Sawfly
Symphyta
also called Gooseberry sawfly, Rose slugworm, Solomon’s seal sawfly
Caterpillar-like larvae (gooseberry, rose, berberis kinds) that can strip a bush in days. Inspect leaf undersides from late spring and pick larvae off by hand.
How to know it
Larvae that look like caterpillars but carry more than five pairs of fleshy prolegs — the reliable tell, and the reason Bt does not work on them. Green with black spots on gooseberries, slug-like and slimy on roses and pears, gray and gregarious on Solomon’s seal, and they rear up in unison when the leaf is tapped.
What you'll see
- A gooseberry or currant bush stripped to bare stems in under a week, usually starting in the center.
- Larvae lined up along the leaf undersides, feeding shoulder to shoulder.
- Rose leaves grazed to a transparent skeleton on one surface, then browning off.
- Solomon’s seal leaves reduced to shredded ribbons in early summer.
- Repeat attacks through the season as successive generations hatch.
Where it comes from
Females saw slits into leaf tissue and lay eggs inside; larvae feed for two to three weeks then drop into the soil to pupate in a cocoon. Two or three generations a year on gooseberries — which is why a bush cleared in May is stripped again in July — and the final generation overwinters as a cocoon in the soil beneath the plant.
Warm late springs and early summers, and bushes with congested centers where the first larvae feed unseen. Damage is fastest on soft-leaved gooseberries and currants.
Dealing with it
- Inspect the center of gooseberry and currant bushes from mid-spring — the first generation always starts low and inside.
- Pick the larvae off by hand; on a garden bush this is quick and completely effective if done early.
- Shake branches over an upturned umbrella or a sheet and collect what falls.
- Do not reach for Bt: sawflies are not caterpillars and it will do nothing.
- A stripped bush usually releafs and survives — water, mulch, and do not write it off.
Keeping it out
- Prune fruit bushes to an open goblet so the center is easy to see into.
- Lightly fork or mulch under gooseberries and currants in winter to disturb the cocoons.
- Encourage birds, which take sawfly larvae readily once they find a colony.
- Check again three weeks after clearing the first generation — a second is coming.
- Grow umbel flowers nearby for the parasitic wasps and tachinid flies that attack them.
What it goes for
Gooseberries, redcurrants and whitecurrants above all; also roses, Solomon’s seal, berberis, pears, plums and aquilegia.
Who hunts it for you
- Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
- Spined soldier bugPodisus maculiventris A predatory stink bug — same shield outline as the pest, but with a sharp spine at each shoulder and a beak it uses to spear caterpillars and beetle grubs. Learn the spines before you squash a shield bug, because this one is on your side. Grow to recruit it: Dill,Fennel,Sweet alyssum,Yarrow
- Tachinid flyTachinidae Bristly, house-fly-lookalike parasitoids that lay eggs on caterpillars, beetle grubs, and earwigs — the maggot consumes the host from within. Adults sip nectar from shallow umbels and daisies. Grow to recruit it: Cilantro,Dill,Feverfew,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com