Injury by Sawfly caterpillars is caused by burrowing in to the fruit, resulting in symptoms similar to those caused by Codling moth, or by foliar feeding, dependent on species and fruit crop attacked. Sawfly attacks often occur on apples, blackcurrants and gooseberries.
Damage in apples is caused by caterpillars tunnelling in to the fruit. Young caterpillars often leave a characteristic scarring on the surface of attacked apples. In blackcurrants and gooseberries damage is caused by caterpillars feeding on the foliage. Heavy infestations can strip bushes completely of all foliage, decimating that year’s crop of fruit and reducing yield in the following year as well.
Apple Sawfly (Hoplocampa testudinea)
Apple Sawfly overwinters as a cocoon in the soil, at a depth between 8cm and 24cm. Pupation occurs three or four weeks before spring emergence. Some caterpillars remain in a pre-pupal stage until the following year.
Each adult female can lay up to about 30 eggs, singly on the underside of open blossoms. Sap exudes from egg laying sites and turns reddish-brown, providing an easy method for assessment of an infestation. Eggs normally hatch after about 14 days, coinciding with a period around four or five days after 80% petal fall.
Apple Sawfly caterpillars have a black head on a white or cream body. The head becomes light brown in colour as the caterpillar grows. Developing fruit is damaged by caterpillars feeding and burrowing in to the flesh. If a caterpillar penetrates as far as the core, the seeds will be eaten, causing the fruit to fall from the tree. The characteristic Apple Sawfly scars on fruit are caused by caterpillars that feed just beneath the skin of the fruit but do not penetrate as far as the core. Each caterpillar can attack two or three apple fruitlets. Feeding continues for approximately four weeks, until the caterpillar falls to the soil and forms a cocoon.
Blackcurrant Sawfly (Nematus olfaciens)
The Blackcurrant Sawfly will attack crops of blackcurrants, whitecurrants and redcurrants. Overwintering occurs as a cocoon in the soil, normally near the surface. Adults first appear in May and are particularly active in sunny weather. The body is black at the front and yellow-orange towards the rear, with transparent wings that can reach 16mm across.
White eggs are laid singly or in small groups, along veins on the underside of leaves. Emerging caterpillars are green with green heads, and have black markings on the body and small black spots on the head. Feeding begins as small holes in leaves progressing to the whole leaf being consumed. Two or more generations of adults appear during the summer, with first adult emergence occurring in May or June. The second generation of adults can normally be found during July and September.
Common Gooseberry Sawfly (Nematus ribesii)
Cocoons overwinter in the soil and give rise to the first generation of adults in the spring. There are usually three generations of Common Gooseberry Sawfly each season. Adults first appear in April or May and are similar in appearance to adults of the Blackcurrant Sawfly.
Semi-transparent, white-green eggs are laid together in large numbers on the lower surface of a leaf, normally along main veins. Eggs hatch after about one week to produce young caterpillars with a green body and some black markings. The black markings become more distinct as the caterpillar grows and matures. The head of these caterpillars is black.
When fully grown the caterpillars lose the black spots and turn pale green with an orange area behind the head and another at the rear. Hatching to full maturity normally takes about four weeks. Brown cocoons are then formed in the soil with further adult emergence in another two to three weeks.
Lorsban* WG (active ingredient chlorpyrifos) has good activity on Sawfly caterpillars. Timing of application varies with crop. Do not apply to crops in flower or to those in which bees are actively foraging. Do not make applications when flowering weeds are present.