Aruncus sawfly

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

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Aruncus sawfly (Nematus spiraeae) on goat's beard (Aruncus dioicus)

Quick facts

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

What is Aruncus sawfly?

Sawflies are a group of insects suborder (Symphyta) of the Hymenoptera (bees, ants and wasps). There are about 500 species of sawfly in Britain. They have caterpillar like larvae that feed on plant material and are named after the saw like egg laying organ used by females to lay eggs in plant material. Adults can come in a range of colours many are black, green orange or striped yellow and black. Most are small (< 1cm) but some species such as the Birch sawfly (Cimbex femoratus) can be over 2 cm long. Several species can be found in gardens and are part of the biodiversity a healthy garden will support. More information can be found at The Sawflies of Britain and Ireland webpages

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

Larvae can be found from May (first generation), with a second generation in July and August and a third generation in September.

Control

Check Aruncus (goat's beard) frequently from spring onwards so action can be taken before a damaging infestation has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Non-pesticide control

  • Where possible tolerate populations of aruncus sawfly, host plants often recover from defoliation and may not be affected every year
  • Encourage predators and other natural enemies of sawfly in the garden, such as birds and ground beetles.
  • Check plants regularly from early may for the presence of larvae and remove by hand where practical
Pesticide control

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

  • If numbers of larvae are too high for hand picking, control may be achieved by spraying with pesticides. Spraying at dusk is likely to give the best results
  • Organic contact insecticides containing natural pyrethrinrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer). Several applications of this short persistence products may be necessary to give good control
  • More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • A systemic containing the active ingredient Flupyradifurone (Provanto Smart Bug Killer) is available for use on ornamentals and selected edibles
  • 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 RHS Gardening Advice. It is a list of products currently available to the home gardener

Download

Pesticides for gardeners (pdf document outlining pesticides available to home gardeners)

Biology

Aruncus 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. 

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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