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The caterpillar like larval stage of aruncus sawfly can completely defoliate goat's beard (Aruncus dioicus) in spring and summer.
Aruncus sawfly (Nematus spiraeae) on goatsbeard (Aruncus dioicus)
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.
Aruncus sawfly larvae feed in groups so damage can occur very quickly and defoliation can be extensive. Keep vigilant for early signs;
Damage can be expected from May (first generation) until September (third generation) with another generation in July and August.
Regulary inspect plants during the growing season and remove larvae from the leaves.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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, 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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