Phyllotreta nemorum (Linnaeus, 1758)
Widespread and locally common throughout the Palaearctic region from Europe to the far east of Russia, China and Korea, the distribution includes most of Asia Minor and Europe from the Mediterranean, including many of the islands but not North Africa, north to the UK and into central provinces of Fennoscandia, it has also become established in Australia from European introductions. In the UK it is generally common throughout England and Wales although much more local and scarce in the north west, and there are a few records further north into Central Scotland and from the south of Ireland. The species is narrowly polyphagous on various cultivated and wild Brassicaceae (etc.) and may occur wherever these are present; grassland, arable land, wetland margins, dunes and open woodland, and they are often common on disturbed sites such as gardens, waste ground and allotments. Larval hosts include many species of Brassicaceae but adults have also been recorded from a range of plant families including Capparaceae, Cleomaceae and Resedaceae, hence they may occur when sweeping vegetation in most situations. Adults occur year-round; they overwinter in soil or among litter etc. and are active over a long season from early spring until September, peaking in abundance during late spring. Overwintered adults reproduce in the spring after a period of feeding and survive into June or July, they are fully-winged, often fly in warm weather and may migrate in numbers in response to food shortages e.g., when crops are harvested. Females begin to oviposit a week or so after mating, they attach small batches of eggs to host stems or foliage and each will produce about 150 eggs. Larvae burrow into older leaves soon after they emerge and produce irregular mines as they feed, these mines usually begin near the leaf base and widen towards the apical margin and include pieced of frass, sometimes bound by silky threads, at irregular intervals, and larvae often leave their mine and begin a fresh one in another leaf. Larval development is rapid; they pass through three instars within 15-30 days, depending on temperature, leaving their cast skins in the mine as they do so, and when fully grown they emerge from the leaf and drop to the ground to pupate. Pupation occurs from late June in an earthen cell a few cm below the surface, this stage lasts for 2 or 3 weeks and new-generation adults appear from July. In cooler northern regions there is a single generation each year but there may be two in warmer regions in the south. Adults feed be scraping the surface of leaves, sometimes producing small holes, and this, coupled with larval feeding, may cause localized damage to crops when large populations are present, but the species is usually considered to be a nuisance rather than a serious pest, especially in domestic gardens where brassicas are cultivated. Adults will occur by sweeping, especially among crops such as oilseed rape or cabbage, they may be sampled with flight-interception or yellow pan-traps and they have even been recorded at light.
2.5-3.0mm. Body black with a faint but distinct green or blue metallic reflection, elytra each with a broad longitudinal yellow or orange stripe, antennae black with 1-3 basal segments partly or wholly brown, legs brown with black femora. Head weakly convex and punctured between large and protruding eyes, antennae 11-segmented with all segments elongate; in males with segments 4 and 5 thickened. Pronotum transverse, broadest about the middle and evenly curved to distinct angles, surface punctured throughout and sometimes uneven but without basal fovea or impressions. Elytra broadly-oval and smoothly curved from sloping shoulders to a continuous apical margin, evenly punctured throughout although these may become weaker towards the apex, each with a longitudinal pale stripe which is almost straight internally, sinuate but not deeply indented externally and curved inwards and narrowed before the apex. Hind femora greatly enlarged tibiae without external teeth; hind tibiae with a small spur below the apical margin. Male front tarsi only slightly dilated compared to those of the female. Similar to the common P. undulata but with broader and more parallel elytral stripes and without bicoloured legs.