Olibrus aeneus (Fabricius, 1792)
This is a widespread and generally common species native to the Palaearctic and, following introductions, now Holarctic in distribution. It occurs throughout England and Wales, including Man, north to southern Scotland. Adults are active from early spring until late autumn and may occasionally be found in winter flood refuse; they occur in a wide range of habitats including wasteland, parkland and grass verges, especially where the host plants occur, but in general may be swept from most fairly dry and open situations. The species is oligophagous on various Asteracaea; the adults will be found on a range of flowers but more especially those of ox-eye daisy (Leucanthemum vulgare L.), scentless mayweed (Tripleurospermum indorum (L.)), chamomile (Matricaria chamomilla Gilib) and tansy (Tanacetum vulgare L.) although in hot and dry weather they fly and may be swept from grass and other low vegetation away from the hosts. The larvae are generally phytophagous, feeding upon seeds as well as the receptacle but also occasionally attacking the larvae of other insects occupying the same habitat. Oviposition begins in early spring and continues for 6-8 weeks; the female will lay a single egg, or occasionally 2 in each flower, generally those on terminal stems, between the bracts and the tubular flowers. The larvae feed within the flowers but may move between flowers to continue feeding and when mature move down the stems to pupate in the ground. New generation adults appear in mid or late summer. In some continental countries and areas of North America and Canada the species has been introduced as a possible biological control agent of scentless mayweed where this can be a serious pest of forage crops.
With experience this species will be recognized in the field; it is small, 2-2.5mm, convex and with a continuous and characteristic outline which is broadly oval but distinctly narrowed towards the apex. Also distinctive is the metallic green reflection seen in most specimens although black specimens are common. The entire upper surface has a mixture of fine and very fine punctures but only the elytra have reticulate microsculpture. The head is general mostly hidden under the pronotum so that it appears very transverse with the eyes touching the pronotal margin. The antennae are pale or darker towards the apex or pale with the club dark; the terminal segment is constricted towards the apex which is characteristic of the genus. Each elytron has 2 well-developed sutural striae; among our U.K. species only 2 have the outer stria ending well before the apex, the present species as well as O. millefolii (Paykull, 1800) but here the head and pronotum are also microsculptured. As well as the 2 sutural striae most specimens have 3 or 4 much more weakly impressed although very distinctly punctured striae on the disc , and in some there are 1 or 2 strongly punctured and well-impressed striae close to the lateral margin. The scutellum is small but distinct. The legs are dark yellow to almost black; the femora are short and hidden in dorsal view, the tibiae are short and broad, weakly curved laterally , distinctly punctured and have a pair of short and stiff spines at the apex. Tarsi 5-5-5, the first segment is small, 2 and 3 at least to some extent bilobed, 4 tiny and often not visible within the lobes of the third, and the terminal segment long and widened towards the apex. The claws are smooth, strongly curved and toothed at the base. The second segment of the pro-tarsi in the male is much more dilated when compared to that in the female.