Search for the larvae and remove them from the underside of damaged leaves.
Spray infested plants with thiacloprid (Bayer Provado Ultimate Bug Killer), deltamethrin (Bayer Sprayday Greenfly Killer) or pyrethrum (Py Spray Garden Insect Killer, Doff All in One Bug Spray) while the larvae are still small.
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Adult Solomon’s seal sawfly are black-bodied insects, 8-9mm long, with two pairs of blackish grey wings. 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.