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Microsprinklers for Management of Spotted Wing Drosophila In Mature Pnw Highbush Blueberry

Volume 5 Issue 10

Beverly Gerdeman, Hollis Spitler and Lynell Tanigoshi

Why Microsprinkler chemigation?

  • Mature, berry-laden highbush blueberry plants reduce row width and inhibit tractor entry (Fig. 1).
  • Any equipment entering the fields causes berry drop, reducing yield, however microsprinklers provide a noninvasive method that does not promote berry drop (Fig. 2).
  • Prior to arrival of SWD in 2009, insecticides were rarely needed during harvest.
  • Since the arrival of SWD, weekly insecticide applications are needed to protect berries.
  • Microsprinkler applications can treat large acreages in a fraction of the time needed to make an airblast application, for example, in 2016 Nelson microsprinklers treated a 5-acre plot in 15 minutes.

Initial Research

  • In 2010, we investigated chemigation as a method to control SWD using Netafim™ Supernet microsprinklers, originally intended for cooling, at a large blueberry grower in Oregon.  Oregon had the infrastructure already in place for cooling, which facilitated our research.
  • The Netafim microsprinklers were positioned on a 20’ x 12.5’ triangular spacing of ‘Aurora’.
  • Leaf bioassays using the WSU NWREC SWD colony, provided a reference for the length of time the residues were effective in killing SWD in the field.
  • How did microsprinklers compare with airblast applications?  Although not directly comparable, bioassays showed trends in efficacy between airblast and microsprinkler chemigation (Table 1).  Results indicated that although airblast residues are very effective, the results were offset by berry drop and time and labor factors.

Microsprinkler Chemigation Basics

  • Different classes of insecticides degrade at different rates.
  • Pyrethroids, such as Mustang Maxx, exhibit a long, gradual decline curve, while organophosphates such as Malathion 8 Aquamul, decline steeply after 3-5 days, regardless of the application method.
  • These differences in decline mean that Mustang Maxx (zeta-cypermethrin) kills SWD quicker and residues last longer than malathion.
  • Zeta-cypermethrin residues are not acceptable in certain countries including Canada.  However other pyrethroids could be substituted but require chemigation on the label.
  • Canopy density acts like an umbrella and can limit droplet penetration into the bush where it is cooler and humidity is higher, preferred by SWD.
  • Insecticide applications kill insects in 2 ways: 1) when droplets contact the flies directly and 2) when flies come in contact with toxic residues.  The duration of a microsprinkler application is longer than a quickly passing airblast application, thus increasing the likelihood that droplets will strike the flies.
  • Some insecticides like pyrethroids can boost the efficacy of lesser efficacious products because of their longer residual (Fig. 3).  Furthermore, as the season progresses, sublethal levels of the insecticides accumulate.  These layers continue to boost activity of each subsequent application.  Lesser efficacious products can then be safely rotated into the program without control loss.

Whatcom County Chemigation Research 2015 – 2016

  • In 2015 we began evaluation of the Nelson R10 Rotator® microsprinkler for its potential in managing SWD.
  • The R10 may have multi-purpose potential for frost protection.
  • Three different triangular spacings I (25’x 30’), II (30’x 30’) and III (33’x 30’) were tested across 5-acres at 45 psi in 2016 to compare efficacy.
  • Spacing 1 – a mature ‘Jersey’ plot with large bushes.
  • Spacing II – a younger, smaller ‘Bluecrop’ plot.
  • Spacing III – a medium sized ‘Bluecrop’ plot.
  • Malathion 8 Aquamul and Mustang Maxx registered for chemigation, were evaluated.
  • Design velocities allowed treatment of a 5-acre plot in just 15 minutes.


  • Bioassays of leaves were taken from 2 locations: opposite microsprinklers in non-sprinkler rows and midway between microsprinklers in sprinkler rows.
  • A total of 162 leaves were bioassayed on each date from 3 positions: upper, middle and lower/interior from each location in the 3 spacings.
  • Bioassays were performed in Petri dishes with 3 leaves and 10-15 SWD for 24 and 48 hours (Fig. 4).
  • 48-hour bioassays were used by Michigan researchers and adopted in 2016 for the Whatcom County studies because they were believed to more accurately represent field conditions.  Differences between Mustang Maxx % mortality at 24 and 48 hours were negliglible but Malathion improved after 48 hours.


  • Only trends between the Oregon and Washington microsprinkler field trials can be compared because of huge variables (e.g. cultivars, microsprinklers, rates, droplet size, spacing, etc.).
  • Both products performed similarly in both locations.  Mustang Maxx outperformed Malathion 8 aquamul (Figs. 5 & 6, Table 2).
  • Efficacy of Netafim and Nelson microsprinklers was virtually the same (Fig. 7), providing confidence in the application method. (Nelson data was an average of the 3 different spacings).
  • 2016 bioassays indicated no differences in % mortality between spacings I and II.
  • Even the lower/interior locations exhibited ~ 70% mortality at 7 DAT with Mustang Maxx (2016).
  • Spacing III differed from the other 2 and results may be related to the large 33’ configuration and/or distance from injector and should be further investigated (Figs. 5, 6, 7).
  • Additional tweaking of the system should provide further improvements but testing overlap areas for potential residual hotspots or wash-off is recommended.

Maximizing Efficacy of Microsprinkler Chemigation

  • Bioassays are a useful tool.  They provide comparable information between products but underestimate actual field toxicity because they only take into account mortality as a result of dried residues during a 24 or 48-hour period and not droplet impact.  In the field however, flies constantly inhabit a toxic environment, come in contact with both wet and dry residues and experience direct impact by droplets, as a result actual field efficacy is always higher than bioassay results.
  • Efficacy, percent mortality of SWD, can be improved by manipulating the order of insecticides in a blueberry management program.  This is also true for regular airblast applications.
  • Leading off the season with a pyrethroid will provide a longer, protective layer that will boost efficacy of subsequent lesser residual insecticides!

Future of Microsprinkler Chemigation for SWD Control

Results of both our Washington and Oregon trials, even with different microsprinklers, indicate microsprinkler chemigation is an effective method of controlling SWD in blueberry. Interest in microsprinkler chemigation for SWD continues to expand in Oregon and interest is growing in Washington and British Columbia.  The bottom line is no berry rejections caused by wormy fruit have been reported from cooperator fields in Oregon or Washington.  Ongoing advances in irrigation technology will continue to increase speed and efficacy of microsprinkler chemigation for SWD control in blueberry.  Irrigation companies should continue working to lower costs, making it economically feasible for all levels of blueberry production.