Large rose sawfly

What is large rose sawfly?

Large rose sawfly is an insect pest with caterpillar-like larvae that eat the leaves of roses, sometimes causing defoliation. The adults have yellow abdomens with mainly black thorax and heads.

Roses can also be attacked by other sawflies such as the rose leaf-rolling sawfly.

Symptoms

You may see the following symptoms:

• The female sawflies lay eggs in soft young rose stems. The stems later split open where the eggs were laid, resulting in elongate scars on the stems
• Whitish green, caterpillar-like larvae with black spots and yellow blotches cause extensive defoliation in early and late summer, as they eat the leaves

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Elongated scars on stems are caused by the female sawflies laying eggs in the shoots and flower stalks.

The larvae cause extensive defoliation in early and late summer, as they feed on the leaves and shoots.

Control

Non chemical

When spotted, the larvae can be removed by hand.

Chemical control

For heavy infestations, spray with deltamethrin (Bayer Sprayday Greenfly Killer), lambda cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer), or spray with the systemic insecticide thiacloprid (Provado Ultimate Bug Killer Ready To Use or Provado Ultimate Bug Killer Concentrate 2).

Organic pesticides, such as pyrethrum (Py Spray Garden Insect Killer, Doff All In One Bug Spray, Scotts Bug Clear Gun for Fruit & Veg) will control young larvae but older ones are more tolerant of these insecticides.

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Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biology

In Britain there are two species of large rose sawfly: Arge pagana and A. ochropus. The adult insects of both species have yellow abdomens with the legs, thorax and heads being mostly black. Arge pagana is the more common species. 

Rows of eggs are inserted into soft young rose shoots and female sawflies are sometimes seen dangling from such stems, attached only by their saw-like egg-laying organs.

After hatching, the larvae feed together in family groups. They are pale green with black spots and yellow blotches, and are up to 25mm (about 1in) long. When fully fed, they go into the soil to pupate.

The large rose sawfly (Arge pagana) will produce two (sometimes three) generations from May to October. Arge ochropus usually has a single generation in early summer, but sometimes there is a second generation in late summer.