Agriotes Eschscholtz, 1829
POLYPHAGA Emery, 1886
ELATEROIDEA Leach, 1815
ELATERINAE Leach, 1815
This is one of the larger elaterid genera; it includes about 150 species (depending on how it is defined) and is very diverse in northern temperate regions, more than 80 species are recorded from the Palaearctic region and of these 35 occur in Europe (the total European elaterid fauna includes about 670 species), the Nearctic region is also diverse with about 40 species (of a total elaterid fauna of about 1000 species) although this includes some invasive species from Europe, tropical regions are generally less diverse and some countries e.g. Australia, have no native species. The genus is notorious as it includes several serious agricultural pests and it is probably fair to say that most of the economically significant elaterids belong to this genus, these are mostly native to Europe, widespread and generally abundant, and several will be familiar to UK coleopterists. The distribution of pest species varies; A. obscurus (Linnaeus, 1758), A. lineatus (Linnaeus, 1767) and A. sputator (Linnaeus, 1758) are widespread but most prevalent in northern regions, while A. sordidus (Illiger, 1807) and A. ustulatus (Schaller, 1783), are also widespread but more prevalent in southern regions. Several other European species are widespread and are occasional pests of crops and turf, but the genus also includes some species of very restricted distribution e.g. A. lundbergi Platia, 1989 occurs in the Near East and on Cyprus, A. siciliensis Pic, 1912 is endemic to Sicily, and A. andalusiacus Franz, 1967 is endemic to Spain. Of the UK species only A. obscurus, A. lineatus and A. sputator are considered as major pest species although various other elaterids, notably Athous haemorrhoidalis (Fabricius, 1801), have their moments.
The life-cycle of most species in temperate regions follows a basic pattern. Adults are active for a few months in spring and early summer, they fly well and spend most of their time on foliage and flowers, mostly various Apiaceae and Asteraceae but they visit blossom and, most notably, may occur on hawthorn blossom in large numbers, they may consume foliage but are not injurious to plants. In certain years they may occur in very large numbers but under natural conditions they suffer heavy predation in the early stages. Mating occurs throughout the season and females lay small batches of eggs into the ground where the larvae will consume a wide range of roots. It is the larvae stage which causes damage to crops, they take several years to develop, and this can be extended over a variable number of instars depending on temperature and nutrition, they can move freely through the soil to find roots if food becomes scarce and they can endure long periods without food. During winter they move lower down into the soil to avoid low temperatures and ascend in the spring to resume feeding, winter is always spent in this stage and fully-grown larvae feed for a while in the spring and then move down into the soil to pupate. They are primarily herbivorous and most are widely polyphagous, each species shows a preference for soil type and humidity, but in general most are fairly eurytopic. Typical habitats are open grassland, pasture, wasteland, parks and domestic gardens; basically they may be abundant on any grassland from coastal dunes to alpine meadows, they are often abundant on disturbed sites and so sometimes occur in sports facilities and among domestic crops and lawns where the larvae, commonly referred to as ‘wireworms’, cause local and very annoying damage, but it is among crops that they can be really destructive, especially where these have large roots or tubers such as beet or potatoes, but a wide range of cereals and soft fruits also suffer significant damage. Because the larvae are difficult to control by conventional methods such as ploughing, a whole industry has evolved to produce insecticides and pheromone traps etc., and the pest species have become among the most studied of all beetles.
Adult Agriotes are very typical elaterids; 4-10 mm, elongate with a broad pronotum and gradually narrowed elytra, drab coloured and finely pubescent and with long and slender appendages. They are classified within the Agriotini Laporte, 1840 of the subfamily Elaterinae Leach, 1815, and as such share the following combination of characteristics: Head hypognathous and longitudinally convex with mouthparts vertically oriented, frons usually truncate, antennae inserted under expanded lateral margins of the frons. Clypeus fused with the frons, edge of the frons incomplete; divided into two antennal keels which may extend to the anterior margin. Antennae slender, almost filiform, at most only weakly serrate. Pronotum articulated with the metanotum, procoxal cavity delimited by the hypomeral projection, prosternal process narrow; towards the apex much narrower than the width of a procoxal cavity and shorter than the mesosternal groove which is plainly visible. Prosternal sutures (which lie more-or-less parallel with the lateral margin) open and deepened anteriorly. Pronotal margin strongly sinuate and deflexed, extending anteriorly to the lower margin of the eye or beneath it. Mesocoxal cavity enclosed by the meso- and metasternum and, on its outer margin, by the mesepimeron. Tarsal claws smooth and without setae at the base. Among our limited UK fauna they may be distinguished by the evenly distributed dorsal pubescence, the second antennomere being as long, or very nearly as long as, the fourth antennomere, and the smooth tarsal claws. Genera likely to be confused with Agriotes are Adrastus Eschscholtz, 1829, which have pectinate claws, and Dalopius Eschscholtz. 1829 and some Athous Eschscholtz, 1829 (among a few others) in which the second antennomere is much shorter than the fourth. A rather simplistic key to our species was provided by Joy in his 1932 Handbook, this remains relevant today and is reproduced below with certain modifications.