Each stem has one very black insect on it that looks (vaguely) like a housefly. Does anyone know what these are and if they'll harm the plant?
They are solomon's seal sawfly. The grubs will eat every scrap of greenery on the plant and just leave bare stems. However, it grows like a weed in my garden and comes up again every year just as strongly, so I just let it be, as the damage occurs after flowering, by which time everything else in that border is growing up and covering the poor stalks.
I try and keep up with picking them off. I know what a lovely yellow colour the leaves go in autumn. Haven't seen those for a long time now
Yes, it's sawfly. Although the leaves will grow back next year, you will get the sawfly back, too. Leaving them will allow a cycle to develop, so it's well worth killing them.
I have too many to pick them off. Yes, the leaves are a lovely colour, but mine are growing under hydrangeas and dahlias and deciduous rhododendrons so it's not such a loss.
Agree that this is sawfly. I just pick them off and drop the pests into my pond where the fish gobble them up. Same happens to the Lilly beetle, dropped into the pond, in fact very many pests, aphids etc, end up as fish food, but not the slugs.
Thanks guys. I seem to be getting the pests but not their predators. Like you skyrunner I've had lily beetles on my fritillaries (beautiful little things actually). My spindle tree is smothered in blackfly but not a ladybird in sight. I'm trying to resist resorting to chemicals but it's not easy for people like me who like a quick solution!