Sawflies

Sawflies (Symphyta) are a very understudied group of insects. They belong to the Hymenoptera, which includes Wasps, Bees and Ants, but the female has a saw instead of a sting, hence the common name. The saw is used to cut slots in leaves, and she then lays her eggs into the slots as an anchor to hold them in place, often laying many eggs on a single leaf. The larvae look very much like the caterpillars of moths and butterflies, but they have more prolegs and are generally more tapered, with wider 'shoulders' and narrower rear ends. They also have very simple ocelli on the larger eyes, only a single dot rather than multiple dots as found on moth larvae.

Larva of Arge gracilicornis

The eggs are laid on particular plants, sometimes a single species of plant, but often within a closely-related family of plants. Some species are leaf miners and some others make galls, although the majority of larvae are free-ranging.

I'm not quite sure why Sawflies are so studiously ignored by many entomologists. True, there are many species and many are very similar to each other, often requiring a specimen and a microscope to identify accurately, but other groups are like this, so I'm not quite sure why they are seen as 'difficult'. I suppose it must be a lack of literature: there is very little out there: the main key (Benson in 3 volumes) is now over 60 years old, but I have found the key to be no more difficult than other groups such as beetles or flies. One difficulty is the larvae: they go through a series of moults (instars) as they grow, and can vary their colour pattern quite substantially when they do so. So in order to identify the larvae we need to know all the variations for each species. Cameron's four-volume monograph, written in the late 19th century, includes coloured drawings of some larvae. A further difficulty is that we don't yet have a full knowledge of which larvae turn into which adults. With moths and butterflies, the match is more or less complete, since a great deal of work has been done with them and they are relatively easy to breed through in captivity, but with sawflies the life cycle can sometimes be a bit more complex, and many attempts to raise them in controlled conditions have failed. So we have a situation where some larvae are as yet unidentified, and the larvae of some of the adults are unknown. Clearly, much work needs to be done here.

Adult sawflies can be difficult to distinguish from other groups, but they always have a thick waist like bees, rather than a narrow one like wasps. The thorax is often strongly sculptured.

Tenthredopsis nassata

 

Tenthredo livida, male

Larvae often adopt a tail-up stance when disturbed. The movement is quite sudden, and I have seen parasitoids being propelled through the air if they approach too closely. This is the larva of Nematus pavidus, which is a common Willow feeder:

Larva of Nematus pavidus

The differences between species are often microscopic, and we need to examine a specimen under magnification. With some species we need to see further details, such as the shape of the teeth on the saw. This is the (2 mm,) saw of a Tenthredo:

Saw of a female Tenthredo sp. Sawfly

A few years ago I studied a series of colonies of Nematus pavidus on my local Willow and it is documented here.

Sawflies certainly need attention, and I intend to focus on them this year.

Leaf-miner recording scheme for Ireland

Last week the National Biodiversity Data Centre in Waterford launched a national recording scheme for leaf miners. I have been working on a national checklist for miners for several months and the scheme allows recorders to enter records of leafminers and upload images for verification.

Leaf-miners are a largely ignored group of insects from four groups: micromoths, flies, sawflies and beetles, and hopefully the scheme will raise awareness and produce many records for these groups and perhaps result in some new species for Ireland.

I also produced an introduction to leaf miners which will help people to identify their finds. The introduction and checklist can both be downloaded from here.

Leaf miners are the larvae of insects, and they can be detected by the track they make as they feed inside the leaf. Since they spend all of their larval life inside a leaf, they are very small: some of the adults are only 2-3 mm long.

Mines are identified primarily by knowing the host plant: many miners feed on a single plant or family of plants, so knowing the host plant immediately reduces the number of miners to be considered. Once we know the host plant, then further identification relies on shape of the mine, pattern of the frass (dung) in the mine and perhaps details of the larva or pupa.

Here's a shot I took of the fly miner Phytomyza ranunculi on Lesser Celandine:

Phytomyza ranunculi on Lesser Celandine

There are two main possibilities for this mine, the other being Phytomyza ranunculivora, which has the frass in more discrete lumps:

Phytomyza ranunculivora on Creeping Buttercup

Although Phytomyza ranunculivora hasn't been found in Celandine in UK or Ireland, it does use that plant in other parts of Europe, so it's always wise to check in case we can add a new foodplant for that species. The photograph above shows it on Creeping Buttercup, which is closely related to the Celandine, and was the first (and still only!) Irish record when I found it in 2010.