Galerucella nymphaeae (Linnaeus, 1758)
Galerucella sagittariae (Gyllenhal, 1813)
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POLYPHAGA Emery, 1886
CHRYSOMELOIDEA Latreille, 1802
GALERUCINI Latreille, 1802
Galerucella Crotch, 1873
These species are morphologically virtually identical and this has lead to some confusion regarding the distribution, it seems likely that they are sympatric over most of their European range, that both occur over most of the Palaearctic region and that one or both may occur in North America. In Europe both are generally common from the Mediterranean to the Arctic Circle and both are both are common throughout England, Wales and southern Scotland, becoming more local and scarce further north and in Ireland. They cannot be separated by DNA comparison and, while they do not naturally hybridize, the F1 generation has been found to be fertile under artificial conditions. Much research has been undertaken regarding the Morphometric differentiation of the species e.g. Hippa & Koponen (1986) but there are no reliable external characters and the genitalia seem to be so (narrowly) variable as to be of no use in this respect. They are readily separated from other members of the genus by the form of the pronotum which is concave behind a median lateral projection and has the disc glabrous and shiny with a few coarse punctures. Differences between the species do become apparent when comparing series; sagittariae is smaller (at most 6 mm), overall paler in colour and has a continuously-rounded elytral apex whereas nymphaeae is larger (up to 8 mm), overall darker in colour and has the elytral apex slightly produced. This seems to be rather clear-cut but it often becomes really difficult assigning a specimen simply from these characters, and as both species usually occur in large numbers it soon becomes obvious that they vary more than the above features might suggest. Despite this morphological ambiguity the species are easily separated by taking note of what plant they were associated with in the field; nymphaeae occurs on water-lilies while sagittariae does not, and because they usually occur in numbers it is easy to find a good series of each for comparison. A few years back we (Dave Murray and David Hodges) wrote a page about these species for the website of the Watford Coleoptera Group, this includes some interesting information and can be accessed HERE.
G. nymphaeae
Occurs almost exclusively in still or slow-moving water bodies where the host plants are common, they fly well and may soon colonize new habitats such as garden ponds hosting exotic lily cultivars during the spring and summer. Adults are present year-round; they overwinter in stems of reeds (Phragmites australis (Cav.) etc) and rushes (Juncus L.) or at the base of leaf-sheaths of a variety of wetland plants e.g. Reed sweet-grass (Glyceria maxima (Hartm.) and are active from March or April until the autumn, peaking in abundance during May. Adults colonize water lilies (Nymphaea L. and Nuphar Sibth & Sm.) in the spring, they feed on the upper surface of developing leaves and mating begins soon after a period of feeding. These overwintered adults will survive into the summer alongside the new-generation, and some mating between the generations will occur but mated females will not oviposit until the following spring, alongside those that mate at this time. Eggs are laid in small groups on the upper surface of floating lily leaves and larvae emerge after a few days. Larvae often feed in small groups, they move slowly making irregular feeding furrows which do not destroy the lower epidermis (although this will later rupture), and develop rapidly; they pass through three instars, each developing within five or six days and pupation occurring after about four weeks. Pupation occurs on the upper surface of leaves, often in small groups, and adults eclose after about two weeks. The entire cycle from egg to adult takes about two months, and as egg laying may continue over a long period in the spring and early summer larvae, pupae and adults are often found together. In the UK the species is thought to be univoltine while in Central Europe several overlapping generations may develop in a single season. Sampling adults from floating lily leaves is a safe way to find the species but very large populations can build up and this, coupled with extensively degraded host plants, may force them to disperse to other plants to feed and under such conditions it has been shown that larvae can feed and develop on some of the usual host plants of G. sagittariae.
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G. sagittariae
This species usually occurs on or near wetland margins or in fens, bogs or peat-cuttings etc. but it will also thrive in drier situations where suitable host plants are abundant. In the UK the usual host plants are various semi-aquatic and terrestrial species of dock (Rumex L.) and a range of Rosaceae, but in Northern Europe the following plants have been listed as regular food plants: Marsh Cinquefoil (Comarum palustre L.), Tufted Loosestrife (Lysimachia thyrsiflora L.), Cloudberry (Rubus chamaemorus L.) and cultivated strawberry (Fragaria x ananassa Duchesne). Larvae less often occur on Arctic bramble (Rubus arcticus L.), Stone bramble (R. saxatilis L.) and wild strawberry (Fragaria vesca L.) as well as species of Alchemilla L. and other species of Rumex. Adults are present year-round, they overwinter in tussocks and plant stems and are active from April until September or October, peaking in abundance during May and June and often again in late summer. They appear on host foliage in the spring and feed on the lower epidermis of leaves, leaving the upper surface intact. Oviposition occurs over a long season from early May until late July; eggs are laid close together in batches of up to 25 on the lower surface of leaves and larvae emerge after about a week. They feed externally and usually in groups on the lower epidermis and they develop rapidly, passing through three instars and becoming fully-grown within four weeks. First instar larvae are unable to feed on water-lily leaves as they cannot grip the surface and soon drown but later instar larvae can do so, at least under artificial conditions. Pupation occurs on the underside of leaves, usually in small groups, and adults eclose after about two weeks. The entire cycle from egg to adult takes about eight months and, at least in the UK, the species is thought to be univoltine. Overwintered adults persist into the summer and new generation adults occur from July; pupae have been found from July until late September. Adults should be sampled from appropriate host plants; they often occur in pitfall or flight-interception traps but taking them from host material is a more reliable measure of their identity.