What is Solomon's seal 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.  The larvae of Solomon's seal sawfly feed on the leaves of Polygonatum. 

Symptoms

Solomon's seal sawfly larvae feed in groups and defoliation can occur very quickly. Keep vigilant for early signs in late spring;

• Greyish white caterpillar-like larvae with black heads and up to 2cm (¾in) long seen on the underside of the leaves, eating elongate strips out of the foliage
• Complete defoliation may occur
• Purplish brown scars, up to 2cm (¾in) long, may be seen on the leaf stems where eggs were inserted

Control

Whilst the defoliation caused by Solomon's seal sawfly can appear severe it normally occurs after flowering and the plants will usually put on healthy growth in the year following an attack. Therefore this insect can be tolerated and the plants will survive.

Check susceptible plants frequently from late May 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 sawfly
• Encourage predators and other natural enemies of sawfly in the garden, such as birds and ground beetles.
• Check plants regularly from May onwards for the presence of larvae and remove by hand where practical

• 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 pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae 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 Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available

Download

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

Biology

Adult Solomon’s seal sawfly are black-bodied insects, 8-9mm long, with two pairs of blackish grey wings. In appearance they are similar to medium sized flies but are in the same group of insects as the bees, ants and wasps, the Hymenoptera. They emerge in late spring at about the time when their host plant is coming into flower.

The female uses her saw-like eggs-laying organ to insert rows of eggs into the leaf stems. This causes vertical purplish brown scars to develop where the eggs were inserted.

The larvae feed together in small groups on the underside of the leaves. Initially they make small elongate holes but as the larvae increase in size, their appetite also increases. By mid-summer, the stems may have been stripped of foliage.

The fully fed larvae go into the soil where they overwinter and pupate in the following spring. Defoliated plants will survive but may produce reduced growth in the following year.