Aruncus sawfly

The caterpillar like larval stage of aruncus sawfly can completely defoliate goat's beard (Aruncus dioicus) in spring and summer.

Aruncus sawfly (<EM>Nematus spiraeae</EM>) on goatsbeard (<EM>Aruncus dioicus</EM>)

Quick facts

Common name Aruncus sawfly
Latin name Nematus spiraeae
Plants affected Aruncus dioicus
Main symptoms The foliage is eaten by pale green caterpillar-like larvae
Caused by Larvae of a sawfly
Timing May-September

What is Aruncus sawfly?

Aruncus sawfly has pale green caterpillar-like larva that reach 20 mm in length and eat the leaves of Aruncus plants. The adult is a winged insect, 5-6 mm long with a yellowish abdomen and darker head and thorax. The wings are clear with brown veination.

Symptoms

Aruncus sawfly larvae feed in groups so damage can occur very quickly and defoliation can be extensive. Keep vigilant for early signs;

  • Pale green caterpillar-like larvae with yellowish brown heads and up to 2 cm long
  • Leaves can be reduced to a network of the larger leaf veins, affecting the appearance and vigour of infested plants

Damage can be expected from May (first generation) until September (third generation) with another generation in July and August.

Control

Non chemical

Regulary inspect plants during the growing season and remove larvae from the leaves.

Chemical

  • If infestations are too heavy for hand picking, control may be achieved by spraying with pesticides
  • Organic pesticides, such as those containing pyrethrum (e.g. Bug Clear Gun for Fruit and Veg, Defenders Bug Killer, ecofective Bug Killer) will control young larvae but larger larvae are more tolerant of these insecticides
  • Synthetic pesticides containing deltamethrin (e.g. Bayer Sprayday Greenfly Killer, Bayer Provado Ultimate Bug Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), cypermethrin (e.g. Py Bug Killer) or the systemic insecticide acetamiprid (e.g. Bug Clear Ultra) may give some control of larger larvae
  • Plants in flower should not be sprayed due to the danger to pollinating insects
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

 

Download

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

Biology

This sawfly defoliates goat's beard (Aruncus dioicus), a plant that was formerly included in the genus Spiraea, so this pest sometimes known as the spiraea sawfly. It occurs in gardens throughout Britain wherever its host plant is grown. 

The adult sawfly is 5-6mm long and has a black head and thorax with a yellow abdomen marked with dark bars on the dorsal surface. Larvae overwinter in the ground and, after pupation, adults emerge in May, lay eggs that hatch after about 7 days, and two or three generations of pale green larvae with brown heads feed gregariously on the foliage during the summer months, reducing the leaves to a network of the larger leaf veins. The pupal stage takes place within a silk cocoon in the soil.

This is a species that reproduces asexually and all the adults are female.

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