amalus scortillum (Herbst, 1795) has been associated with docks in Europe but seems to occur exclusively on Knotgrass (Polygonum aviculare L.) in the UK. Conversely some beetles will be found on other species of Rumex but not on docks e.g. the chrysomelid Mantura obtusata (Gyllenhal, 1813) occurs on Common Sorrel (R. acetosa L.) and Sheep’s Sorrel (R. acetosella L. while Gastrophysa polygoni (Linnaeus, 1758) occurs on knotgrass (Polygonum aviculare Linnaeus, 1753) in the UK but is more widely polyphagous in Europe and North America. A few beetles are generally associated with the three docks discussed above and only rarely develop on other plants, these include the chrysomelid Gastrophysa viridula (De Geer, 1775), the brentid, Apion frumentarium (Linnaeus, 1758) and the weevil Rhinoncus pericarpius (Linnaeus, 1758). They are all common and widespread and should soon be found when searching docks. Gastrophysa adults and larvae feed openly on the leaves and are easily seen while larvae of Rhinoncus feed inside stems and roots. Apion frumentarium is present year-round and adults may often be found overwintering among host foliage or under nearby debris, they feed on foliage while larvae develop within stems and roots. Three beetles associated with the docks discussed here that are usually only adventitious on other plants, at least in the UK, are:
Hypera rumicis (Linnaeus, 1758)
Our largest dock-feeding weevil and very distinctive although there are a few very similar species on other plants. Widely distributed and locally common throughout England and Wales though rarely found in numbers, both adults and larvae feed externally on foliage and larvae construct delicate round netted cocoons on the upper surface of leaves in which to pupate, these are often easier to find than the adults and may be numerous in mid-summer.
Perapion hydrolapathi (Marsham, 1802) and P. violaceum (Kirby, 1808)
These two small, metallic brentids are common and widespread and both may occur in numbers on the same plant. They are very similar in appearance, differing very subtly in the shape and relative proportions of the head, pronotum and rostrum but they may be distinguished by the form of the metasternum; in violaceum it is strongly and rather densely punctured while in hydrolapathi the punctation is sparse and very fine, this character is readily appreciated with a little practice even in the field. Adults are common over a long season from early spring and are easy to find on the underside of leaves and low down on stems while larvae mine within stems, leaves and roots and occasionally induce gall formation.
Species found on dock plants
Galerucella sagittariae (Gyllenhal, 1813)
Mantura rustica (Linnaeus, 1767)
Chaetocnema concinna (Marsham, 1802)
Pelenomus quadrituberculatus (Fabricius, 1787)
Rhinoncus pericarpius (Linnaeus, 1758)
Hypera rumicis (Linnaeus, 1758)