Gooseberry sawfly Nematus ribesii

Damage. This is an important pest on gooseberries, redcurrants and whitecurrants, but not on blackcurrants. Extensive damage to foliage may be caused by the caterpillars. In some cases, the leaves of the whole bush may be skeletonized.

Life cycle. The adults emerge from their overwintering soil-borne cocoons in late April. Adults measure about 1 cm in length and resemble flying ants. The male has a black abdomen, the female a yellow abdomen. The female, in early May, lays elongate 1 mm long, light-green eggs in rows along the veins of the leaves situated low down in the host bush. Emerging young caterpillars eat small holes in these leaves. Maturing caterpillars (reaching up to 2 cm in length and identifiable by their green appearance with numerous black spots and with a black head) consume whole leaves and move outwards and upwards from their original positions. After 3-4 weeks of feeding, the mature caterpillars drop to the soil and pupate. There are 2-3 more lifecycle generations of the sawfly from the second emergence in early June to last soil pupation in late September.

Figure 14.22 Viburnum sawfly damage

Spread is by the flighted adults.

Control. Private gardeners often pick off the first few young caterpillars found on the base leaves of the bush at egg-germination times (in late April, early June, early July and late August). In autumn and winter periods, the removal of any mulch from around the bushes and disturbance of soil in the area encourage birds to seek out overwintering pupae. Insecticide products containing pyrethrins control this pest.

Professional growers use two ingredients derived from plants. The insecticides, derris or nicotine, are effective against the young caterpillars if directed on the under-leaf surface, and with special attention being paid to the lower, central leaves.

No biological control is currently available for gooseberry sawfly, and the insecticide formulated from the bacterium, Bacillus thuringiensis, whilst effective on many caterpillars of moths, is ineffective against this sawfly species.

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