Use of Chemical Pesticides to Control Spotted Wing Drosophila

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.