Psylliodes chrysocephala (Linnaeus, 1758)
Throughout much of its range e.g. France, Switzerland, Sweden and the U.K. this species remains the most important pest of a variety of Brassica crops but especially of oil seed rape where infestations can cause up to a 40% reduction in yield and this despite many decades of intensive research, and to make matters worse it is known that many populations are now resistant to many forms of the most widely used group of insecticides, the pyrethroids The natural range includes most of the Palaearctic, north to Scandinavia and the U.K., south to North Africa and extending through The Middle East, Asia Minor and Siberia. Host plants include a range of both cultivated and wild Brassicas as well as members of other families but more especially Brassica spp. (rape, mustards, cauliflowers, turnip rape and crucifers generally), Nasturtium (watercress), Reseda spp. (white mignonette etc.), Raphanus (radish), Sinapis (mustards) and Tropaeolum (Nasturtium). Adults occur year-round but are most common in the spring and autumn, they occur in just about any situation where the hosts thrive; agricultural land, parks, gardens and wasteland etc. and they are common on coastal dunes and shingle where they develop on Beta vulgaris (sea beet) among other plants. Prior to overwintering and sometimes before dispersal in the spring the adults may be found on a range of woody plants including oak, hawthorne, poplar and birch. The adults generally overwinter away from crops, e.g. among tussocks and litter or under bark or logs on arable margins, hedgerows and wooded borders etc. and they become active early in the year, February or March. After emerging they feed for a while on any suitable foliage before dispersing by flight to find crucifer seedlings, and at this time they may be found in large numbers as they assemble prior to dispersal. Adults feed on the seedlings, producing small holes in the foliage, but cause little physical damage although this has been demonstrated to increase the likelihood of fungal and viral infections, and they soon begin to lay eggs upon or within the soil around suitable host plants. The eggs are laid singly or in small groups and each female will lay between 70 and 150 eggs over her lifetime. The larvae climb the seedlings and penetrate the stems above the upper surface of the petioles of the lower leaves, they then mine the stems, working up towards growing tips and at this stage the damage caused is significant; when several larvae are present they may cause
the death of the plant, and in large infestations areas of dead plants will be present throughout a crop. They develop rapidly, within 6-8 weeks, and when fully grown move into the soil below the plant to pupate in the top few cm, and new generation adults eclose from late May. Following a period of feeding these will aestivate during the warmest part of the summer and emerge to continue feeding in late summer, mating and oviposition begins in the autumn. Each female lives for up to 18 months and will oviposit twice; during the autumn and again in the spring, in some sense there is a continuous period of egg-laying from October to March or April, interrupted by low temperatures and the need for the adults to shelter during the winter, hence they may occasionally be swept from sheltered sites during mild winter spells. Eggs laid in the autumn may produce larvae that will remain in stems through the winter or they may remain in the soil and produce larvae in the spring.
In the U.K. the typical form of this species has a brilliant metallic blue or blue-green pronotum and elytra, an extensively brown head and, but for the black hind femora, pale appendages. In var. anglica Fab. the elytra vary from pale to dark yellow with the base and the suture variously darkened. There are many named colour varieties on the continent and it is probably fair to say the species varies from entirely dark to entirely pale; var. collaris is opposite to our anglica in having the pronotum orange and the elytra dark metallic, and var. anglicollis is entirely pale. A single subspecies has been described, ssp. inopis (Peyerimhoff, 1915), from Algeria but this may be sufficiently different to constitute a distinct species. 3.0-4.7mm elongate-oval and compact with the rounded elytral shoulders forming a distinct angle with the pronotal base. Head brown, often dark metallic on the vertex, finely and densely punctured, the eyes more-or less continuous with the outline and the frons only weakly impressed. Antennae 10-segmented, pale with segments 3-10 darkened towards the apex. Pronotum broadest at the base and curved to weakly raised anterior angles, finely bordered laterally and with fine and dense punctures across the disc. The elytra are elongate and broadest in front of the middle, with broad but only weakly raised shoulders, and large punctures arranged in distinct striae which are complete to the apex but becoming weaker in the apical third. Interstices flat and very finely punctured, epipleura dark metallic, strongly bordered and with sparse short pubescence in the basal third. Front and middle legs entirely pale or with the tarsi infuscated. Hind femora greatly enlarged, black or very dark brown, tibiae and tarsi pale or vaguely brown. The outer margin of the hind tibiae are weakly curved from the base to the tarsal insertion, and then narrowed to a truncate apex bearing a single small spur. The first meta-tarsomere is long and slender, at least as long as half the tibial length, and the second segment is inserted at a distinct angle. The third segment is strongly bilobed on all the tarsi. The male pro-tarsi are broader when compared with the female, with segments 1 and 3 equal in width.