Phaedon tumidulus (Germar, 1824)
Celery Leaf Beetle
The only member of the subgenus Paraphaedon sharp, 1910, this species has a very restricted European distribution; beyond the UK it is known from Tunisia and a few scattered locations in southern France, here it is a generally abundant lowland species throughout the mainland north to the Scottish Highlands and on the Isle of Wight and Man, it is also widespread in Ireland though very sporadic and rare and known from only a few records. Hosts include a range of herbaceous plants such as ground elder (Aegopodium podagraria L.), parsnip (Pastinaca sativa L.), carrots (Daucus L. spp.) and a wide range of other Apiaceae but they are mostly associated with hogweeds (Heracleum L.) and, to a lesser extent, cow parsley (Anthriscus sylvestris (L.)). Adults occur in a range of habitats where the hosts are common, woodland, parkland, hedgerows and field margins etc, they are also common on wasteland, gardens and other disturbed sites, they are present year-round and become active from early February or early March when they may be found on freshly emerging host foliage. Mating occurs in the spring and oviposition continues through the spring and into the summer, eggs are laid singly or in small numbers on the underside of host foliage and larvae emerge within about a week. Larvae pass through three instars and are fully grown within a few weeks, the cycle from egg to adult taking three to four weeks, pupation occurs in a subterranean cell below the host and adults emerge within two weeks; freshly emerged adults feed alongside larvae on host plants for a week or two before aestivating during the warmest part of the summer although at least some adults will be found throughout the summer on host foliage. By late spring larvae may be seen feeding in large number exposed on host foliage, often accompanied by numbers of feeding adults, and the resulting damage may be severe, in extreme cases, which are common, whole plants may be defoliated. The common name derives from occasional large populations which may cause severe damage to cultivated celery (Apium graveolens L.) although nowadays this is very infrequent. The species is univoltine but both adults and larvae overwinter in tussocks and among litter etc around host plants, all current year adults overwinter while only larvae from eggs laid later in the summer will do so. Adults and larvae are easily sampled by sweeping or beating host material, the larvae sometimes in huge numbers, and new habitats are likely to become infested as the adults are good fliers. Adult numbers peak in May and June as overwintered adults are joined by those from overwintered larvae.
3-4mm. Easily determined among our UK species by the smooth and very finely punctured pronotal disc. Entire body, including underside, shiny black with a strong metallic green or brassy reflection, appendages entirely dark metallic except the underside of the two basal antennal segments and the claws which are red. Head coarsely punctured and with a transverse impression between the antennal insertions, eyes strongly transverse and coarsely faceted, distance between antennal insertions much wider than the length of segment 1. Antennomeres 8-11 finely pubescent and dull, contrasting with 1-7. Pronotum broadest across the base and narrowed to rounded (from above) anterior angles, disc very finely punctured (just visible at X20), and surface towards the basal and lateral margins with scattered large punctures. Scutellum large, impunctate and microsculptured as the pronotal disc. Elytra with regularly punctured striae to apex; the ninth weakly impressed and less densely punctured, interstices very finely punctured and microsculptured. All tibiae smooth in externally and lacking terminal spurs. Male pro-tarsi broader than those of the female, the basal segment as wide as the bilobed third, in the female a little narrower.