Sawflies

Count the legs

Behind the three pairs of jointed true legs at the head end, both caterpillars and sawfly larvae carry stumpy fleshy prolegs along the abdomen. A butterfly or moth caterpillar has five pairs of those at the very most, and often fewer. A sawfly larva has six, seven or eight.

The head gives a second reading. A sawfly larva has one large simple eye on each side, which reads as a single dark bead. A moth caterpillar has a cluster of tiny ones that no eye picks out in the field.

Behaviour is the third. Touch a leaf carrying sawfly larvae and the whole row lifts the back half of the body into a raised S at once, held for a few seconds.

Getting this right changes the advice. Sawflies are related to wasps and bees rather than to butterflies, so Bacillus thuringiensis, which handles caterpillars on brassicas and box, does nothing whatever to them. So do the nematode products sold for caterpillars. Anyone who treats a stripped gooseberry with a caterpillar product and watches the damage continue has met this distinction the expensive way.

Which ones you will meet

Gooseberry sawfly is the destructive one. Larvae hatch low in the middle of a gooseberry, red currant or white currant, work outwards, and can take a bush to bare ribs in a fortnight. Three generations run from late April into August, so a bush cleared in May is stripped again in July by a gardener who stopped looking.

Rose slug sawfly grazes the surface off rose leaves and leaves a lace of veins that dries to brown parchment. The large rose sawfly instead eats whole leaves and cuts rows of slits into young green stems to lay its eggs, which splits and browns the shoot along the line.

Pear and cherry slugworm looks like a small black slug on the upper surface of the leaf, coated in its own slime, and skeletonises pear, cherry and hawthorn foliage from midsummer.

Solomon’s seal sawfly and aquilegia sawfly strip their host to bare stems in early summer, both after the plant has flowered.

Birch sawfly larvae feed in ranks along a leaf edge on birch and alder, and do no meaningful harm to a tree.

What the damage costs

Very little of this kills anything. A gooseberry stripped in June ripens its crop and refoliates, an aquilegia eaten in July has already set seed, and a birch loses a fraction of one summer’s leaves.

The cost falls on the following year rather than this one. A fruit bush spends the second half of the summer building the buds it will flower from next spring, and it cannot do that with no leaves and no water. That is what makes a defoliated gooseberry worth watering and mulching, and why repeated stripping year after year thins a bush out.

Ornamental damage is a separate question and it is your own to answer. A rose grazed to lace at the front of a border matters in a way that a birch at the end of the garden does not.

What to do about it

Look under the leaves weekly from late April, starting at the centre and the base of gooseberries and currants.

Pick the larvae off, or take off the whole leaf where twenty are feeding together, and tread on it. Hold a bucket under a branch and tap it, since most species let go.

Go back in July. The second and third generations arrive on plants that were clean in June.

Cut out rose shoots that carry a line of laying slits, and the eggs leave with the prunings.

Keep a stripped fruit bush watered and mulched through the rest of the summer.

Leave the birch, and the Solomon’s seal after it has flowered, to the wasps.

What it gets mistaken for

Caterpillars are the constant confusion, and the proleg count settles it in five seconds. Getting it wrong sends people to treatments that cannot work.

Slugs on pear and cherry leaves would leave slime trails across the soil and feed at night. Slugworm sits on the leaf in daylight, has a head and legs underneath the slime, and never leaves the tree.

Leaf miners tunnel between the two surfaces of a leaf, leaving both intact over a hollow blotch or squiggle. Sawflies remove tissue from outside.

Vine weevil adults cut small clean notches out of the leaf margin at night and leave the rest of the leaf whole.