Spotted Wing Drosophila—It’s That Time of Year
Posted: July 1, 2016
Though it’s possible the larvae were in fact SWD, in other cases in the past, larvae found in late season strawberries have proven not to be SWD in Pennsylvania, instead being offspring of other types of fruit flies or sap beetles. Larvae of other fruit fly species do look identical in appearance to SWD larvae, being typical “maggots” with no noticeable head. Larvae of sap beetles are larger, and have a head. If other fruit flies or sap beetles were the problem, there would be some sign of berry damage or softening from a disease or moisture, or holes (however tiny) in the fruit.
If the fruit had just turned ripe and was completely intact except for damage from the ovipositor, the likelihood that the larvae were that of SWD is much higher. The only way to tell for sure, however, is to take a sample of the fruit, and allow the adults to hatch out. This is done by placing the fruit on an absorbent layer such as sand or paper towels in an airtight container from which the flies cannot escape. The sand or paper towels absorb moisture from the decomposing fruit that would otherwise drown the insects. Then after a week or so, the adults hatch out. The container and contents can be frozen to keep the adults from flying away when you open it.
Spotted wing drosophilia male wing spots. Photo: David Biddinger
There are some improvements in lures, should growers wish to monitor for SWD. Research trials in New York and Ontario found a new lure by Scentry to be more specific and capture SWD adults earlier than other baits used in the past. A summary of work conducted there and elsewhere can be found at Cornell's spotted wing drosophila blog.
There are number of good fact sheets and web sites available to help with identification and control. A 2015 factsheet that discusses monitoring methods and pesticides is available online from the University of Maryland.
We’ll be monitoring at a number of sites in PA, as in years past.