Oulema melanopus (Linnaeus, 1758) and O. rufocyanea (Suffrian, 1847) - The Cereal Leaf Beetle

O. rufocyanea Male

O. melanopus Male
These two species are very similar in appearance and, so far as is known, undergo a broadly similar life cycle (Cox, 2007). Because of this and the need to discuss their identification, which is not straightforward, we are featuring them as a pair, rufocyanea has only recently been added to our list (Cox, 1995) and so many existing records, where the specimens are not available for confirmation, have been rendered invalid. A separate distribution map, based on verified records only, is given by Cox for each species and we confine our discussion to these. A further, aggregate, map is given which contains many more records and, while it contains unverified records, it does suggest a less local and slightly more widespread distribution than either of the specific maps alone. Oulema melanopus occurs throughout England and Wales including the Isle of Wight, Anglesey and Man and into southern Scotland north to Edinburgh with a single island record from Arran, further north there are a few records east from Inverness to Aberdeen. A more restricted distribution is shown for rufocyanea which occurs across England north to north Yorkshire and there is a single island record from the Isle of Wight. Records for Wales and the West Country are much sparser but it must be remembered that this is a recent addition and that identification is difficult. Both species are common throughout our Watford area although during 2008 and 2009 rufocyanea seems to have been by far the more so, this is based on a few series of twenty or so specimens taken from across the area on various dates through the spring and summer. Host plants are various grasses and the typical habitat is unmaintained grassland in a wide variety of situations e.g. we have found both species throughout Whippendel wood, Common moor, all local parks and wasteland, town centre gardens, road verges and riverside vegetation etc. and on a hot summer day they are likely to turn up just about anywhere. Sweeping low vegetation will soon produce them, on several occasions we have recorded both species from the same locality on the same day but whether they occurred in the same sweep is not known.

The following brief notes on the life cycle are taken from Jones and Jones who referred to the species’ simply as melanopus. Adults overwinter in grass tussocks etc. (We extracted rufocyanea from leaf litter among grass in Whippendel wood during November 2007.) and become active during April, or a little earlier depending on season, and commence feeding on various grasses. They are considered as pests at this time as they feed on the leaves of oats and various cereals, when disturbed they readily take to flight. After a period of feeding they begin mating towards the end of May. A week or so after mating oviposition begins; cylindrical yellow eggs (about 0.9mm long) are laid singly on or near to the midribs of the leaves in a glutinous secretion which hardens on contact with the air. Larvae emerge after about two weeks and feed on the upper epidermis leaving long skeletonised strips. The eruciform larva is yellow with the head black, legs and spiracles dark, and has many stiff, curved hairs; it is generally obscured by a covering of excrement and appears as a small shiny blob on the leaf surface. There are four instars and when fully grown the larva measures 4-6mm. Pupation occurs 7-8cm into the soil, initially the pupa is bright yellow but darkens rapidly to the adult colour. Adults emerge in late summer and feed on grasses until September or October when they hibernate. Our latest record of a swept adult was from Cassiobury Park for an O.rufocyanea on 14/10/2008.

As already mentioned these species are outwardly very similar and so we now give a general description based upon a male and female rufocyanea and a male and female melanopus examined side by side. There are, apparently (Cox, 1995), slight differences between the species but these are relative and, from our observations and several ‘blind runs’ where we have tried to identify by comparison with named specimens, probably inadequate even when comparing with a named series. Supposed differences include size; rufocyanea being on average a little smaller and narrower, elytra; which may be more elongate and less convex in melanopus, and small differences in pronotal shape. Both males and females are readily identified by dissection (see below).

Among our fauna these species are unmistakable; the colour, pronotal constriction, strongly punctured elytral striae and pseudotetramerous tarsi (etc.) are distinctive. 4-5mm. Head dark; metallic blue to black, vertex finely punctured with a median longitudinal furrow (varies in strength), slightly concave and roughly sculptured between eyes, frontal tubercles shiny and flat; sometimes a little obliterated by surrounding sculpture. Clypeus finely pubescent and raised into a median ridge between antennal insertions. Eyes very convex and emarginate on anterior edge. Antennae entirely dark; basal segments metallic blue. Pronotum testaceous to dark rufo-testaceous, anterior and posterior margins (often very narrowly) black. Quadrate or nearly so and without lateral margins; curvature of side margin varies but usually parallel at least in part between front angles and basal constriction. Front angles with a small tubercle which is just visible from above, anterior margin finely bordered. Hind angles slightly acute, hind margin strongly bordered. Surface convex with sparse large punctures and very fine punctures which are just visible at x40. Cuticle within the basal constriction more densely punctured, cuticle between this and basal margin transversely wrinkled or ridged (varies). Scutellum finely punctured (x40), shiny and coloured as surrounding elytra. Elytra entirely bright metallic blue; comparing a series of each species side by side we see no appreciable difference in colour between species, each with ten complete rows of strong punctures, these appear to be a little stronger in melanopus, and an abbreviated scutellary row which may consist of weaker punctures. Interstices, especially the inner one or two, with some finer punctures which may run in a single line in places. Side margin sinuate and smoothly rounded to apex. Femora pale, as pronotum, or slightly infuscate towards apex. Tibiae pale with apex dark. Tarsi dark.

For a certain identification these species will need to be dissected in order to examine the genitalia. The structures of interest are figured by Berti (1989) and by and Cox (1995) who also provides interesting discussion on the nomenclature and the species’ discovery in the U.K. These should be consulted before proceeding in order to gain familiarity with the structures concerned. The abdomen will need to be removed and placed in warm KOH (10% i.e. saturated solution) for a few minutes; our method is to place a drop or two of solution onto a cavity slide and leave this, along with the immersed abdomen, on the warm (not hot) surface of a radiator for a minute or two. If a significant amount of water has evaporated and crystals have formed a drop or two of fresh water can be added. Following this the abdomen will have swollen and can be opened, we find it best to do this at x20 and then use a higher power, around x40 or x50, to examine the structures. Place the abdomen ventral side down and insert a pin into the basal opening to hold it steady, then use another pin or a sharp needle scalpel to gently cut along one side from base to apex so that the tergites can be folded over away from the exposed contents. The mass of soft and mostly dissolved fatty tissue can now be gently removed with a pin into the surrounding solution. When the solution has turned dark it can be removed by placing a small twist of tissue at the edge to soak it up while introducing clean solution at the opposite side using a fine pipette, take great care all the while to avoid soaking any small structures into the tissue. The median lobe of the male will be immediately obvious and can be placed to one side. The female structures are tiny and will need to be looked for very carefully as you work through the abdominal contents, the spermatheca is well sclerotized and will be found first but before removing it you need to find the spermathecal duct and the part of the bursa copulatrix to which it is connected, these structures are not sclerotized and so are more or less invisible, they are also tiny and very fragile and so will need to be moved away from the rest of the contents carefully. At this stage we have tried using a tiny amount of nigrosin aqueous stain in order to see the structures but have come to the conclusion that it is not worth the bother, better to persevere as follows. After removing most of the fatty tissue cut away the bulk of the bursa copulatrix leaving the end attached to the spermathecal duct, the whole intact structure; spermatheca, spermathecal duct and part of the bursa copulatrix, can now be pulled with a fine pin towards the edge of the cavity where the solution is very shallow, be very attentive at this stage because at some point while the structure is being moved the light will catch it just right and all the detail will be revealed.

In melanopus the spermathecal duct is short and attached to the side of the bursa copulatrix and obliquely at the end of the spermatheca. In rufocyanea the duct is much longer and attached end on to both the spermatheca and the bursa copulatrix.

Differences in the males are much more obvious albeit a little tricky to appreciate. Having removed the median lobe it will need to be opened (try to work at x40 or x50) up or manipulated in order to extract the endophallus, this is a sclerotized structure and will immediately be recognised. Sometimes (not often) the endophallus will be extruded if a little pressure is applied to the centre of the median lobe, often only the sclerotized flagellum pops out but the rest can be teased out by inserting a fine needle into the tip and pulling repeatedly until the whole structure emerges. The long, slender and pointed form of rufocyanea is very different to the shorter, broad and blunt form of melanopus.

References

Berti,N. 1989 Contribution á la Faune de France. L’ identité d’ Oulema (O.) melanopus (L.) (Col. Chrysomelidae Criocerinae). Bull. Soc. Ent. Fr. 94: 47-57.

Cox, M.L. 1995 Identification of the Oulema ‘melanopus’ species group (Chrysomelidae). Coleopterist 4:33-36.

O. rufocyanea Male

O. melanopus Male

O. rufocyanea Male

O. melanopus Male

O. rufocyanea Male

O. melanopus Male

O. rufocyanea Male

O. melanopus Male

O. rufocyanea Male

O. rufocyanea Median Lobe

O. rufocyanea Median Lobe

O. rufocyanea Flagellum

O. melanopus Flagellum