Fruit Flies and Raspberries

 You have just picked a juicy red ripe raspberry and are about to pop it into your mouth when you notice a small white wriggling grub (Photo 1). You suddenly lose your appetite! In all probability, this is the larva of the fruit fly, Drosophila susukii; more commonly referred to as the Spotted Wing Drosophila (SWD).

Photo 1: SWD Larva in Raspberry Fruit

As the name suggests this is an invasive species, originally from Japan. The female has a long sharp serrated (i.e. saw-like) ovipositor which is its secret weapon. The common fruit fly (Drosophila melangaster), often seen around the fruit bowl with the overripe banana, can only lay its eggs in rotting or fermenting fruit. The SWD, however, uses its serrated ovipositor to pierce the skin of unripe fruit thereby nullifying the simple pest control method of picking soft fruit before it is fully ripe.

You can make your own SWD trap using apple cider vinegar as the lure or purchase a commercial trap (here and here) with specially-formulated solutions to attract the fruit flies. These are generally sold as monitoring traps for identifying the presence of SWD. However, they also help to control an infestation when used in conjunction with good crop hygiene (e.g. remove & dispose of any ripe/mouldy fruit, regular crop picking) and crop protection techniques (e.g. insect-proof mesh).

I opted for the Drosal Pro Kit which includes five traps and 1L of attractant. Simply add some of the attractant liquid to the trap, fit the lid and hang up (1 metre high or more) near your raspberries (and/or other soft fruit bushes/canes). I spaced my traps about 5 metres apart in among the raspberry canes (Photo 2).

Photo 2: Drosal Pro Trap in Position

The liquid attractant will evaporate over time (Photo 3) so will need to be replaced every few weeks or so depending on how hot the weather and how shady the trap location is.

Photo 3: Fruit Fly Graveyard

The entrance holes of the traps are just the right size for SWD but too small for other beneficial insects. The commercial attractant (DrosaLure) is very effective but fairly expensive at around £15 per litre. Early days yet, but 1 litre should last the season. You can save some money by making your own attractant using apple cider vinegar and a few drops of washing up liquid; lower efficacy but also lower price!

After a few days, the trap had collected a good number of SWDs (Photo 4), mostly females (no spot on the wing) but also the odd male (Photo 5).

Photo 4: Spotted Wing Drosiphila, RIP

Photo 5: Male SWD (deceased)

I picked 250 g of fresh ripe raspberries yesterday and could find no evidence of the SWD larvae. So far, very happy with the protection these traps offer.