Beetles on stinging nettles
Stinging nettles are native to the Palaearctic region, including North Africa, but are now established in various regions worldwide, they are members of the Urticaceae and may be recognized by the square section stems and stinging hairs, dead-nettles are sometimes superficially similar, they belong to the Lamiaceae and also have square-section stems but they lack stinging hairs and have very different flowers, both groups of plants attract a range of beetles and both will quickly become familiar. Beds of stinging nettles are ideal places to look for beetles, especially for the beginner, because although many species may be swept from nettle foliage there are only a few that use nettles as host material, and these are generally common and should be expected wherever nettles are growing in abundance. There are many advantages to spending a while studying nettles; they are easily identified, they are generally abundant across the UK, will grow in just about any habitat and on any soil type and they are perennial so that any studies can continue over a few years in a convenient habitat. The square section stems and leaves are clothed with hollow stinging hairs (trichomes) which are finely tipped with silicon, rather like glass, and will easily penetrate human flesh, they inject a mixture of chemicals including histamines that produce an immediate stinging sensation (contact urticaria) which varies between individuals but is usually painful, produces local swelling and lasts for about 15 minutes. Rarely there may be individual plants or local populations that lack stinging hairs, most notably at Wicken Fen in Cambridgeshire, but these usually occur at low frequency and there is no evidence to suggest they are
either spreading or that they represent a different species. Stinging hairs may have evolved as a defence against grazing mammals but they seem to offer no defence against insect larvae, notably those of a few butterflies, that may cause extensive defoliation (at least formerly when every nettle-bed hosted numerous larvae.) There are two species of stinging nettle in the UK but the one of interest is the perennial common nettle, Urtica dioica L. and its varieties, while the smaller annual U. urens L. is more local and tends to grow in isolation; it also hosts many insect species but is often disappointing to sample as it tends to grow in damp shady conditions and plants rarely reach any substantial size. The common nettle will colonize new habitats from seeds dispersed by the wind or by insects and other animals, but once they become established they produce vigorous rhizomes which soon give rise to dense stands of plants which tend to exclude other species. Common nettles thrive on disturbed ground, the natural habitat was probably open woodland on moist or peaty soils and the present day abundance of the species is largely due to human habitat modification, they grow vigorously on phosphate-rich soils and so thrive on farm land and pasture, they quickly invade allotments and gardens and are often associated with rabbit warrens or ground disturbed by moles and other small mammals. Nettles are also very resistant to physical damage and grazing as the rhizomes continue to grow and produce abundant growth during spring and early summer. They are among the first plants to grow in the spring and quickly produce dense carpets of low tender foliage, although this may new growth may emerge in the autumn and persist through the winter in sheltered situations or mild seasons, they recover quickly from physical damage or fire and may be transported to new sites as fragments of rhizome in top soil but they cannot withstand regular mowing or ploughing etc.
The specific name doica refers to the fact that the common nettle has separate male and female plants, they both produce flowers in arching arrays along the tips of main and larger lateral shoots, individual flowers are small but they are produced in abundance and so attract insects, some of which are more attracted to flowers of a particular sex. Flowers on male plants are brilliant yellow from the pollen produced on the anthers while those on female plants have a finely pubescent white or silvery appearance. Rare hermaphrodite flowers are occasionally produced that contain parts of both sexes, and sometimes hermaphrodite plants occur where the upper and lower parts of the plants produce flowers of different sexes.
The smaller U. urens also stings and some people find it more painful than the common nettle, it is typically a darker green and has a characteristic appearance that quickly becomes familiar, but it may be distinguished by the leaf stalk being longer than the leaf it supports, at least in fully developed leaves, and, as it is an annual that develops from seed over a single season and then dies off, it lacks the yellow rhizomes of the common nettle. Both male and female flowers always appear on the same plant. Plants tend to be small and are easily overlooked, they host the same range of insects as the common nettle but because they rarely produce abundant growth they are not so attractive to insects and often lack them altogether, nonetheless they may be worth investigating, especially when the leaves display feeding signs.
Stinging nettle bed, Cassiobury Park ,Watford
Species found on nettles
For many naturalists the obvious attraction of nettles is as a food source for a few nymphalid butterflies, whose larvae tend to occur communally and are readily identified, but nettles also host large populations of various aphids and so are attractive to many predatory bugs, lacewings and ladybirds etc, this is why some carabids regularly inhabit nettle-beds, the most frequently recorded is Demetrias atricapillus (Linnaeus, 1758) but Anchomenus dorsalis (Pontoppidan, 1763) should also be found, especially near farmland and pasture, and ground-dwelling species are common among litter in established nettle-beds. Larger rove beetles such as various species of Quedius and Philonthus sometimes occur when sweeping nettles (the abundant Philonthus cognatus Stephens, 1832) is sure to appear sooner or later), and various Tachyporus are common through the spring and summer among the foliage. Various chrysomelids are often associated with nettles e.g. Neocrepidodera ferruginea (Scopoli, 1763) and Chrysolina polita (Linnaeus, 1758), which both feed on foliage as adults but their larvae develop elsewhere; on grasses and dead nettles respectively, these two should be expected if surveying nettles extensively, but a range of other polyphagous leaf-beetles will also occur Occasionally a species will use nettles as a secondary host e.g. Chrysolina fastuosa (Scopoli, 1763) where both larvae and adults usually feed on dead-nettles, or Psylliodes attenuata (Koch, J.D.W., 1803) which usually develops on various Cannabaceae. Many beetles will be found basking on nettles in warm sunshine and these include some of our larger and easily identified species such as Athous haemorrhoidalis (Fabricius, 1801), Clytus arietis (Linnaeus, 1758), Pyrochroa coccinea (Linnaeus, 1761), P. serraticornis (Scopoli, 1763) and Rhagonycha fulva (Scopoli, 1763), but many others may be found on the foliage including elaterids, nitidulids, lycids, malachids, cerambycids and chrysomelids. In many cases these will be seen basking but often they will be found on the flowers e.g. some Meligethes and cantharids or Rutpela maculata (Poda, 1761), or simply sheltering among the foliage. Ladybirds visit nettles to feed on the abundant aphids, these are usually easy to see and many are distinctive enough to record without taking specimens, any of the aphidiophagous species might be expected but the more usual species on nettles include Adalia bipunctata (Linnaeus, 1758), A. decempunctata (Linnaeus, 1758), Coccinella septempunctata Linnaeus, 1758, C. undecimpunctata Linnaeus, 1758 and Propylea quattuordecimpunctata (Linnaeus, 1758). From the above discussion it will be seen that a wide range of species can be recorded by sweeping or beating nettles; many common and obvious species such as Cantharis rustica Fallén, 1807 or Agriotes lineatus (Linnaeus, 1767) will sooner-or-later appear but as the season progresses there will be a good number of challenging species such as various Longitarsus Berthold in Latreille, 1827 and a range of staphylinids, many of these will be dispersing from nearby host plants or maybe sheltering from rain or strong sun, in any case nettles will provide a range of interesting species for a few seasons but there are a few species (6 in the UK) that are more-or-less restricted to nettles; the adults may e.g. visit umbels or blossom in order to feed, or they may occur adventitiously as they disperse, but they develop on stinging nettles and in this sense are restricted to this host. These species are:
All of these are common in the spring and into the summer and should be expected when working nettles. The Green Nettle Weevil, Phyllobius, is large and conspicuous on stems and foliage and usually occurs in numbers, adults feed on host foliage while larvae grow and pupate among the roots. The small and inconspicuous brentid, Taeniapion, is a local species of southeast England, it is sometimes quoted as rare but we have found it commonly in many locations across the south and it is probably under-recorded. Adults will need to be looked for very carefully in the sweep net but they often occur in numbers in spring and early summer, larvae develop and pupate within stems and adults eclose in the autumn and overwinter in situ. Parathelcus and Nedyus are generally common and often abundant, Nedyus usually more so, adults feed on foliage and larvae develop in lower stems and roots, pupation occurs in the ground and both adults and larvae overwinter. Both are common in spring and early summer and they usually appear again in much smaller numbers in the autumn. Brachypterus adults and larvae occur in the flowers, adults of both species are abundant in late spring and early summer and larvae develop in the flowers in early summer. Both stages may be obtained by placing flowering stems in water for a week or two and then tapping fully-grown larvae onto soil where they will pupate and produce adults within a week or two.
Species found on nettles
Coccinella undecimpunctata Linnaeus, 1758
Neocrepidodera ferruginea (Scopoli, 1763)