Oulema melanopus (Linnaeus, 1758)
Cereal Leaf Beetle
This is a widespread Palaearctic species occurring throughout Europe and east through Siberia to Mongolia, it also occurs sporadically in Mediterranean Africa and is widespread in the Middle East. Following accidental introductions it is now an established and widespread pest of cereals and wild grasses in the United States and Canada where the eulophid wasp Tetrastichus julis (Walker, 1839) has been introduced in an attempt at biocontrol. In the 1990’s the UK population was found to consist of two distinct species and so older data are unreliable but since that time it has proved to be widespread and common throughout England and Wales, including the islands, though more scattered in Scotland with many records from the highlands. Adults occur year-round, peaking in May and June and again in August, they feed in late summer and autumn prior to overwintering and at this time may occur in arable situations in very large numbers; hosts include a range of wild grasses but also cultivated cereals; the favourite host is wheat but it will develop on almost any cultivated crop and has a strong affinity for oats, barley and rye, they overwinter in sheltered situations near the host, among stubble and litter, under hedgerows, in crevices and under bark and in tussocks and moss etc. and become active early in the spring. Typical habitats are arable land, parks, gardens and wasteland in general but they are likely to occur wherever the hosts thrive, they are quick to colonize new areas and seem very tolerant of human disturbance. They commence feeding early in the year, consuming the upper epidermis and mesophyll and leaving long translucent strips along the underside of leaves, and mating occurs once the temperature reaches 9 or 10 Celsius. Oviposition begins in April and continues over 45-60 days; eggs are laid on the underside of the leaves along the mid-vein, they are cylindrical, 0.9mm by 0.4mm and bright yellow but fade to white as they mature. Larvae hatch within a week or two and cover themselves with excrement and debris as a defence against predators and parasites, they then feed voraciously on the upper layers of the leaves and so stunt the growth of the plant; development is rapid and they pass through four instars within two or three weeks. Fully grown larvae ascend the plant or drop to the ground where they
enter the soil and form a pupal chamber lined with secretions, the pupal stage lasts between 20 and 25 days and newly eclosed adults appear from July. Adults feed and disperse by flight in the summer but may be repelled from areas hosting large populations of the species as compounds released during larval feeding attract a wide range of parasites and predators. Adults will occur when sweeping vegetation in general and grassland habitats in particular, they will also appear among suitable extraction samples during the winter.
4.3-5.5mm. Head dark metallic blue to black, vertex finely punctured and with a median longitudinal furrow (varies in strength), slightly concave and roughly sculptured between the eyes, frontal tubercles shiny and flat or sometimes a little obliterated by the surrounding sculpture. Clypeus finely pubescent and raised into a median ridge between the antennal insertions. Eyes very convex and emarginate anteriorly. Antennae entirely dark with the basal segments metallic blue. Pronotum testaceous with the anterior and posterior margins narrowly dark; quadrate or nearly so and without lateral margins; evenly curved from rounded anterior angles to the sub-basal constriction. Surface convex, with sparse large punctures and very fine punctures which are just visible at x40, more densely punctured within the basal constriction and the cuticle between this and basal margin transversely wrinkled or ridged (varies). Scutellum finely punctured (x40), shiny and coloured as the surrounding elytra. Elytra entirely bright metallic blue, each with ten strongly punctured striae and an abbreviated scutellary striole which may consist of weaker punctures, interstices with some finer punctures which may run in a single line in places. Side margin sinuate and smoothly rounded to the apex. Femora pale or slightly darkened towards the apex, tibiae pale with the apex dark, tarsi, including claws, entirely dark.
Among the UK fauna this species might only be confused with the closely similar O. duftschmidi (Redtenbacher, 1874). The genitalia are distinct in both sexes and the habitus is subtly different but in many cases diagnostic; O. melanopus is narrower and more elongate, each elytron >3.3X longer than wide, or the body is <1.46X longer than the elytra; in duftschmidi the elytra are broader, ≤3.3X longer than wide or the body length is ≥1.46X the elytral length (Duff, 2016). In general most specimens will need to be dissected and the genitalia examined to be sure of the identification. The abdomen should be removed and soaked in KOH (or NaOH which is more readily available) until it is soft and swollen, then opened with a lateral cut and laid out flat with the contents exposed on a cavity slide. Among the dark fatty tissue the male structure will be obvious and can be removed onto the slide, more solution can be added and soaked off with a small twist of tissue to remove the mass of fatty substance but this needs to be done very carefully with females as the structures of interest are small and delicate, the median lobe can then be opened to reveal the endophallus which is the diagnostic bit; long and slender in melanopus; thick and truncate in duftschmidi, in some cases the endophallus can be extruded by applying pressure on the lobe, it will often appear at the apex and then slide back in but a glance at the apex is all that is needed. Females are more delicate, the spermatheca is well-sclerotized and will be seen first, once found be sure to identify the spermathecal duct and the bursa copulatrix to avoid any damage when sliding the structure off to the side of the slide. During this delicate operation the KOH may start to crystallize, especially after the structure has been transferred to the flat part of the slide, in which case a drop of organic acid can be added to neutralize the solution; colourless vinegar works well. The form and attachment of the spermathecal duct is diagnostic; in melanopus it is short and attached laterally before the apex of the spermatheca and bursa copulatrix, in duftschmidi it is long and curved and attached to the apex of both structures. After neutralizing with acid and washing with a drop of water a few times then allowing to dry the various structures can be mounted on the same card as the specimen using a drop of DMHF.
Flagellum - lateral view