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Spotted Wing Drosophila – More You Should Know

Posted: September 2, 2011

Given that spotted wing drosophila (SWD) has been found in PA, many growers are scrutinizing their berries a little more closely. The main concern is that there could be SWD larvae in the fruit. Blackberries and raspberries are two favorite foods of SWD, and fall-harvested cultivars are the most at risk since SWD populations increase throughout the growing season. However, there are other types of larvae that could be in fruit, including those of fruit fly species that lay eggs in overripe fruit.

With raspberries, some folks have noticed that if they see juice on the inside of the cap, and then pull apart or squish these fruit, they find larvae (not necessarily SWD larvae).  They keep an eye out for red staining on the receptacle from the juice as they harvest. No juice inside the cap = white receptacle = no larvae.  I don’t know if that’s always the case, but it held true with berries I’ve checked.  With blackberries, finding suspicious fruit is trickier, as you need to find the oviposition hole and damaged flesh around it.  You can also check suspicious fruit by squishing it in a plastic zip-lock bag, adding a 1:16 solution of salt to water (1/4 cup salt to 4 cups water), and looking for larvae floating at the surface of the water after about 15 minutes.  Earlier recipes used sugar, but apparently salt makes the larvae exit the fruit faster.  Here are a few other questions people seem to be wondering about.

If I find larvae in the fruit, does this mean my farm has SWD present?  While larvae could be SWD larvae, they also could be larvae of common fruit flies especially if the fruit was very ripe or overripe.  SWD larvae are very tiny white maggots - they do not have a visible head.  The larvae are actually in the flesh of the fruit – not on the surface, though they do extend their breathing apparatus to the surface.  It’s essentially impossible to tell types of maggots apart, unfortunately, without raising them to adulthood and then identifying the adults. If the larvae in your fruit flesh have a discernable head, you probably are looking at larvae of sap beetles or picnic beetles.  If the larvae you find have a discernable head and are inside the cap, but not actually in the fruit flesh itself, you may have raspberry fruitworm.
 
How do I know whether the fruit flies I am seeing are SWD?  First, it’s important to note that the vast majority of fruit flies you see are not SWD.  The identifying characteristic on the SWD males is a large black spot on the outside edge of each wing near the tip, but not right at the very tip.  If you see fruit flies with a small spot on each wing at the very tip, they may belong to a different genus of fruit flies that includes organic matter decomposers and a type of pea leafminer. Females are much more difficult to tell apart from other fruit flies.  Another tiny insect I’ve noticed on raspberry fruit that could potentially be mistaken for SWD is a predatory insect of thrips that, when magnified, is obviously not a fruit fly.  It is about the size of a fruit fly though, and without magnification, its markings give it the appearance of having black spots towards the outside edge of its wings.  You can set out vinegar traps to trap adults – this is especially important if you are finding larvae in order to determine whether SWD may be present.  A good set of directions for making traps with excellent photos is at http://www.ipm.msu.edu/SWD/SWD-monitor.htm.

What should I do next if I think my farm has SWD?   It’s important to correctly determine whether you have SWD.  You can contact your county extension office to ask for help.  The folks there can contact me if needed.  If it looks likely that you have SWD, we will contact PDA as the personnel there will need to confirm the ID and presence of SWD.

What cultural steps can I take to minimize problems?   Note that most of these steps are the very same steps we recommend for minimizing problems with sap beetles and diseases. 

1) Pick fruit as soon as possible. With raspberries, this means harvesting as soon as you can pull the fruit from the plant, and with blackberries, harvesting when the fruit is fully colored.  If you notice any raspberry fruit with juice inside the caps, discard these berries, preferably after checking for larvae. 

2) Harvest thoroughly.  Even if you need to pay someone to pull off old fruit, keeping the planting clean will be worthwhile for a number of reasons in addition to this one.

3) Dispose of unwanted fruit in a way that will keep fruit flies from using it as a food source or from hatching from it.

What insecticides work if they become necessary?  Effective insecticides for which 2(ee) labels specific for spotted wing drosophila have been issued that are registered on raspberries and blackberries are: Danitol (fenpropathrin); Delegate (spinetoram); Entrust and Success (spinosad); Mustang Max (zeta-cypermethrin); and Pyganic (pyrethrins).  Other materials registered on caneberries for other pests are also effective on SWD.   Our big concern is development of SWD populations that are resistant to certain insecticides. This pest has a very short life cycle, so please be sure to use materials from different activity groups for subsequent sprays.  Here is a listing of insecticides registered for use on caneberries in PA that have a PHI of 3 days or less, grouped by activity group.  Ratings and length of expected residual activity are from information found at http://www.berriesnw.com/SFU/2011/SFUdocs11/SWD-Caneberry-MngtPlan.pdf .   It should be noted that the residual activity has sometimes been reported to be shorter than what is listed here, so close watch for return of adults will be needed. 

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