Plagiodera versicolora (Laicharting, 1781)
This very widespread Palaearctic native is now Holarctic following its introduction and establishment in the United States during the first world war; it is now widespread and locally common, in some areas a pest of nursery stock, throughout the central and northern Nearctic region, it is generally common throughout the Palaearctic region from Portugal to the far east of Russia and Japan, extending to the north of Fennoscandia and south across North Africa, much of southern Asia including Pakistan, India Taiwan, Korea and other parts of the Oriental region. In the UK it is generally common across south and central England north to the Wash and sporadic and rare further north to the Humber, it is generally absent from the West Country and in Wales it occurs mostly in southern coastal areas. Here it is a lowland species but throughout much of its range it occurs to low mountain altitudes; in Morocco to 2000m and in central Russia to 3200m. A wide range of host plants have been recorded including, occasionally, Black Poplar but they typically occur on various species of Salix L. including S. alba L. (White Willow), S. purpurea L. (Purple Willow or Purple Osier), S. caprea L. (Goat Willow), S. x pendulina (Weeping Crack-Willow), S. viminalis L. (Common Osier) and S. udensis Trautv. & Mey (Sachalin Willow). Adults occur year-round in habitats typical of the hosts; mostly wetlands situations e.g. river, pond and reservoir margins, marshy areas, fens, river valleys and flood-plains but they also occur on suitable trees in parks, gardens, roadsides and sand dunes etc. sometimes far from water. Adults overwinter among loose bark and fallen timber or in tussocks or under leaf-litter close to the host and may become active in mild spells, they have been recorded from all months but are generally active from early spring until late autumn, peaking in abundance during May and June. In the UK it is univoltine but across much of its continental and North American range there are two or even three generations through the spring and summer. Overwintered adults feed on the underside of leaves producing small holes and as they often feed communally and in numbers leaves may become skeletonised and the damage may be extensive. They may have mated during the previous season but usually mate again in the spring, oviposition begins during May and continues into the summer, and through this period females
may mate and oviposit repeatedly ; eggs are laid in small batches on the underside of leaves and hatch within 4 to 10 days. Larval emergence is usually synchronized and at this time they are cannibalistic and will consume unhatched eggs, mortality may thus be high, they feed in groups and display defensive behaviour; forming feeding rows or rings with the heads outermost, they usually feed from the underside of the leaves but may also feed in numbers from above. They develop rapidly, passing through 3 instars and becoming fully grown within about 3 weeks, pupation occurs on the underneath of leaves and adults eclose after 2 weeks. New generation adults occur from July until August, they feed and mate through the summer before overwintering but so far as is known there is not another generation in the UK, overwintered adults survive into early summer but have generally perished before the next generation arrives. Adults are readily sampled by beating or sweeping host foliage, they are good fliers and may suddenly occur in numbers in new environments, and through the colder months often occur in extraction samples.
This small and convex species might only be confused with various species of Phaedon Latreille, 1829, but here the elytra are striate while in Plagiodera they are confusedly punctured.
2.5-4.5mm. Entire upper surface dark metallic blue or green, rarely black. Head rather flattened or variously depressed between small and very convex eyes, surface variable; usually smooth, shiny and finely punctured but may be rugose and coarsely punctured, antennae inserted under the lateral margin in front of the eyes and separated by at least twice the length of the basal segment, black with basal four or five segments to some extent pale, at least beneath. Pronotum strongly transverse, smoothly rounded and finely bordered laterally, with prominent anterior angles and very finely bordered basal margin, surface variable, smooth, shiny and very finely punctured to almost entirely coarsely punctured, always more strongly punctured or rugose toward the lateral margin. Scutellum smooth and shiny, central area often raised or with various depressions. Elytra randomly punctured throughout; slightly elongate and evenly rounded to a continuously curved apical margin, with conspicuously convex and gently sloping shoulders and a small depression inside the apical angles. Legs dark with indistinctly paler tarsi and yellow claws. Tibiae gradually thickened towards the apex, without apical tooth on outer edge. Tarsi 5-segmented, the third segment strongly bilobed.
Found on herbaceous plants.