What is this red swelling on this leaf of crack willow, Salix fragilis? The tree is on the public footpath by the silt lagoons and has these swellings on several of it's leaves. The red structures themselves are around 10mm long and 5mm wide. Answer to follow next week....
I'm sure you will have guessed that this is another gall and this time it is caused by a species of sawfly called Pontania proxima, also known as the willow redgall sawfly. Sawflies are members of the insect Order Hymenoptera, they are related to bees, wasps and ants and there are around 500 species in the UK.
Pontania proxima is a common and widespread species throughout much of the UK, occurring anywhere where it's two host species of tree are present - crack and white willow (Salix fragilis and Salix alba respectively).
The female sawfly lays her eggs inside the leaves of willow trees, using a long, needle-like tube at the end of her abdomen, known as an ovipositor. On hatching, the tiny larvae feed on tissues inside the leaf, causing the plant to form the gall. The galls protrude out both the underside and upperside of the leaf, can reach up to around 8mm in length and can be either red, yellow or green. The larva itself is green in colour and reaches up to 5mm in length. The larva overwinters inside the gall (a single larva in each gall), where metamorphosis occurs.
Imago (adult) insects emerge the following spring through to August and are rarely seen, due to their small size (3-5mm) and inconspicuous appearance, being all black in colour.