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.