A common pest of gooseberry, the gooseberry sawfly (Nematus ribesii) also attacks red and white (but not black) currants.
Typical sawfly damage
It is the caterpillars of the sawfly that do the damage, feeding on the leaves of gooseberry, white and red currants. They start feeding low down in the centre of the bush, and move outwards, stripping the leaves. The tiny, newly-hatched caterpillars initially make pinhead sized holes in the leaf, but very soon entire leaves are devoured. Sometimes a "skeleton" of leaf veins are left behind. Whole bushes can be stripped in a very short time. Defoliated plants are weakened and tend to produce a poor crop the following year.
Description of pest
Sawfly larvae are green with black spots and a shiny black head. When fully grown they reach approximately 30mm. The eggs are about 1mm long and pale greenish white in colour.
The sawfly over-winters as a cocoon in the soil. Adults emerge from April onwards. They lay torpedo-shaped eggs along the main veins on the undersides of leaves; eggs are usually found on lower leaves in the middle of the bush. These hatch and the larvae remain in the centre of the bush feeding for 1-2 weeks until about half grown. They move through the bush feeding voraciously. It is at this stage that they are usually noticed for the first time. Approximately 3 weeks after hatching the larvae drop to the soil and form cocoons. There are 3-4 generations in a year between late April and September.
Prevention and control
- Hand picking: Be vigilant! The time to catch this pest is when it is still small and feeding low down inside the bush. Inspect bushes carefully in late April, and again around early June, early July and late August/September, as these are the times when new generations will be hatching. Yellow sticky traps hung in bushes in early spring are useful for both control and monitoring of sawfly. Crush any eggs and larvae on the underside of leaves, or remove the entire leaf if heavily infested. It helps to look out for leaves in the centre of the bush with the characteristic "pin-holed" appearance, as this indicates newly emerged larvae. If the pest has already moved to the outer areas of the bush it may be too late to do much. The larvae should still be picked off by hand, or sprayed (see details below) to reduce future generations. If sawfly are a recurrent problem, growing the bushes as cordons allows easier access.
- Cultural control: Remove mulches in late autumn/winter and cultivate lightly round the bushes to encourage birds to clear up the cocoons in the soil.
- Biological control: – Steinernema carpcapse is a nematode control that is used like a contact insecticide, as in, it needs to be directly applied as a spray to the sawfly larvae. Very effective if used when larvae are first seen.
Organic gardening products are available from The Organic Gardening Catalogue.
Spray flowering crops at dusk when bees are not active. Always follow the manufacturers instructions when making up and using sprays.