Small World Discoveries
by Tony Enticknap - tickspics
Taking a close look at insects and other small species that can be found in and around East Dorset and the New Forest ...
HYMENOPTERA | Stinging Wasps
Ectemnius spp.
HYMENOPTERA > APOCRITA | Aculeata > Crabronidae > Crabroninae
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Although there are ten Ectemnius species on the British list, I believe there are only four, possibly six at most, that I could encounter locally. The most likely are Ectemnius cavifrons, Ectemnius cephalotes, Ectemnius continuus and Ectemnius lituratus, and an outside chance of Ectemnius rubicola and Ectemnius sexcinctus.
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All of these species are extremely difficult, if not impossible, to positively separate from photos and, as such, any attempts on my part to identify and name individuals have to be treated as ‘likely’ at best, hence the use of the cf. qualifier on each accompanying photo. The ‘species accounts’ should be accurate though, and having descriptions for the four main contenders on one page together with a selection of images is useful for comparison purposes.
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The species in this genus form their nest tunnels in dead wood, such as stumps, fallen trunks or rotting logs, either by digging new tunnels or utilising existing holes from a previous season. Wooden fence posts and old building timbers may also provide a suitable nesting site. Whilst these wasps are solitary species, large nesting aggregations can occur and, in some of these situations, common entrance holes may be used by different females who will gnaw out their own corridors. The individual nest cells are provisioned with various flies from several Diptera families. When each brood cell is stocked with sufficient prey species, the corridor will be sealed with chewed wood pulp. Accounts vary for each species, but typically the eggs hatch after a few days and the larvae develop in their cell feeding off the dead flies over the next two or three weeks. The pupal development takes a similar amount of time followed by the emergence of the adult, which will live for two to three months.
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Ectemnius cavifrons
One of the slightly larger (10-16mm body length) species with yellow bands on each abdomen segment. The clypeus has a golden pubescence, and the ocelli form a more-or-less equilateral triangle rather than an anteriorly obtuse triangle that is generally characteristic in this genus. Vertex clearly indented around ocelli. Antennal segment 3 is more than 4x longer than broad and, noticeably in the female, 2x longer than segment 4. In the male, segment 3 has a bulge or notch between two teeth, and further teeth on 4 and 5 that should be visible from good photos. The mesoscutum is frontally punctured, the remainder punctuated and wrinkled. The side (mesopleuron) of the thorax is also wrinkled. The yellow bands on the abdomen can be complete or interrupted when viewed from above.
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The descriptive information for this particular species that has been carefully gleaned from various sources. The listed characteristics are those that may assist id from photos. As yet, I have not been able to attribute these features to any individual I’ve photographed.
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Ectemnius cavifrons is commonly found in woodland, hedgerows, parks and gardens. This species seems to favour syrphids (hoverflies) when looking for prey to provision its nest cells. It is sometimes referred to as the Garden Ectemnius, which tends to imply that it is the most likely Ectemnius species to be found in gardens.
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Ectemnius cephalotes
Although marginal, this is the largest (9-17mm body length) Ectemnius species, which can be identified in both sexes by the densely striate mesoscutum with transverse striae on the front half, longitudinal to the rear. The clypeus hairs are silver, and the ocelli form an anteriorly obtuse triangle. Antennal segment 3 is normally shorter than 3x broad. The black and yellow striped abdomen has a glossy appearance, with S1 being long-haired and, if visible in the male, S7 grooved in the centre. The abdomen bands can be complete or interrupted when viewed from above.
Ectemnius cephalotes is a widely distributed species across much of England and Wales, although notably absent in some areas. It is most likely to be found around fairly large pieces of dead wood where there are umbellifer flowers nearby. It does not favour a particular habitat as it has been recorded in both woodland clearings and open countryside, as well as gardens and at coastal locations. Whilst Ectemnius cephalotes has a peak flying period of July and August, the species may be seen from early June through to the end of September.
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It is one of the species that often nests in dense aggregations where different females may be seen using the same common entrance hole to their tunnels and nesting cells.
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Paul Brock, author of the 'Britain's Insects' field guide, refers to the species as the Large Ectemnius.
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Ectemnius continuus
This is the larger (8-15mm body length) of the two species that have reduced yellow markings on abdomen segments S1 and most notably S3; the other species being the much rarer Ectemnius rubicola. Antennal segment 3 is clearly twice as long as broad and, in the male, segment 6 is noticeably grooved beneath - features that can be clearly seen from the top view of the male (third photo) as well as the anteriorly obtuse triangular shape of the ocelli.
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Ectemnius continuus is a widespread species especially in the south that can be seen from May to September in a variety of habitats ranging from woodland rides and clearings to grasslands, wetland and coastal locations. Adults show a strong attraction to the flowers of umbellifers as they act as a source of both nectar and prey, which are mainly cyclorrhaphan flies such as syrphids, muscids and calliphorids. This species is referred to by some authors as the Common Ectemnius.
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I'm as certain as I can be that the individuals in the following photos are all Ectemnius continuus, despite the cf. qualifier.
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Ectemnius lituratus
Apart from being fractionally smaller (9-15mm body length) which isn’t apparent in the field, this species is extremely similar to Ectemnius cephalotes such that separation of the two becomes near impossible from photos alone. The mesoscutum is described in various keys as either “having pits set in a network of fine lengthwise and feint transverse wrinkles”, the “front half not striate, but with the rear having longitudinal striae with punctures among them” or “sideway stripes on the front half”. Personally, I find that rather confusing and contradictory. Abdomen segment S1 is long-haired as in Ectemnius cephalotes, but in the male S7 is not grooved in the centre.
That above doesn’t provide anything really to help with id, but the species seems to be far more strongly associated with woodlands where there is dead wood and umbellifer flowers, hence the common name of Woodland Ectemnius that is used by at least one respected author. Most of the records for the species are from southern England and, from what I can deduce, it is probably the most likely Ectemnius species I’ll encounter locally in wooded habitats. It can be seen from June to September.
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