Aruncus sawfly

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

Aruncus sawfly ( Nematus spiraeae ) on goatsbeard ( Aruncus dioicus )

Aruncus sawfly (Nematus spiraeae) on goatsbeard (Aruncus dioicus)

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?

Sawflies are in the same group of insects as bees, ants and wasps (the Hymenoptera). They have caterpillar like larvae that feed on plants the adults are winged insects that can appear fly like. 

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 defoliation can occur quickly. Keep vigilant for early signs from May onwards;

  • 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), with a second generation in July and August and a third generation in September.

Control

Non-pesticide control

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

Pesticide control

  • If infestations are too heavy for hand picking, control may be achieved by spraying with pesticides. Spaying at dusk is likely to give the best results
  • Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer). Several applications of these short persistence products may be necessary to give good control
  • More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
  • Follow label instructions when using pesticides
  • Plants in flower should not be sprayed due to the danger to bees and other 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 insect is 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 soil and, after pupation, adults emerge in May and lay eggs that hatch after about 7 days. 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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