Turnip Sawfly Athalia rosae
The Athalia species are extremely hard to tell apart. All species, of which some 10 species are present in Britain, are yellowish red and have a variable amount of black markings and light ringed legs. The Turnip Sawfly however is the only species easily identified in the field and from pictures. The thorax is yellowish red with a black oval or triangular spot on both sides, the so-called shoulder patches. None of the other Athalia species shows these shoulder patches. Adult males measure 6 to 7 mm, adult females 7 to 8 mm.
When young the larvae are greenish grey. Later they turn dark grey to black on top and grey below. They are rather wrinkled and may reach a length of 16 to 18mm. Young larvae eat round patches out of the leaves of the host plant causing little damage. Grown larvae however eat the whole leaf, leaving just the veins. When appearing in great numbers they often destroy the whole plant. They are considered a pest, especially in the growing of mustard. In moderate Europe, such as in Britain, there are two generations per annum. In very favourable summers a small third generation may appear. In autumn the fully grown larvae go underground. There they make a cocoon in which they hibernate and, later, pupate. The first adults may appear in April, in Britain usually in May. Adults eat nectar and in our garden the spring generation is very keen on ground ivy. The last adults are seen in October.
The Turnip Sawfly is very common all over Britain. It inhabits a vast area: Europe, Northern Africa, Asia and Northern America.