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 (32^o F).

As is noted in the two graphs below, at constant temperatures of up to 35^o 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 35^o 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 35^o 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.

Introduction:  An August 2009 field trial demonstrated the efficacy of  malathion and zeta-cypermethrin (Mustang) against the spotted winged drosophila (SWD), Drosophila suzukii in raspberries.  The following summary reports efficacy of additional materials trialed against SWD in December 2009.

Materials and Methods:

Treatments:

Products were applied on December 3, 2009 with a Maruyama 056 gas powered back pack sprayer at maximum labeled rates in 179 gallons water per acre and 150 psi.  Each plot was 1,173 sq ft of a proprietary raspberry variety under polyethylene-covered tunnels.

Pre- and post-treatment estimations of SWD were made with a D-Vac Model 122 (hand carry) gas-powered insect sampling device operated at an airflow capacity of 280 cu ft per minute per square foot at the opening of the collecting head.  Each side of a 36-ft section of 4-ft high hedge row was sampled.  Collected samples were transferred from the D-Vac into Ziploc bags for transport back to the lab where they were frozen for approximately an hour prior to counting.

Because male SWD (easily distinguished by the black spot on their forewings) comprised over half the total Drosophila captured in most samples, we assumed the remaining fraction of Drosophila to be female SWD (which do not have spotted wings).  However, since the species of the females was not confirmed, they will be given in results as "females". Results are reported as total (male + female) SWD.

In order to evaluate the impact of treatments on incidence of fruit infestation by larval SWD, thirty marketable fruit were collected from the center of each treatment replicate prior to treatment and then 7-, 14-, 21- and 28-days post-application.  Fruit samples were placed in 4x6x12-inch plastic bags fixed with a 1x ¾-inch PVC screened vent tube sealing the open ended of the rearing bag (see photo below).  After two weeks, the numbers of larvae, pupae and adults were enumerated.

Results:

Table 1: Total numbers of male + "female" SWD captured in D-Vac samples

Means followed by the same letter do not significantly differ (P=0.05, Student-Newman Keuls)

Table 2: Infested fruit

Means followed by the same letter do not significantly differ (P=0.05, Student-Newman Keuls)

The effect of Delegate and Malathion was most notable in the evaluation of infested fruit (Table 2).  Although all treated fruit collected one week after application demonstrated significantly lower levels of infestation than the untreated control, Delegate and Malathion treatments continued to show significant levels of control up to three weeks after application.

Conclusion: This study demonstrates that Delegate is as effective as malathion in controlling SWD at relatively low population levels as those encountered during December 2009.

There are several insecticides mentioned for control of vinegar flies in this article.  Before using any insecticides, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.

We are very grateful for the cooperation of Dutra Farms and Chris Hogan for providing the test site.

The following is the summary of results of a recent pesticide efficacy trial to control spotted wing drosophila (SWD) in raspberries.  All materials in the test are registered for caneberries and have a one day pre-harvest interval.

Treatments:

First mix of materials was made with the addition of 32 oz/A LI700 acidificant.

First application was made on August 19, 2009 with gasoline powered backpack sprayers in 1173 sq ft plots inside of tunnels.   Second application of malathion was made to the first treatment begun with Mustang on August 24.  The gallonage of water carrier was very much sufficient to achieve full coverage of the approximately six foot high raspberry hedgerow.

Sampling for SWD was done as per the “sweep net method” in which a not to be resampled 20 foot section of hedgerow is agitated with a stick and flies collected by insect net.  This method avoids tearing and soiling of the net, breaking canes and laterals, while collecting a good portion of the flies residing in the hedgerow.  Most samples resulted in male SWD (distinguishable by the single large spot on the back of each wing) being a large proportion of the sample, in most cases at least half, so the assumption is that the other half were female SWD.   While the chart below uses total numbers of vinegar flies collected, males and what were assumed to be females (but could in fact be regular vinegar flies since they were not sexed) were counted separately for later reference if necessary.

Means followed by the same letter do not significantly differ (P=0.05, Student-Newman Keuls)

The organically registered Pyganic and Entrust have efficacy over the short term, but the conventional materials show efficacy out to two weeks post application and offer a good solution to a serious outbreak of large numbers of SWD.

Additionally, three registered horticultural oils, Azadirect, Organicide and Ecotrol were tested in non-replicated fashion in the same area as the trial above.  None offered any control of significance, however.

The collaborating grower applied Diazinon 50W to raspberries of discontinued harvest at the maximum rate of 2 lbs per acre to devastating result.  While having similar levels of flies than the untreated check in the study before application, there were very few flies left after with many samples coming up zero, and this continued to be the case a week after.

There are several important topics to discuss concerning the results of these chemical efficacy trials.

Resistance:  With such outstanding results as those above, it is easy to decide that one will go with one of the cheaper chemistries and continue until the flies are all gone.  While this sentiment is understandable, it is imperative that pesticide users rotate the chemicals they are applying. Since one is pitting the chemical against what is quite likely several million flies per acre and the subsequent genetic diversity such a large number offers, the likelihood of the pest population becoming resistant and not controllable after continued use of the same chemical application after application is very high. History with other pests tells us that it can take only a few years to reach complete resistance.  Rotate.

Effects on Non-target Species:  Malathion and Mustang are broad spectrum pesticides, meaning along with killing flies they will kill most beneficial insects such as predators and parasitoids.  While for the time being, these chemicals are providing a solution to a critical acute situation, over the long term other solutions integrating other methods (more in continuing posts, enhanced sanitation comes to mind first and foremost) will have to be sought. Although one may tend to think that the high numbers of SWD indicate that there is very little biological control going on, remember that SWD is only recently established here so it may take several years for native predator and parasite populations to adjust to this new and large source of food.  The high densities of this fly in one place are just begging for disease outbreaks.  One expert with experience with insect invasions in California predicts with a high level of confidence that the large numbers of SWD currently experienced will be much lower in two to three years because of natural control.  

Raspberry and blackberry growers who use bees for pollination must be aware of what pesticides will do to foraging bees.  Malathion and Mustang are unquestionably toxic to bees, Entrust less so once it has dried.  Pyganic, while less toxic than the others, still requires some precautions.  It is best not to apply at all when plants are flowering, and generally this period of time will not have many vinegar flies present anyway, since they are drawn to ripening fruit rather than flowers.

The following link by Pacific Northwest Extension is very useful and includes a pesticide safety chart when referring to pesticides and honey bees:

https://catalog.extension.oregonstate.edu/sites/catalog.extension.oregonstate.edu/files/project/pdf/pnw591.pdf

There are several insecticides mentioned for control of vinegar flies in this article.  Before using any insecticides, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.