Protapion trifolii (Linnaeus, 1768)
This native Palaearctic species is common and often abundant through Europe from North Africa to Scandinavia and the UK, across the Middle East and Caucasus to western Russia, and in many places is a significant pest of agricultural clover. Here it is common throughout southern and central England and Wales, more scattered and scarce further north and absent from most of Scotland although it is present in the Outer Hebrides. The species is likely to occur wherever the host is common, wooded borders, wasteland, roadsides and grassland of all types but more especially arable areas. Hosts include a range of Trifolium species e.g. T. alpestre L. and T. repens L. but, at least in the UK, T. pratense L. (red clover) and T. medium L. (zigzag clover) seem to be preferred, and T. hybridum L. (alsike clover) is most resistant to attack. Adults occur year-round and are generally diurnal, they overwinter among leaf-litter and tussocks etc. along wooded borders and hedgerows and become active during March and April when they assemble on trees and shrubs prior to migrating in the evening to their feeding areas. They spend up to a week feeding upon host foliage before mating begins in April, oviposition begins soon after and lasts for about 3 months, coinciding with the largest abundance of flowerheads, and each female will lay between 80 and 130 eggs, open flowerheads are generally chosen; the female chews through the corolla and lays a single egg between the stamens. Larvae emerge after 2-11 days and pass through 3 instars, developing within the flower or unopened bud, this stage lasts between 19 and 27 days and each larva will consume 7 or 8 ovules or seeds. When fully grown the larva chews a cavity into the receptacle and pupates. Adults eclose after 6-11 days and females generally constitutes just under half the population, they remain on the host until late summer when they migrate in the evening to nearby trees and shrubs to assemble prior to overwintering. The entire cycle from egg to adult takes about a month.
Adults are tiny, 1.7-2.1mm and typical of the genus with a narrow forebody and very convex elytra which forms a distinct angle with the prothorax in lateral view. Entirely dark leaden grey and glabrous above, head densely and strongly punctured with large convex eyes and long temples diverging towards the base. Male rostrum shorter than the head and pronotum combined and narrowed beyond the antennal insertions to the apex, antennae inserted about the middle; female rostrum longer than the head and prothorax and not narrowed to the apex, antennae inserted in the basal third. Antennal colour variable; from almost entirely dark to extensively pale, antennomeres 5-8 quadrate to slightly elongate and without long erect setae. Pronotum quadrate or nearly so, densely and deeply punctured and with a distinct longitudinal basal impression which usually extends to the middle. Elytra with distinct shoulders and evenly curved to the apex, striae deeply impressed and punctured to the apex, interstices flat on disc becoming convex towards the base and apex and very finely punctured. Pro-coxae pale red or yellow, all trocanters black or at least dark, femora extensively pale but usually darkened towards the apex. Tibiae usually bicoloured; pro-tibiae darkened towards the base and apex, meso- and meta-tibiae entirely dark or variously pale towards the base.