Introduction

While it is not possible to distinguish SWD larvae from those of other common vinegar flies, the presence of larvae in intact fruit prior to harvest should be viewed as suspicious. Signs of infestation by SWD may be confused with normal aging of mature fruit. Most uninfested ripe fruit will show mild overall softening after several days.

Recognizing fruit damage

When fruit has been attacked by SWD, early mould, wrinkling and softening can be seen at 2-3 days following egg-laying, and most fruit show obvious damage after 3–4 days (Figure 7, 8, 9).

Raspberries show damage quickly, with scarring and collapse appearing as soon as 1–2 days after egg-laying.
Figure 7. Raspberries show damage quickly, with scarring and collapse appearing as soon as 1–2 days after egg-laying.
Strawberries deteriorate quickly following egg-laying. Fruit wrinkles and fruit softens, with mould appearing after only 3 days.
Figure 8. Strawberries deteriorate quickly following egg-laying. Fruit wrinkles and fruit softens, with mould appearing after only 3 days.
Blueberries start to soften after about 3 days.
Figure 9. Blueberries start to soften after about 3 days. Photo credit: V. Walton, Oregon State University.

Pin-prick sized scars or holes (Figure 10) from which sap exudes may also be evident.

Infested cherry showing pin-pricks and damage directly under the skin. In this shot, it is possible to see larvae and pupae which have exited the fruit.
Figure 10. Infested cherry showing pin-pricks and damage directly under the skin. In this shot, it is possible to see larvae and pupae which have exited the fruit. Photo credit: V. Walton, Oregon State University.

Small holes created by larvae for breathing may also been seen, sometimes with larval breathing tubes visible (Figure 11).

Larval breathing holes on blueberries.
Figure 11. Larval breathing holes on blueberries. Photo credit: BC Ministry of Agriculture.

Eventually, fruit softens in spots and the skin collapses, with a wrinkled appearance that becomes very obvious after about 5 days. Fruit left hanging following harvest may show signs of heavy infestation (Figure 12).

Fruit left on the tree following harvest is often heavily infested. Note presence of pupae on the cherry clusters.
Figure 12. Fruit left on the tree following harvest is often heavily infested. Note presence of pupae on the cherry clusters. Photo credit: V. Walton, Oregon State University.