Athalia rosae L Turnip sawfly

The turnip sawfly is a sporadically important pest of radish and turnip; it also damages Chinese leaf, mustard, oilseed rape and swede crops. Larvae skeletonize the leaves of host plants, and heavy infestations can be of considerable economic importance. Populations of this pest declined throughout northwestern Europe in the late nineteenth century, and the sawny then became extinct as a breeding species in the British Isles. Since the 1940s, however, numbers have increased and the sawfly (which is renowned as a migrant) has been reported on occasions in England, usually in the extreme southeast.

BIOLOGY

Adults occur from May to October, with up to three generations annually. They require warm conditions to be active, and fly only at temperatures above approximately 18°C. Eggs, up to 300 or more per female, are deposited singly in the leaf margins of host plants, and they hatch in about a week. In common with many related sawfiies, fertilized eggs give rise to female offspring and unfertilized eggs to males. At first, larvae mine within the leaf tissue; later, they graze externally, typically feeding gregariously. Individuals are fully grown in about 3 weeks. They then enter the soil, where each forms a cocoon within which to pupate; adults of the next generation appear 2-3 weeks later. Fully fed larvae of the final brood enter the soil to overwinter; they pupate in the spring and adults appear shortly afterwards.

DESCRIPTION

Adult 6-8 mm long, mainly yellow to reddish-yellow; head and antennae black. Egg 1mm long, white. Larva up to 18 mm long; body distinctly wrinkled, mainly grey or greenish-grey to velvety black, with a pale underside, a pale lateral band and a dark-edged dorsal stripe; head shiny black; abdominal prolegs eight pairs present.

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