Eat or be eaten: pollen beetles on pak choi

During a recent visit to our local community farm (Highbridge, Hampshire, southern England), I was taken to the pak choi (Brassica rapa chinensis) plot and asked to look at the large numbers of small insects on this crop as there were concerns that they might be significant pests.

Insects on a head of pak choi

On seeing these, my first thought was that most were pollen beetles (Nitidulidae), but that there were also other groups including some flea beetles (Chrysomelidae: Galerucinae: Alticini) and thrips (Thysanoptera). However, all of these are small (below 3mm in length) and in groups where species can be hard to separate, so a selection were collected and taken away to be invetigated more closely.

Dorsal view of the pollen beetle (approx 2.5mm long excluding appendages)

The dorsal view clearly shows this to be winged, dark in colour with a metallic blue-green tinge, and with orange-brown clubbed antennae. Without delving too deeply into diagnostic morphology, the presence of small curved indentations on the underside of the last abdominal segment, plus 3-segmented antennal clubs shows this to be the genus Meligethes. Then, features such as simple tarsal claws, front tibiae with tiny regular teeth, a pronotum with a narrow rim, Brassicaceae host plants and elytral punctures no more than two diameters apart confirm it as the common M. aeneus. For more detail on the identification of British Nitidulidae (Meligethinae), see Kirk-Spriggs (1996).

Pronotum (to the right) showing the side rim, plus punctures no more than two diameters apart.

Teeth on the front tibia

Having also collected some samples of the host pak choi plants, I investigated some young pods which appeared to show feeding damage. Inside these I found some small (2-3mm) beetle larvae which from their general form I believe to be Nitidulidae, presumably M. aeneus, although I hope to raise these to adult for confirmation.

Dorsal view of a beetle larva from pak choi, noting brown sclerotised (hardened) head and legs (and first segment to a lesser extent) and overall 'rice grain' shape.

A closer view showing the segmentation of the antennae (towards the outside) and palps, with curved mandibles towards the centre.

Returning once more to Kirk-Spriggs (1996), M. aeneus adults emerge from hibernation in spring and (sometimes having fed on pollen of other species) fly to, and lay eggs on, developing buds of a range of host plants, on which the larvae then feed once hatched. Pioneering work by Heeger (1855) noted larvae feeding on unripe seeds, though more recent larval research and observations have tended to focus on feeding on flower buds, especially on the commercially important oilseed rape (Brassica napus). Certainly, adults feeding on flower buds can cause them to abort with subsequent loss of pods. This may be important for commercial oilseed growers, but for a community farm where the interest is in cropping the leaves of pak choi, this is not a problem - in fact, a reduction in flowering and seed-setting may even be desirable! Once feeding is complete, the mature larvae drop to the ground, bury themselves and pupate within an earthen cell.

Having determined the species of the most numerous beetle, I moved onto the flea beetle - again a group often considered to contain a number of pest species.

A flea beetle collected from pak choi

In this case, for more about identification, try Hubble (2010), though note that this is a draft and hence a work-in-progress; key features are the dark antennae with segments 4 & 5 unmodified, the lack of paler patterning, the lack of a pronotal groove/furrow, and the apical spur of the hind tibia being located at the middle of the lower edge. These features lead to an identification as Phyllotreta nigripes.

Hind leg of P. nigripes showing the position of the apical spur (at the 'knee').

As noted by Cox (2007), this species is now widespread in the southern half of England (scattered elsewhere in the country) and can be found on a wide range of Brassicaceae, both wild and cultivated. Adults feed on leaves making small holes, while larvae feed on the roots, usually at around 15-20cm depth (Balachowsky 1963). Clearly such feeding behaviour could impact on plant productivity, but even with the large numbers of M. aeneus feeding alongside what appears to be less abundant P. nigripes, the pak choi crop appears to be doing well.

So, moving onto the last of the three species I collected - a thrips - the smallest at no more than 2mm in length.

Not the best photo I have ever produced, but the long wings with black and white banding is indicative of the genus Aeolothrips. Despite their tiny size, this banding is visible with the naked eye.

These thrips were numerous and highly active, and are likely to be the species A. intermedius which is common and known from a wide range of plants in Britain (Kirk 1996). Although thrips (which is spelled the same in the singular - there is no such thing as a 'thrip') are often considered to be pests and may be found in huge numbers on cereals, many are beneficial. This may well be the case here, as Aeolothrips is one of the genera containing predatory, rather than plant-feeding, species - hence they may target the potential (even if not currently problematic) pest species noted above. This predatory behaviour has been documented in recent work by Conti (2009) who found that both larvae and adult females are generic predators and may even help control other plant-eating thrips species.

So, what can be concluded here? My thoughts can be summarised as follows:

• 
  
• A large population of potential pests (mainly M. aeneus) are present and breeding on crops at the community farm, but these do not appear to be affecting yields.
• One reason for this lack of impact is that the stages being cropped are not those primarily affected (i.e. leaves are being cropped and there is no requirement for the crops to set seed - something that may actually be undesirable). It is clear from the literature that commercial oilseed crops may be impacted.
• It is possible that the wildlife-friendly ethos of the farm is helping as the field margins are allowed (indeed encouraged) to support a wide range of 'weed' species. Observations may confirm this suggestion, but it is possible that these 'weeds' act as 'companion plants' providing alternative food sources for potential crop pests. In a monoculture, this would not be the case.
• Alongside these species are what appear to be a healthy population of at least one small invertebrate predator which has been suggested as a natural (and native) control of potential agricultural pests. It is likely that this would be impacted by attempts to chemically control M. aeneus even if chemicals were chosen to avoid impacts on other key groups such as bees. Fortunately the farm project is chemical-free.

References

Balachowsky, A.S. (ed.) (1963). Entomologie Appliquee a L'Agriculture. Tome 1. Coleopteres (second volume). Masson, Paris.

Conti, B. (2009). Notes on the presence of Aeolothrips intermedius in northwestern Tuscany and on its development under laboratory conditions Bulletin of Insectology, 62 (1), 107-112 Other: 20093173454

Cox, M.L. (2007). Atlas of the Seed and Leaf Beetles of Britain and Ireland. Pisces, Newbury.

Heeger, E. (1855). Beitrage zur Naturgeschichte der Insekten. Sitzungsberichte der Akademie der Wissenschaften. Mathematisch - Naturwissenschaftliche Classe. Wien. 14: 273-281.

Hubble, D. (2010). Keys to the Adults of Seed and Leaf Beetles of the British Isles (Coleoptera: Bruchidae, Orsodacnidae, Megalopodidae & Chrysomelidae). Test Version 2010. Field Studies Council, Shrewsbury.

Kirk, W.D.J. (1996). Thrips. Richmond, Slough.

Kirk-Spriggs, A.H. (1996). Pollen Beetles. Coleoptera: Kateretidae and Nitidulidae: Meligethinae. Handbooks for the Identification of British Insects 5(6a). Royal Entomological Society, London.

Diversity: it's the talk of the chalk

The south-east of England is known for a number of things - too many rich bankers in over-priced houses, an excessive number of golf clubs (there's a correlation here I think!) and easy access to France by tunnel. However, from an ecological point of view, it is also well known for its calcareous grasslands growing on the cliffs and Downs pushed up when the ripples of the 'Alpine orogeny' (formation of the Alps) buckled what is now the south coast of England.

These grasslands are often exposed to the weather, hot when sunny, slightly alkaline (pH around 7.5-8), windswept, free-draining (so, often dry), low in nutrients due to leaching, and often closely grazed (by rabbits and sometimes sheep). Hence, though conditions are too challenging for more ruderal plant species, millennia of such conditions mean that there is a diverse array of plant and animal species adapted to grasslands of this type. Now, I don't intend to attempt a summary of the ecology, natural history and conservation management of southern England's chalk grasslands - these are covered elsewhere (e.g. Rodwell 1992, Crofts & Jefferson 1994). Instead I simply wish to provide a snapshot of what is visible during a brief visit - in this case during an Open University field trip in July 2011.

The site was the top of the cliffs just east of Birling Gap in East Sussex (near the more famous headland of Beachy Head). The cliffs are chalk with flint beds and have their own interesting flora and fauna such as the splendid lygaeid bug Henestaris laticeps - a specialist of cliff-faces where Buck's-horn Plantain (Plantago coronopus) grows, and easily identified by its stalked eyes.

H. laticeps (5-6mm long) showing stalked eyes. The similar H. halophilus is found in saltmarshes and has shorter eye-stalks. The function of the stalks remains unknown though noted by Southwood & Leston (1959).

However,cliff specialists might one day form a post (or series) of their own, so to keep the focus squarely on the cliff-top grasslands, let's take a walk uphill and see what can be found...

As this is a grassland, it seems sensible to start with the plants. Chalk grassland is noted for its high diversity - often a mosiac of mat-forming and rosette species with a scattering of upright species. These low-growing species are well placed to spread their leaves to maximise photosynthesis as well as spreading their roots to capture water as it swiftly drains through the soil. Those with rosettes are low-growing only until it is time to flower, at which point a spike grows quickly, flowers and sets seed, hopefully avoiding grazing or other potential hazards. Rosettes of some species such as P. coronopus may have downy hairs in the centre which trap moisture (including dew) and may also disrupt airflow sufficiently to reduce water loss through evapotranspiration.

Squinancywort (Asperula cynanchica), a prostrate mat-forming plant with small pinkish-white flowers and tiny linear leaves.

In the face of limited water and nutrients, other species manage the energy balance a different way, growing a larger stem and becoming upright. Some of these, such as Viper's Bugloss (Echium vulgare) shown below protect their investment in biomass (which may be offset against less rapid reproduction and/or reduced energy reserves) by developing bristles or spines to deter herbivores.

A clump of Wild Mignonette (Reseda lutea)

The small yellowish flowers of R. lutea.

The blue and pinkish flowers of E. vulgare (bristly hairs can be seen).

In the absence of abundant nutrients, ruderals (which are stronger competitors than chalk grassland plants when conditions are favourable) are absent, or heavily disadvantaged, and so those species which compete poorly but can tolerate chalk grassland conditions are able to form the high-diversity mosaic mentioned, in some cases taking advantage of opportunities to colonise bare patches of soil (a thin rendzina) produced by erosion and other processes. In this case, for those interested in the NVC, or National Vegetation Classification (Rodwell 1992), although I have not formally surveyed/analysed the site, it appears to be close to a CG2 Festuca ovina-Avenula pratensis grassland; with abundant Carex flacca, Lotus corniculatus and Thymus polytrichus (among others) it may be CG2a, the Cirsium acaule-Asperula cynanchica sub-community. This is an informal assessment, so apologies to the site managers if it's wrong!

Leaving the NVC aside, such a high diversity of plant species naturally suggests a high diversity of invertebrates. I'm not aware of a species list for the site (I'd love to survey it!), but even a single visit on a moderately warm, fairly cloudy day with a little rain provided numerous interesting sightings.

Yes, I know, more beetles in copula... this time it's the Bloody-nosed Beetle Timarcha tenebricosa on the gorse fringing the grassland along its northern edge, parallel to the southern cliff edge. These are large (up to 18mm) leaf beetles which get their common name from their ability to ooze red liquid when disturbed. This pair was too busy to bother (as were several others).

A shell of the 'Garden Snail' Cornu aspersum (formerly Helix aspersa) which has been used as a nest by one of the Osmia bee species that make shell-nests.

These Osmia shell-nesters use the internal coils as they might any other handy crevice and there are three species relevant here; O. aurulenta, O. bicolor and O. spinulosa (also known as Hoplitis spinulosa). Although somewhat anecdotal, Andrewes (1969) notes that O. aurulenta closes its shell-nest with a plug of felt-like plant material as seen here (it is not dung, although it does appear so) while O. bicolor uses a fine 'rubble'  of shell and stone fragments and covers the shell with a mound of plant material (Edwards 1998) which was not seen in this case, though a shell hidden under the edge of gorse had been chosen, assuming no movement of it by humans. Edwards (1998) also lists H. aspersa as a shell used by O. aurulenta, but not O. bicolor; similarly, Edwards & Roy (2009) note that H. spinulosa uses smaller (i.e. medium-sized) shells such as Cepaea nemoralis and is not suited to the hard-grazed grassland seen at Birling Gap. So, my tentative suggestion is that this is a nest of O. aurulenta (a scarce/Notable species and probably the least common of the three mentioned here) - however, observations of adults would be needed to make a certain identification to species. Maybe on my next visit!

References

Andrewes, C. (1969). The Lives of Wasps and Bees. Chatto & Windus, London.
Baldock, D.W. (2008). Bees of Surrey. Surrey Wildlife Trust, Woking.
Crofts, A. & Jefferson, R.G. (eds.) (1994). The Lowland Grassland Management Handbook. English Nature, Peterborough/Wildlife Trusts, Lincoln.
Edwards, R. (ed.) (1998). Provisional Atlas of the Aculeate Hymenoptera of Britain and Ireland. Part 2. BWARS/BRC, Huntingdon.
Edwards, R. & Roy, H. (eds.) (2009). Provisional Atlas of the Aculeate Hymenoptera of Britain and Ireland. Part 2. BRC, Wallingford.
Rodwell, J.S. (ed.) (1992). British Plant Communities 3: Grasslands and Montane Communities. CUP, Cambridge.
Southwood, T.R.E. & Leston, D. (1959). Land and Water Bugs of the British Isles. Warne, London.

Bee-boys makin' with the freak freak

OK, yes, I admit it - I've given this post a title derived from a terrible pun of a Beastie Boys track... but, as you will see, it's kind of appropriate...

This is one of those posts triggered by a single, initially bemusing, observation. In this case, a cluster of smallish-to-medium (approx 10mm long) bees in the empty seed head of a Snake's-head Fritillary (Fritillaria meleagris) flower.


Here you can see four bees clustered into the old seed head - at most I found eight (five in one head and three in another, with only the most upright stems chosen). I had to wonder what they were doing - roosting, some type of reproductive behaviour, or something else entirely? Also, I wanted to know why there appeared to be two forms - one with creamy-white hairs, another with orangey hairs. So, first stop - identification.


This bee has clear abdominal hair bands and has posed helpfully so that its wing cells can be seen. I don't want to delve too deeply into diagnostic morphology on this occasion, but there are some features that need to be looked at. Firstly, there are three submarginal cells (below the costa or 'lump' on the front edge of the forewing, and the 3rd one (nearest the tip) is longer than the middle one. Another feature might be the grooves below the antennae but these are obscured by the hairs on the front of the face. However, what about the eyes? Are they hairy or not? The photo above suggests they are, but...


Here, you can see that the eyes are not hairy - it's important to look from the right angle! Now, we need to look at the antennae and tarsal ('foot') segments.

Looking closely, it is clear that the antennae seem knobbly (in other species they are smooth and cylindrical).


OK, not the clearest picture ever, but the last tarsal segment (the one that has the claws) is broadened, a bit like the shape of a teardrop, rather than being narrow and elongate - here it also had a reddish-brown colour.

So, what does this tell us? Well, working through the various options, it brings us to the genus Melitta and not the other genera that look very similar such as Andrena. There aren't many species of Melitta in southern England:

• M. haemorrhoidalis has only faint abdominal bands in the male and red hairs at the abdominal tip in the female. It isn't this (as it happens I know this species so could discount it easily).
• M. tricincta only feeds on Red Bartsia (Odontites verna) - there isn't any here.

This leaves us with M. leporina - it has the clear abdominal bands, and is yellowish when fresh, becoming paler as it ages - hence the two 'colour forms' - they are simply different ages. It feeds on clover and vetches, and our garden has plenty of white clover, which is handy because separation from M. tricincta would otherwise need a microscope...

So, to confirm, I watched for a while and saw the males eventually move from the seed head and start scouting around the clover, though they didn't land or feed. Then, an individual - a female I think - did appear and fed for some time.

This specimen of M. leporina on white clover shows not only the white hair associated with old age (I know the feeling...), but also the wear on the wings - you can see the tattered edges.

So, what's going on? Well, I did a bit of reading and it turns out that M. leporina males form little temporary roosts near clovers and vetches which they use as a base to seek females. The males rarely stop and feed, but the females do - then in the evening when the temperature cools, they return to the roost. I imagine the clustering keeps them warm, and using only taller stems provides some protection from nocturnal ground-dwelling predators. In the end, it turns out that this behaviour involves both roosting and reproduction - it's also good to see in our bee-friendly garden as this species, in Britain, is found mainly in SE England and is declining - it isn't currently listed as 'scarce' or 'rare', but as Edwards (1998) says, this status should be reviewed. I'll keep looking out for them, and their unsurprisingly scarce parasitic bee Nomada flavopicta, and hopefully the seed heads won't stay empty...


References

Baldock, D.W. (2008). Bees of Surrey. Surrey Wildlife Trust, Woking.

Edwards, R. (ed.) (1998). Provisional Atlas of the Aculeate Hymenoptera of Britain and Ireland. Part 2. BWARS/BRC, Huntingdon.

Falk, S. (1991). A Review of the Scarce and Threatened Bees, Wasps and Ants of Great Britain. NCC, Peterborough.

Diary of a farm pond: July 2011

If you are a regular here, you'll know that I'm writing a range of occasional series - 'what's in a gall', 'wildlife-friendly gardening' and so on. One of these is the diary of the pond at Highbridge Farm (Hampshire, southern England) - for pictures of the pond itself, see the April diary entry.

Events have moved on - from a summery spring to an autumnal summer, and the combination of rain and warmth has produced a lot of plant growth, plus of course the change of species with the seasons and the development of a seeded meadow around one side of the pond. It's also important to remember that this is a 'working' pond i.e. it is used for small-scale irrigation. So, what has changed recently?

Well, as expected there are more dragonflies and damselflies (Odonata) such as the blue-tailed damselfly (Ischnura elegans) below with its bright blue 'tail-light' - plus some large nymphs which must be close to emergence. Fingers crossed for some good photos of these, including some exuviae (shed skins).

Blue-tailed damselfly Ischnura elegans

Within the water itself, a number of molluscs have appeared, including the impressive (by British standards) Great Pond Snail Lymnaea stagnalis - this one is around 4cm long...

Lymnaea stagnalis feeding on algae and plant material.

Personally, I hope the mollusc community develops a taste for the algae in the pond - with warm temperatures comes blanketing algal growth; last year this was removed by hand (aided by rakes, grapples and a small boat) and I suspect this will need to be again soon. A small amount is not a problem - in fact many invertebrates seem to use it as an important moist substrate - but for the development of aquatic plants, light penetration is required which the algal can block out. Back to the aquatic molluscs - I did notice that they have an important role in the terrestrial food chain; where stranded on land, they appear to form a source of small-scale carrion, used by invertebrates that like that sort of thing...

A fly (with the silvery jowls, I think it's the common Greenbottle Lucilia caesar) guarding a dead water-snail. It almost appears to be challenging me to try and take the snail, and certainly didn't want to abandon its prize.

Once I was no longer considered a threat, the fly resumed its business - presumably feeding as I saw no evidence of egg-laying behaviour.

Staying at the pond edge, the marginal vegetation (reeds etc) is gradually developing and the structure is clearly important with a range of invertebrates using it in different ways - hunting, mating, egg-laying, pupation and emergence, feeding and so on.

A pair of 2-spot ladybirds Adalia bipunctata busily making more ladybirds on the head of a reedmace plant - important given the threat posed by the invasive Harlequin ladybird Harmonia axyridis.

Following the ladybirds' example (no, I'm not obsessed, honest - they just don't move much when mating, and so are easier to photograph), a pair of Donacia reed beetles.

Moving onto spiders, ventral view of a female Tetragnatha extensa - note the large divergent jaws.

Cautiously approaching (remember the female's jaws), a male T. extensa - as well as his own large jaws, he has two swollen palps at the front - modified structures used for sperm transfer. Onward brave spider!

I'll be following the development of the pond and its inhabitants - after all, many ponds have been lost from the British landscape and they are of key importance for nature conservation - for example, see Pond Conservation, the British Dragonfly Society, Froglife and the Freshwater Biological Association. More soon!

Hairy eyes and yellow knees: the Broad Centurion

Lately, aside from the wildlife-friendly gardening and farming (more of that soon), I've been taking an interest in diagnostic morphology - using the fine detail of invertebrate structures to determine why they are what they are. This isn't needed for some species if they are distinctive in some way at a macro level, but for a lot of species, a close-up is needed for identification, often including dissection. Personally I find that the best way to learn how to do this is to have ready-identified specimens so you can work through a key with the beastie under the microscope and see if you get to the known right answer. If not, work bask to see where you went wrong and try again. Once you've conquered this, it's time to try specimens you're not sure about.

Before I start with today's species, one minor caveat - the images here, although I'm pleased with them as they do the job they are required to, are not necessarily what might be considered professional quality. However, my aim is to show what can be done by an amateur, or (like me) freelance professional at home, without the latest laboratory/museum set-up. All my work here is done with a good quality, but old, binocular microscope (acquired for free during a university clear-out from a lab tech who didn't want it to go into a skip), a new but inexpensive high-power monocular microscope, and a point-and-shoot compact digital camera.

So, enough background - today's beastie is the common Broad Centurion (Chloromyia formosa), a soldierfly (family Stratiomyidae) known from umbellifers in a range of habitats throughout lowland Britain, and found from May to August, and occasionally September. This one was found dead in our back garden and although we have no umbellifers there are plenty nearby.

Dorsal view of C. formosa showing hairy eyes, green pronotum & scutellum, and bronze abdomen, colours seen to varying extents in many soldierflies.

Ventral view of C. formosa showing the darker underside (gold tufts on the abdomen) and dark legs with yellow 'knees' and tarsi.

So, the first stage is to identify the family the fly is in. It is about 9mm long (excluding wings/appendages) and superficially not unlike many hoverflies (family Syrphidae). The key here is to look at the wing.

Wing with the front edge at the top. The discal cell (small and shaped as a rough pentagon) is near the centre of the wing. There are several gently curved veins pointing to the rear edge and these become quite faint. The two long veins nearest the body converge and join just before the rear edge of the wing and hence enclose the 'anal cell'. In hoverflies there would be two cross-veins running parallel to the rear edge of the wing.

The haltere (the upper 'drumstick') is a modified wing found near the wing in all true flies (which is why they have only one pair of wings). The hairy paddle-shaped structure below is a flap near the base of the wing.

There are other clues on the head and legs.

The head showing large eyes which meet on top, indicating that this is a male. The antennae are short and clearly show the banded 'flagellum' and hair-like 'arista' (hence the front of the head is at the top of the photo). On top of the head (just below the centre of the photo), the shiny ocelli (simple eyes) can be seen. The hairiness of the eyes is also clearly visible.

The leg (femur at the top, tibia to the side) showing the yellow 'knee'. The tibia has small yellowish hairs but no large bristles.

This antennal structure is typical of the soldierflies - also, the legs with hairy (but not stoutly bristly) tibiae, along with the wing features above means that it is certainly in the family Stratiomyidae. To work out the genus and species, we need to look again at the dorsal view.

The scutellum, a rounded flap just behind the pronotum has long pale hairs and clear punctures, but the key feature is that the rear edge (towards the top) has no protruding spines. The presence (and then number) of spines would indicate a different genus of soldierfly. The slightly out-of-focus second green structure is the post-scutellum which lies just under and behind the scutellum itself.

The abdomen detached in dorsal view - note the bronze colour (bluish and wider in females) and short stocky shape.

This combination of features - overall length, scutellum without spines, hairy eyes, and stocky abdomen bring us to the species-level identification, Chloromyia formosa. As far as identification goes, in Britain at least, this is all that is required - however, it can be useful (or simply interesting) to investigate the organism more thoroughly, so here are a few more images of different structures.

The underside of the head showing the groove where the mouthparts lie. These are short which is why they feed on umbellifers which have small flowers and easy available nectar.

A close-up of the abdomen showing punctures and hairs.

The tip of the abdomen showing the hair-fringed reproductive opening.

I hope that was a worthwhile overview of soldierfly morphology (for one species in any case) - more to come and the next instalment is likely to be either a moth or a ground beetle.

Reference

Stubbs, A. & Drake, M. (2001). British Soldierflies and Their Allies. BENHS, Reading.
An excellent book and essential for the study of soldierflies in Britain - it also includes the Acroceridae (hunchback flies), Asilidae (robber flies), Athericidae (water snipeflies), Bombylidae (bee flies), Rhagionidae (snipe flies), Scenopinidae (window flies), Tabanidae (horse flies), Therevidae (stiletto flies), Xylomyidae (wood soldierflies) and Xylophagidae (awl flies).