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A Guide to the British Metallus species (Hymenoptera: Tenthredinidae)

 

The three species of the sawfly genus Metallus are reputedly rare in the UK. Their mines and larvae are easily identified, and possibly a 
survey of the mines will show that this rarity is only apparent. The hostplants at least are not at all rare: brambles (Rubus fruticosus), raspberry (Rubus idaeus) and wood avens/herb bennet (Geum urbanum).
In all three species the mine starts with a short (rarely long and narrow) corridor that quickly widens to a transparent blotch in which 
the frass grains are loosely scattered. The initial corridor often is  overrun by the later blotch.
Often an inspection of the larvae is necessary for a definite identification. This can be done without opening the mine. Like in 
almost all sawfly species, Metallus larvae are positioned belly-up in the mine, nicely exposing the underside of thorax and abdomen that 
shows diagnostic characters.
There is a complication. The very last larval instar in the mine, the so-called prepupa, looks like an altogether different animal. It differs in many morphological details from the previous stages, and in  particular almost all black pigmentation is lost. Presently, it is not 
possible to discriminate prepupae. (Rearing sawflies from this stage is not easy, because adults emerge only after hibernation.)

 

Metallus lanceolatus:

This species  poses least problems in identification as it is  the only Metallus species on wood avens/herb bennet (Geum urbanum). It also is the only species that forms a real blotch mine on this plant. In the extensive database of British sawfly data compiled by David Sheppard there are  only nine records of this species, between 1929 and 1998, from Devon, Oxfordshire, Cambridgeshire, Surrey and Yorkshire. In Holland the species is rather common.
The mine (below) is large and, because of the thin leaves of the hostplant, exceptionally transparent. The larva is much less pigmented than the  other two species (the larva illustrated below is actually a fairly dark one).

 

 

 

Confusion is only possible with mines of the fly Agromyza potentillae, that forms a gradually widening corridor. These mines however are not full-depth (and thus greenish, when held against the light).

In the first half of the mine the frass is clearly deposited in two rows along the sides of the mine (as  below) and of course, when the larva is still there the typical morphology of a dipterous larva is seen (below) -without the clearly defined head region and feet.
 

 

Metallus pumilus:

Metallus pumilus occurs on all members of the genus Rubus: raspberry and the countless micro species that we loosely call brambles. 

Its mine consists of a large blotch that may occupy a sizable part of a leaflet. 


On David's database there are 17 records dated between 1923 and  1999.  It is the commonest of the three species but still rather local occurring in Hertfordshire, Middlesex, Buckinghamshie, Surrey, Yorkshire, Bedfordshire, Devon, Cambridgeshire, Oxfordshire, Wiltshire and Kent.

In Holland it is the commonest bramble miner by far.

 

The larva has big dark brown plate on the prosternum, and much smaller markings on the meso- and metasternum and the first abdominal segment

 

Here too, confusion may arise with Agromyza potentillae, which is even more common on Rubus (especially on raspberry) than on herb bennett.
However, even in empty mines the distribution of the frass, and the fact that the mines are not full-depth will make an identification easy.

A potentially  cause of confusion is the rare Rubus miner Emmetia heinemmanni (trapped only as a moth in the UK at present).

This species too makes a wide, transparent blotch. The larva, however, is very different. It is positioned belly-down in the mine, and, when seen from the leaf upper side looks
totally different ( as opposite)
 

Metallus albipes:  
 

Metallus albipes lives on the genus Rubus, like M.lanceolatus, but  probably is restricted to R.idaeus, the raspberry. According to Ewald  Altenhofer's experience in Austria it must be looked for on raspberry plants growing in deep shade, in which the branches are not yet  lignified. The mine illustrated below was located at the edge of a public park, in a patch of raspberries which were shaded by the branches of overhanging trees.


The number of British records is only ten, dated between 1902 and  1983.  The counties where these were found are Devon, Hertfordshire, Bedfordshire, Huntingdonshire, Surrey, Derbyshire, Yorkshire and Northumberland.

As you may have  read, recently mines of this species were discovered in Hampshire. In Holland only one record of this species is available (two adult specimens in 1975).


The mines of M.albipes resemble those of M.pumilus (Hering even says they are indistinguishable), but Altenhofer has the experience that the  initial corridor is much longer and narrower. The two British mines found by Rob Edmunds (one of them illustrated below) do not confirm this. Yet, M.albipes larvae can be distinguished with certainty from M.pumilus larvae by the absence of the dark spot on the ventral side of the first  abdominal segment. Even in a photo of the larva in its mine this detail is clearly visible.

 

 

For a better understanding of the ecology of the two Rubus-feeding Metallus species it is necessary to record the hostplant with some 
precision. In this plant genus this is not so easy, because it consists of hundreds of asexually reproducing microspecies. Leaving out a number of cultivated, escaped, boreal and rare species, and lumping all microspecies in one aggregate (R. fruticosus), a Rubus-key would run like this:

1a. fruit red, easily removed from the receptacle; leaf underside whitish; stem erect, not rooting at the tip; not very pricky: ........R. idaeus 
(raspberry)
1b. fruit black when ripe, not so easily removed; leaf underside often green; stems decumbent, rooting at the tip; strong thorns ........... 2
2a. fruit with glaucous bloom, with only a few drupes; leaflets  alternate: ..............................................................R. caesius (dewberry)
2b. fruit without glaucous bloom and with numerous drupes; leaves often  with more than 3 leaflets: ...............Rubus fruticosus (bramble)

 

Willem Ellis  - September 2004

Images: Willem Ellis, Rob Edmunds 2004

Newsletter of leafmines.co.uk                                                                           September 2004