Atomaria linearis Stephens, 1830
POLYPHAGA Emery, 1886
CUCUJOIDEA Latreille, 1802
ATOMARIINAE LeConte, 1861
ATOMARIINI LeConte, 1861
Atomaria Stephens, 1829
Atomaria Stephens, 1829
This generally common species occurs throughout Europe and North Africa extending east through temperate Asia and north to Scandinavia and the UK where it is common and often abundant throughout the south in a wide variety of habitats. Larvae develop in the soil feeding upon a range of plants, mostly Chenopodiaceae, Poaceae and Fabaceae but the species is best known because the adults may cause serious damage to mangold and beet crops and, to a lesser extent, carrot and potato plants. Away from agricultural habitats the adults occur among decaying vegetation generally, in compost, hay, straw and accumulated leaf-litter. On the continent it remains an occasional serious pest of beet crops in many areas e.g. Belgium, Denmark, Sweden and Holland, but following changes in UK legislation in the 1930’s, which enforced compulsory beet-crop rotation in an effort to control beet cyst nematode, it has rarely been a problem here. Adults occur year-round; they overwinter among leaf-litter, under debris or in the soil, sometimes to a depth of a metre, and become active during mild days in early spring when they aggregate in large numbers on the soil surface prior to dispersing over short distances when the temperature reaches 17°C. Dispersal continues into June as more overwintered adults emerge from the soil, they feed initially during the day on remains of the previous year’s crop or on organic debris etc. but soon become crepuscular and nocturnal and then they may occur at light in numbers. It is the adults dispersing in numbers among crops that do the damage; they feed on the roots and hypocotyls, and bore holes and pits into cotyledons and developing stems, often killing seedlings and sometimes entire areas of crops, in large infestations summer populations of several million per hectare. Such numbers usually follow cold and damp springs when the adults remain in the soil longer than usual. When seedlings have developed past the four true-leaf stage they can usually compensate for the damage and resist further attack. Oviposition begins in May and continues until September and so populations overlap, and there is a second generation during the summer and autumn; each female will lay about 50 eggs in the soil among the host root
system between 20 and 30cm below the surface. Larvae emerge after 5 or 6 days and develop entirely in the soil, generally at a depth between 40 and 60cm, and are fully developed within 4 to 7 weeks; they feed on fine roots and are thought not to cause significant damage to growing plants due to the plants being sufficiently developed before they begin feeding. Pupation occurs in the soil from May or June and the beetles eclose within 10 to 16 days. Adults are active from spring to late autumn and may be swept or attracted to light at any time but huge numbers may occur in both the spring and autumn, especially following rain sufficient to leave the ground damp for several days.
The eggs are tiny, 0.40-0.45mm in diameter and milky white in colour. Fully grown larvae are about 2.3mm long, finely pubescent and pale grey with a broad yellowish head, dark mandibles and protruding labrum. The terminal abdominal segment has two long and stiff spines.
Adults are tiny, 1.2-1.5mm, narrow and elongate, parallel-sided and moderately flattened dorsally, when mature they are dark brown or have the head and pronotum darker than the elytra, and the entire dorsal surface is clothed with fine and pale recumbent. Head transverse from above and with weakly transverse eyes that touch the anterior pronotal margin. Antennae thin and relatively long, basal segment long, curved and thickened towards the apex, segment 5 elongate, 4 and 6 or 6 and 7 quadrate or slightly elongate, 9-11 form a distinct club with the terminal segment as wide as the penultimate. Pronotum quadrate to weakly transverse, with the posterior margin finely and evenly bordered and the disc smoothly convex and closely punctured. Scutellum widely transverse. Elytra completely covering the abdomen and continuously rounded apically, without striae and with punctation random and less dense than on pronotum. Antennae and legs pale orange, contrasting against the body.