Posts Tagged: Drosophila suzukii
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Spotted Wing Drosophila, Drosophila suzukii, on raspberry. (Photo by Kathy Keatley Garvey)
As any grower knows, vinegar flies, Drosophila spp., have been a major deal in strawberries the last few years. It seems the slightest delay in picking (especially in the fall) is met with an onslaught of vinegar flies followed by a huge loss of fruit.
I have the good fortune to be funded this year by the North American Strawberry Growers Association (NASGA) on a study looking at predaceous nematodes as a way of controlling vinegar flies. More on that at a later date, but having my research assistant Monise Sheehan and I spend some time looking closely at vinegar flies in strawberry brings up some observations that I would like to share here.
First of all, regular vinegar flies, as opposed to the spotted wing drosophila, Drosophila suzukii, do not oviposit in fruit, rather they oviposit on the surface. I am going to underline that this doesn't at all mean they won't damage the fruit, since it is only 24 hours each one spends in the egg stage before becoming an active larva fully capable of penetrating into the fruit.
It is interesting to see where the egg laying is taking place. It's uncommon to see eggs just deposited on a healthy fruit (Photo 2); it is rather in wounds (Photo 3) and cracks (Photo 4), bruises and any other kind of damage which softens and compromises the integrity of the fruit where the bulk of the eggs are found.
I think the take home message for strawberry growers and supporting professionals is to know how very fast the life cycle of vinegar flies goes and the big role that damage, be it wounds, cracking, bruising or general softness has in promoting their development.
Photo 1: Vinegar fly eggs under the microscope. Long fibers are respiratory tubes.
Photo 2: Mass of vinegar fly eggs on an ostensibly healthy fruit. The immaturity of this fruit is notable.
Photo 3: Mass of vinegar fly eggs in fruit wound, rendering it totally unmarketable.
Photo 4: Vinegar fly eggs nicely lined up in crack under calyx. This fruit is lost.
The agricultural pest, native to southeast Asia and now found in many parts of North America, was first observed in California in the fall of 2008 in the central valley, but was not identified until early 2009. It is reported to attack soft-skinned, ripening fruits, and has been a particular problem for raspberry and cherry growers in California.
The research team – composed of Phaff Yeast Culture Collection curator Kyria Boundy-Mills of the UC Davis Department of Food Science and Technology; Frank Zalom UC Cooperative Extension specialist in the Department of Entomology at UC Davis and integrated pest management expert; doctoral student Kelly Hamby in the Zalom lab; and UC Davis visiting professor Alejandro Hernandez of University of Extremadura, Spain – published their work in a recent cover article of the journal Applied and Environmental Microbiology, the No. 1 cited journal in microbiology.
"This represents the first look at potential yeast associations of Drosophila suzukii," said Hamby.
“Often Drosophila have interactions with yeast communities, and communities often vary between host plant species, so it was a bit of a surprising that we found Hanseniaspora uvarum so often, though, we used only culture-based methods so other yeasts may be present that are hard to culture,” Hamby said.
Hannah Burrack, a former Zalom lab graduate student (now an assistant professor and Cooperative Extension specialist in the Department of Entomology, North Carolina State University) originally found that olive flies were more attracted to yeasts that were isolated from the flies themselves than to the Torula yeast bait that is commercially available for olive fruit fly management, Hamby said.
In their journal article, the authors wrote that “D. suzukii is unique in that it oviposits on marketable fruit relative to overripe or damaged fruit, and its injury facilitates colonization by other Drosophila species. If untreated, it is capable of causing a potential $860 million of revenue loss annually to blackberries, raspberries, and cherries in California, Oregon, and Washington. Knowledge of potential yeast associations could be used in lure development.”
Zalom said the pest is a major problem in the area for backyard cherries. “Many residents have not been able to harvest cherries for several years now.”
Deep in the bowels of Briggs Hall on the UC Davis campus, entomology graduate student Kelly Hamby...
Close-up of Drosophila
In some of the first literature written in Japan in 1939 (Kanzawa, T.) about spotted wing drosophila, Drosophila suzukii, (SWD), experiments were made regarding the sensitivity of the egg and larval stages of spotted wing drosophila to periods of temperatures above and below freezing (32o F).
As is noted in the two graphs below, at constant temperatures of up to 35o F, 96 hours or more of cooling resulted in total mortality of spotted wing drosophila eggs and larvae. This was also anecdotally confirmed in tests conducted in 2009 in California.
While temperatures below freezing are not useful to fruit shippers, temperatures in the area of 35o F are. However, it is important to note that for success the constancy of the temperature is critical. So, while in an ideal situation constant temperatures of 35o F or a little below are effective in SWD egg and larvae suppression when extended for periods longer than 96 hours, the reality can vary significantly from the ideal. Shipped fruit ordinarily do not experience lengthy regimes of constant temperature as they are moved from place to place. Temperatures of a refrigerator truck can vary by location inside and placement of the produce (ie on the side, towards the bottom etc.), and certainly the temperatures at the point of sale can vary from the ideal to room temperature to even warmer.
Additionally, while initial damage from SWD on raspberries, blackberries and strawberries can be difficult to detect, this is not the case for other fruits such as cherries or blueberries, where the activity of SWD will leave an unsightly blemish.
The take home message from this information is that while extended cooling can be suppressive of SWD, growers should not rely on cooling alone. It will still be important to manage SWD in field.
Thanks to Shinji Kawai for making the information from the 1939 Kanzawa paper available.
Graph courtesy of Ed Show
Graph courtesy of Ed Show