SWD appears with a vengeance at Upper Mountain Research Station

Male D. suzukii on raspberry leaf, Upper Mountain Research Station.  Photo taken 30 September 2010 by Absalom Shank, NC State Horticulture Department

Yesterday, I spent the day at the Upper Mountain Research Station near Laurel Springs confirming the first spotted wing drosophila (Drosophila suzukii) larval infestation in North Carolina.  Spotted wing drosophila (SWD) is a potentially devastating invasive pest of soft skinned fruit which has been present in North American since late 2008.  See here for more background.  Blackberry, raspberry, and strawberry research plots are heavily infested.  SWD were first observed on 30 September in high tunnel primocane fruiting raspberry plots by Absalom Shank, research technician in the NC State Horticulture Department.

Up until last week, no larval SWD had been observed in North or South Carolina, despite the fact that adults have been captured in high numbers.  This is likely due to the fact that most of our trap captures occurred after the majority of susceptible host fruit had been harvested.  It is likely that larvae have been present but in low enough numbers that they escaped detection.

The first thing I did after arriving at the station was to place 6 SWD traps in 3 of 4 high tunnels, where the highest number of flies had been observed.  These tunnels contain primocane and floricane fruiting blackberries and raspberries. Three of the traps were baited with apple cider vinegar, and 3 were baited with a yeast and sugar mixture, as are being used in our regional monitoring program.  Within 20 minutes, the first adults appeared at the apple cider vinegar baited traps.  A total of 4 flies were captured in 3 hours.

Apple cider vinegar baited SWD trap in primocane fruiting blackberry tunnel.  Photo: HJB

Male SWD at apple cider vinegar baited trap entry.  Photo: HJB

SWD larvae were observed last week in fruit from guard plants and caneberry seedling plots, both of which are not regularly harvested.  This week, larvae were found in trial plots, which are harvested twice per week, so flies are not restricted to unharvested plantings.

Fallen raspberries in breeding plots.  These fruit can harbor SWD populations. Photo: HJB

Adult SWD were readily observed and collected in high tunnel plots (and the most of those were in primocane fruiting raspberries), but were not as numerous in outside plots.

Female SWD on ripe raspberry fruit in a high tunnel.  Photo: HJB

Primocane fruiting blackberries in high tunnel.  Photo: HJB.

Primocane fruiting raspberries outside. Photo:; HJB

Many fruit in raspberry breeding/seedling plots were heavily infested, which is not surprising considering that these plots have never been harvested.  Most of the fruit collected contained multiple larvae.

Ripe yellow raspberry from breeding plot.   This single fruit contained 5 SWD larvae.  Photo: HJB

Strawberry plots at the station also contained large numbers of SWD larvae.  Because it was very windy, adult fly presence could not be confirmed in these plots.  Flies which appeared to be SWD females were present, but these are more difficult to sight identify than males.  The fruit damage observed, however, was consistent with SWD.  Sound appearing fruit contained multiple maggots when cut open.  Larvae were collected to rear out adults.

Strawberry containing multiple SWD larvae.  Photo: HJB

These strawberry plots had also not been harvested for several months and contained sound, rotting, and decomposed fruit, all of which are attractive and suitable for SWD larval feeding.

Strawberry plots containing heavily SWD infested fruit.  Photo: HJB

Strawberries with recently laid SWD eggs were also found.  These fruit appeared sound except for a slightly abraded area where the eggs were.

Strawberry fruit with slightly abraded area containing SWD eggs.  Photo: HJB

Enlarged view of SWD eggs.  Photo: HJB

I collected several adult flies and numerous infested fruit to bring back to laboratory to start a research colony.  These insects will be used to test host fruit preference, monitoring tools, and control strategies.  This afternoon I am heading to check my traps at the Sandhills Research Station and check the caneberry breeding plots there for larval infestation. Update: I visited the Sandhills Station today.  See here for a summary of what I found.


SWD Monitoring
We have been monitoring SWD in NC, SC, and VA for 5 months and have detected flies in Montgomery County, NC; Lee County, NC; Randolph County, NC; Davidson County, NC; Edgecombe County, NC; Saluda County SC; Spartanburg County, SC; and Lexington County, NC.  We can now add Ashe County, NC to that list.

Small fruit and tree fruit growers should be aware that SWD populations are present at potentially damaging levels and should carefully monitor their plantings for adult and larval presence.  Caneberry growers with primocane fruiting berries and strawberry growers with fall fruiting or day neutral plantings should be especially aware.  Adult traps can be constructed from plastic containers or bottles (see here for a description of trap construction and here for videos demonstrating trap construction and use).  Sticky cards for use in traps can be obtained from a variety of sources.  We get ours from Great Lakes IPM.  Male SWD are readily distinguished from other flies by their spotted wings.  Females are more difficult to distinguish, and growers should consult their county extension agent for identification confirmation if males are not present.  See here for examples of SWD and non SWD flies on sticky traps.  Traps should be checked at least weekly.

Growers should also monitor fruit for larval infestation.  Large larvae will be visible in fruit, but small larvae may not.  Larvae can most easily be detected via the "fruit dunk" method.  Fruit are gently crushed and floated in sugar water.  After several minutes, larvae float to the surface and can be counted.   See here for a video and here for a description.  A hand lens may be necessary to observe small larvae.  It is important to remember that many native Drosophila feed on rotting fruit.  Observing infestation in relatively sound fruit and confirming adult presence minimizes the likelihood that you will confuse SWD with native relatives.

SWD Management
Sanitation is extremely important in managing SWD.  In areas where SWD is present, all ripe fruit should be removed from the field or plants should be treated with insecticide regularly, with the interval depending upon the material used.  Unmarketable fruit should be destroyed or removed from the site.  Composting may not be sufficient to kill SWD larvae and may attract adults to rotting fruit.  This presents a challenge to fruit breeding activities at research stations and will need to managed with care.

SWD populations in the western US (California, Oregon, and Washington) appear to respond well to a number of insecticides, which is encouraging news.  I am preparing a document listing the registered materials that are effective against SWD by crop and will have this available early next week.  Growers who have confirmed SWD should contact their county cooperative extension agent or myself for management recommendations.  It is especially important to rotate insecticides used to treat SWD to minimize the likelihood of resistance development.

More information
OSU SWD Website
Spotted Wing Drosophila Updates - WSU Mount Vernon Research Center
Strawberries and Caneberries - Blog by Mark Bolda (UC Farm Advisor)
NC Small Fruits IPM SWD Posts



Sponsored by the Southern Region Small Fruit Consortium, Project 2010 E-01.