Placing some decaying logs up to a couple of metres high in a sunny spot in a garden can provide excellent opportunities for the ‘aerial’ nesters among our bees and wasps. Beech is the best wood, since it has exactly the right texture; oak is reasonable but pine and birch are not quite so productive. In addition, insect boxes, including those made by the German company Schwegler, tend to increase the activity splendidly and the species which have nested in those (some of which have also nested in the logs) are shown on the Insect Boxes page. Images of aerial activity are on the Bees & Wasps in flight page.
The habitat of decayed wood is, of course, ephemeral, since once the substance is suitable for burrowing, it has only a relatively short time to go before falling apart. That’s one reason why sensible conservation practices in woodland, allowing dead wood to decay in its own time unless there is a definite danger to people, are vital, since they ensure continuity.
About 20 logs I placed near the greenhouse and against a hedge in the garden have been well utilised both by those species which use former beetle burrows and those which do their own digging. One highlight in September 2010 was watching two female Ectemnius cavifrons (15mm) nesting in a chunky beech log.
This species is fairly common, resembling Ectemnius cephalotes in many respects, and it preys principally on fairly large hoverflies from June to October, having two broods in some years. The burrows can contain up to 14 cells in a branched system which presumably does little for the stability of the log. Each cell has one egg and the larva has up to 12 flies to feed on. It follows that if nesting in good numbers this species can have quite an impact on hoverfly populations locally.
Interestingly, Ectemnius cavifrons is on record abroad as being parasitised by the fly Eustalomyia hilaris (9mm) and a member of that genus entered the burrow towards the end of nesting. She is pictured emerging.
By accident, many solitary wasps do good work for gardeners by catching plenty of species regarded as pests as prey for their progeny, especially aphids. In the garden in 2010 and 2011 around 20 of the so-called Mournful Wasp Pemphredon lugubris (up to 12mm) nested in the logs and two in bamboo canes.
A female of this species can put up to 40 good-sized aphids in each cell in its branched nest system in decayed wood. The cells are constructed in rows. There are eight species in the genus in Britain, and they look very similar. Both Pemphredon inornata (8mm) and the scarce Pemphredon morio (7mm) nest in the garden. The Red Data Book species Pemphredon rugifera is another from the genus. All are shown in the Woodland pages of this website.
Nest, or entrance, sharing is not common with solitary wasps – an exception is Ectemnius cephalotes, shown on the Dead Wood page of the Woodland & Hedgerow section – but usurpation is found much more often. In October 2007 (pretty late for starting a nest) an Ectemnius continuus female took over a Pemphredon lugubris burrow. The latter, who had more reason than most to be mournful, tried to keep possession but as a smaller species there was no chance of her succeeding.
Given the greater size of prey Ectemnius continuus catches – the picture above shows the female usurper with one of her ‘catch’ – and the fact that they have one cell with up to eight flies per ‘branch’, the nest required considerable extra excavation, the results of some of which are shown above. Despite the late start, the female completed her nest.
Various other species have nested in the logs. The winners in sheer numbers are Stigmus solskyi and Stigmus pendulus (6mm), which also prey on aphids. The latter is expanding its range, having first been identified in Britain in 1986.
Crossocerus annulipes (7mm) is a decayed-wood burrower preying on Hemiptera and at least eight nested in the logs in 2010. It is in flight from June to October and the nest can contain 20 cells with anything up to 25 prey items in each, a formidable total. The nests are branched and this can involve a significant amount of woodwork. The female who did the drilling pictured above in 2010 was stocking a nest as late as the second week of October and on one warm day she managed to take 18 prey items in an hour. The next year a female was still catching prey on 26 October and attending her nest on 29th.
From the same genus, and the same size, Crossocerus distinguendus uses aphids or Diptera to stock its nest which can be in a fairly wide range of sunny locations including sand or gravel slopes, vertical banks, decaying timber or in the soft mortar of old wall.
There were at least four Crossocerus distinguendus females nesting in the logs in 2010 and although officially this species is nationally scarce that assessment must be questionable. They used to be restricted to the south-east of England, especially Kent, but appear to be expanding their range, having been found in such widely-separated counties as Somerset, Shropshire and Yorkshire. Clearly a species that is doing pretty well.
Mimumesa dahlbomi (7mm) deals with Homopteran bugs and is fairly common. Rhopalum clavipes (7mm) can take a wide variety of prey, including Diptera, though in Britain Psocidae (book lice) and Psyllidae tend to be used as with the female shown above heading for her nest.
This is a widespread species seen at any stage from spring to autumn and despite the books stating that they usually nest in plant stems, at least three nested in one of the logs in the garden in 2007, including one in September and October. She was a tremendous huntress, catching an average of five prey items an hour for the two afternoons I watched her.
From the same genus and also common is Rhopalum coarctatum (7mm). This species uses similar locations and can have very long nests, up to 29 cells, though the average is half a dozen. Interestingly, old nests sometimes are re-used, quite possibly by the generation which emerged from them. Males, as in the image, have colourful legs and antennae.
The Passaloecus genus tends to have pale colouring on the mandibles, catch aphids and be in flight from May to September, nesting in pithy stems or old beetle borings. They are all of similar dimensions, about 6mm long, and the three shown here, all of which nest in the garden, are relatively common. Passaloecus corniger is perhaps the most interesting as it often steals prey from other wasps. Passaloecus gracilis and Passaloecus singularis are not noted for doing this to the same extent if at all. The use of resin to make partitions in the nest and combination of resin and stones to effect closure is standard.
Spilomena beata was one of the most interesting sightings during 2010, with two males and one female turning up on one of the logs. At only 2.5mm or so these were the smallest solitary wasps I've ever come across, and indeed they are the smallest in Britain. There are four species nationally, all catching thrips or possibly aphids and nesting in stems or beetle holes. Males uniformly have much brighter markings, chiefly on the face as the yellow on the pictured wasp confirms.
Pseudomalus violaceus (7mm) is a cuckoo wasp which preys on Pemphredon and Passaloecus species among others. Eggs are laid in the cells with the resulting larvae eating the food store of the host. This is a scarce species.
Images © Jeremy Early. All rights reserved.