Phyllotreta cruciferae (Goeze, 1777)
Widely distributed throughout the Palaearctic region, including parts of India and North Africa, this species is locally common throughout Europe north to the UK and southernmost provinces of Sweden, it was first discovered in North America in the 1920s and is now established and widespread across the northern United States and Canada. In the UK it is locally common in south eastern England, especially in the Home Counties, and much more local in the west and across the midlands; there are old records further north and in Wales but the species is not known from Ireland. Adults are present year-round; they overwinter among roots and litter etc. or on moss, tussocks and even dead wood, they become active from March or April and new-generation adults appear from August and so they tend to be common over a long season, they peak in abundance during May and June and again in August and September. In Northern Europe there is a single generation each year but in warmer southern regions there may be two or more. Host plants include a wide range of both cultivated and wild brassicas and in some regions the species is classed as an economic pest but it is rarely of concern in the UK. Brassica species vary widely in their susceptibility to attack and there may be regional or seasonal preferences, but common hosts include Oilseed Rape (Brassica napus L.), cabbages etc. (B. oleracea L. and cultivars) and nasturtiums (Tropaeolum L.), and among wild plants they sometimes occur on e.g. yellow cresses (Rorippa Scop), Hairy Rocket (Erucastrium gallicum (Willd.) and Wild Mignonette (Reseda lutea L., Resedaceae), but it is probably fair to say that most Brassicales are worth sampling for the beetles. Typical habitats are open and often dry grassland, heathland, arable land, coastal dunes and disturbed areas such as roadsides and wasteland but they may often be common in woodland clearings or wetland margins where suitable hosts are abundant. After emerging in the spring adults feed on host foliage before mating, here they usually produce holes about 3 mm in diameter in the epidermis of leaves, petioles, stems and developing buds, they usually do not perforate leaves etc. but the tissue below the feeding point becomes necrotic and assumes a typical ‘shot hole appearance. Where seedlings are developing they may sever cotyledons or apical meristems so that all aerial parts of the plant die off. Later in the season adults may cause more severe damage by disfiguring crops and making them unmarketable or by destroying seed pod epidermal tissue which can cause seeds to dry out and die and may cause fungal infections to affect damaged tissue. Mating occurs over a long season from early spring and females oviposit from May until July. Small batches of eggs are laid in the soil among host roots and larvae emerge after two weeks or so, they feed externally on roots near to the surface and are fully developed within about four weeks. Pupation occurs in an earthen cell and adults emerge after three and four weeks. Adults may be sampled by sweeping likely host material; they usually occur in small numbers but large numbers sometimes occur in the spring and when they migrate from headlands to arable fields. Placing yellow-pan traps among crops can produce numbers of adults, especially during warm spells in spring and early summer when they tend to be active among crops.
Phyllotreta cruciferae 1
1.8-2.4mm. Broadly elongate and depressed, body dark metallic green or bluish, sometimes with the forebody and elytra contrasting, antennae black with segments 2 and 3 pale or sometimes with the apex of the first segment pale also, legs black or faintly metallic. Head with large and convex eyes and strongly converging frontal furrows, vertex and frons finely and evenly punctured throughout. Antennae 11-segmented and filiform in both sexes; males without modified segments. Pronotum transverse, broadest slightly in front of distinct posterior angles and narrowed to obscure anterior angles and a more or less straight apical margin, surface moderately strongly but not densely punctured, without basal impressions. Elytra long and smoothly curved from rounded shoulders to separately-rounded apical margins, surface punctured throughout, a little more strongly so than the pronotum; forming distinct striae across the disc nut otherwise randomly so. Hind tibiae greatly enlarged in both sexes, tibiae narrow and smooth externally, hind tibial spur situated in the middle of the apical margin. Tarsi pseudotetramerous; the widely bilobed third segment partly concealing the diminutive fourth segment, basal segment of the hind tarsus elongate but not more than one-third the length of the hind tibia c.f. Longitarsus. Basal segment of front tarsi in males only slightly wider compared to that of females. Males are usually narrower and more elongate when compared with females, and this is usually obvious, at least in many specimens, when comparing a series of specimens.