Tuesday, June 30, 2020

Management of Spotted-Wing Drosophila in Berry Crops

Since its introduction and spread in the US from 2008-2012, spotted-wing drosophila (SWD), Drosophila suzukii (Matsumura), SWD has presented a huge problem for berry growers.  SWD is a difficult pest to control.  Because of its high fecundity and number of generations, there is a high risk of insecticide resistance.  Consequently, growers should use a combination of tactics.  Because of the need to rely on non-chemical approaches as much as possible, there will be overlap in approaches between conventional and organic growers as they work to manage this common problem.  Nevertheless, because of specific concerns and constraints between the two groups of growers, specific comments will be made.

Fig.1. Spotted-wing drosophila adults. a. male showing black spots near wing-tip. b. serrated ovipositor of female.

Hosts: Hosts include caneberries, blueberries, strawberries, grapes and cherries.  Caneberries and blueberries are at greatest risk.  Grape is not an ideal host, with lower attack and survival rates than other berry crops.  Nevertheless, attacks by SWD can give rise to larval infestation and resulting sour rot.  There are differing levels of susceptibility among different winegrape cultivars, depending on variety (Shrader et al. 2019), ranging from Viognier (more susceptible) to Petit Manseng (less susceptible).

Description:  By now, most berry growers are familiar with this insect and its appearance.  The adult fly looks similar to native vinegar flies, with the main exceptions of a black spot on the leading edge of the wing in the male (Fig. 1a), and the large ovipositor of the female (Fig. 1b).  This large, serrated ovipositor allows the insect to insert eggs into the flesh of a ripening fruit, allowing larval establishment ahead of other drosophilids, which colonize rotting or overripe fruit material. Eggs are white, elongate oval, inserted under the skin of the berry, with long respiratory filaments from one end (Fig. 2a).  These respiratory horns may be found protruding from an oviposition site with magnification (Fig. 2b). 

Fig. 2. Spotted-wing drosophila eggs. a. exposed egg with respiratory filaments. b. Respiratory filaments protruding from oviposition hole in a blueberry.

Larvae are translucent maggots 2-3 mm long, with black mouth hooks visible at the anterior end (Fig. 3).  Silvery white tracheal tubes may be visible through the dorsal cuticle with magnification.  Respiratory projections are present on the posterior end, giving an appearance of being pointed at each end.  Pupae are brown, elliptical, about 3 mm long, with respiratory projections from the hind end.

Larvae are translucent maggots 2-3 mm long, with black mouth hooks visible at the anterior end (Fig. 3a).  Silvery white tracheal tubes may be visible through the dorsal cuticle with magnification.  Respiratory projections are present on the posterior end, giving an appearance of being pointed at each end.  Puparia (covering of the actual pupa) (Fig. 3b) are brown, elliptical, about 3 mm long, with respiratory projections from the hind end.

Fig. 3. Spotted-wing drosophila larvae in a raspberry (a), and puparium (b)

Biology: In eastern Asia, there are up to 13 generations.  A life cycle can be completed in 8-14 days, but adults can live up to 9 weeks.  Females use the atypically large ovipositor to lay eggs in fruits as they are ripening, earlier than other drosophila species.  Eggs are inserted under the skin of ripening fruit; each female lays 7-16 eggs/day. Eggs hatch in 1-3 days, and larval feeding on the flesh causes a collapse of localized tissue after another 2 days, followed by growth of fungal or bacterial organisms.  Updates will be posted in the spotted-wing drosophila page within the Virginia Fruit web site:  https://www.virginiafruit.ento.vt.edu/SWD.html

Monitoring:  In general, traps are not powerful enough to serve as a control.  Traps should be used to detect activity, and when flies are detected, make sure that other control measures are in place.  Several commercial traps are available (Trece and Scentry).  A trapping guide has been posted (Wallingford et al. 2018), with discussion of several baits, and comparing commercial with homemade traps.  Traps using homemade baits of either yeast or apple cider vinegar may be used for monitoring.  Traps using apple cider vinegar (ACV) alone are attractive to flies and less odorous to work with than with yeast added; traps with added yeast may be somewhat more sensitive, but fluid should be replaced with each service of the traps.  A combination of ACV and red wine (60:40) is more attractive than ACV alone (Shrader 2017).  If available, brown rice vinegar is more attractive than ACV (Willbrand and Pfeiffer 2019).  Traps should be checked at least weekly.  Most of the Drosophila flies collected will not be SWD, so the flies collected must be checked carefully.

  Chemical control:  Control measures are directed against the adults; there are no effective controls for larvae in the fruit.  As vulnerable fruit approach ripeness, weekly spray applications should be made.  Because of the high number of offspring and number of generations, there is a high risk of development of insecticide resistance.  Consequently, insecticides with different mode of actions should be rotated to prolong the effective life of insecticides.  Organophosphates (malathion and phosmet) are effective (check labels for registrations on specific crops), as are pyrethroids (be wary of induction of secondary pests).   Spinosyns offer an additional mode of action class, with spinetoram having greater efficacy than spinosad.  See Table 1 for a list of insecticides available for SWD on the most vulnerable crops.  Included are the maximum number of applications (or amounts of material) allowed per season and the IRAC class of mode of action.   This is important in designing rotations - it will be helpful to reserve materials effective against SWD until properly timed for that pest.  Organically approved treatments are included in the table.  It should be noted that the organic materials generally have a shorter residual life, and lower efficacy that synthetic insecticides.  It is also worth remembering that it may be worth keeping a material in a rotation even if of lower efficacy, to help keep the SWD population from adapting to other products.

Blueberry         Caneberry               Grape                Strawberry
IRAC 1A – carbamates
Lannate 90SP
              Rate                         0.5-1 lb                         --                                   --                                --
              PHI                         3d                                  --                                   --                                --
              Applics/Season       4                                    --                                   --                                --
IRAC 1B - organophosphates
Malathion 8F (malathion)
Rate                        2.5 pt 3                          2 pt                             1.88 pt                        1.5‐2 pt
PHI                         1d                                 1d                                3d                               3d
Applics/Season       2                                   4                                  2                                 4
Imidan 70WSB (phosmet)
Rate                        1.3 lb                            ‐‐                                  1.3‐2.12 lb                  ‐‐
PHI                        3d                                  ‐‐                                  7‐14d (label)               ‐‐
Applics/Season      5                                    ‐‐                                  6.5 lb                          ‐‐
IRAC 3A – pyrethroids and pyrethrin
Brigade 10WSB (bifenthrin)
Rate                       16 oz                            16 oz                              8‐16 oz                     16 oz
PHI                        1d                                 3d                                   30d                            0d
Applics/Season      80 oz                            2 (32 oz)                        16 oz                          80 oz
Danitol 2.4EC (fenpropathrin)
Rate                    10.6‐16 fl oz                    ‐‐                                   10.6‐21.3 fl oz          10.6 fl oz
PHI                     3d                                     ‐‐                                   21d                            2d
Applics/Season    2                                      ‐‐                                   2                               2 (2.6 pts)
Mustang Maxx 0.8E (zeta‐cypermethrin)
Rate                    4 fl oz                              4 fl oz                             2‐4 fl oz                    ‐‐
PHI                     1d                                     1d                                  1d                              ‐‐
Applics/Season    24 oz                               24 oz                             24 oz                         ‐‐
Tombstone 2EC (cyfluthrin)
Rate                      ‐‐                                      ‐‐                                  2.4‐3.2 fl oz               ‐‐
PHI                       ‐‐                                      ‐‐                                  3d                               ‐‐
Applics/Season2     ‐‐                                      ‐‐                                  12.8 fl oz                    ‐‐
                 (3.2 fl oz/14d)
4PyGanic 5EC (pyrethrin)
Rate                     4.5‐18 fl oz                         4.5‐18 fl oz                4.5‐18 fl oz               4.5‐18 fl oz
PHI                      0d                                        0d                               0d                              0d
Applics/Season    n/a                                      n/a                              n/a                             n/a

IRAC 5 – spinosyns
Delegate 25WG (spinetoram)
              Rate                     3-6 oz                                 3-6 oz                          3-5 oz                        --
              PHI                      3d                                       1d                                7d                              --
              Applics/Season    19.5 oz                               6                                  19.5 oz                      --
Radiant 11.7SC (spinetoram)
Rate                     ‐‐                                        ‐‐                                  ‐‐                              6‐10 fl oz
PHI                      ‐‐                                        ‐‐                                  ‐‐                              1d
Applics/Season    ‐‐                                        ‐‐                                  ‐‐                               5
4Entrust 80W (spinosad)
Rate                    1.25‐2 oz                           1.25‐2 oz                       1.25‐2.5 oz                1.25‐2 oz
PHI                     3d                                      1d                                  7d                              1d
Applics/Season    31                                                          6                                    5                                5
IRAC 28 – diamides
Exirel 10.2EC (cyantraniliprole)
              Rate                    13.5-20.5 fl oz                    --                                    --                              13.5-20.5 fl oz
              PHI                     3d                                        --                                    --                              1d
              Applics/Season   0.4 lb ai                               --                                   --                               0.4 lb ai

IRAC 6/28
Minecto Pro (abamectin/chlorantraniliprole)
              Rate                     --                                         --                                     --                            10 fl oz
              PHI                       --                                        --                                     --                            3d
              Applics/Season     --                                        --                                     --                            40 fl oz
IRAC Unknown
4Surround 95WP (kaolin)
              Rate                     25-50 lb                               25-50 lb                         25-50 lb                  --
              PHI                      see label                               see label                        see label                 --
              Applics/Season     --                                         --                                    --                            --
4Grandevo (Chromobacterium subtsugae strain PRAA4-1)
              Rate                    2-3 lb                                   2-3 lb                             2-3 lb                       2-3 lb
              PHI                     0d                                        0d                                   0d                            0d
              Applic/Season     n/a                                      n/a                                   n/a                           n/a

1 3 applications per crop, 6 applications per calendar year.
2 See label. Maximum seasonal rate depends on use of other products.
3 This rate may be available as a 24C registration; check for state labels. Full Sect 3 label rate is 1.25 lb.
4 OMRI-approved, suitable for organic production.

Our Southern Region Small Fruit Consortium provides recommendations for SWD in the pest management guides for caneberries, blueberries, strawberries and bunch grapes.  In addition, SWD is addressed in our Virginia VCE Small Fruit recommendations (https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/456/456-017/ENTO-337B.pdf).

If SWD needs to be controlled in a vineyard setting, it will helpful to make an application just before berries close in clusters, since many oviposition strikes are in the protected inner surfaces of the cluster.  Surround has been shown to reduce injury by half, even though this time is well before normal SWD timing.

The flowable formulation of malathion is safer than the EC formulation, but the flowable formulation may be in shorter supply.  When using the EC (oil-based) formulations, use caution if also applying the fungicide captan.  The oil can act as a penetrant, potentially causing phytotoxicity).

Cultural control:  Netting of 80g weight was effective in controlling injury (McDermott and Nickerson 2014, Riggs et al. 2016, Ebbenga et al. 2019).  Lighter grades (larger mesh) are not effective.  While netting is initially expensive, it becomes cost effective because it may be used for several years.

Harvest fruit promptly and thoroughly to eliminate breeding sites. It is important to harvest all fruit, including those in the interior and lower parts of the plant canopy.  This can be problematic in pick-your-own operations.  This issue should be kept in mind once SWD established in an area, since at times grape growers may leave berries on the vine to allow greater development of some harvest parameters.   Any overripe or rotten fruit nearby should be destroyed.  In vineyards, pomace produced during the crushing process should not be dumped near the producing vineyard block.  This can become a source for many SWD.

When berries are harvested, it will be helpful to get them as cool as possible, as soon as possible.  There is complete mortality of larvae in fruit held for 96 hours at 35˚F, and below 40˚F, eggs and larvae don’t develop (Bolda 2010, Burrack 2016).  In most cases, such uniform holding conditions are not maintained; fruit cooling should be considered a component of SWD management and not a sole control tactic.

Biological control: Because of the ability of SWD to encapsulate and kill the eggs of our native parasitoid wasps, biological control has not been successful.  Research is underway to find parasitic species that are able to attack this species.

Bolda, M. 2010. Length and magnitude of fruit cooling and spotted wing drosophila mortality, Strawberries and Caneberries. Univ. Calif. Agric. Nat. Res. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=2474.
Burrack, H. 2016. Preventing and managing spotted wing drosophila infestation. NC State Extension.  Entomology - Insect Biology and Management.  https://entomology.ces.ncsu.edu/2016/06/preventing-and-managing-spotted-wing-drosophila-infestation/.
Ebbenga, D. N., E. C. Burkness, and W. D. Hutchison. 2019. Evaluation of exclusion netting for spotted-wing drosophila (Diptera: Drosophilidae) management in Minnesota wine grapes. J. Econ. Entomol. 112: 2287–2294.
McDermott, L., and L. Nickerson. 2014. Evaluation of insect exclusion and mass trapping as cultural controls of spotted wing drosophila in organic blueberry production. N. Y. Fruit Quarterly 11: 25-27.
Riggs, D. I., G. Loeb, S. Hesler, and L. McDermott. 2016. Using insect netting on existing bird netting support systems to exclude spotted wing drosophila (SWD) from a small scale commercial highbush blueberry planting. N.Y. Fruit Quarterly 24: 9-14.
Shrader, M. E. 2017. Drosophila suzukii (Matsumura) (Diptera: Drosophilidae): Risk assessment for an invasive vinegar fly in Virginia vineyards.  Ph.D. dissertation. Virginia Tech, Blacksburg. 141 p.
Shrader, M. E., H. J. Burrack, and D. G. Pfeiffer. 2019. Drosophila suzukii (Diptera: Drosophilidae) oviposition and adult emergence in six wine grape varieties grown in Virginia. J. Econ. Entomol. 112: 139–148.
Wallingford, A., B. Sideman, and G. Hamilton. 2018. Monitoring spotted wing drosophila (SWD) with traps. Univ. New Hampshire Extension. https://extension.unh.edu/resource/monitoring-spotted-wing-drosophila-swd-traps.
Willbrand, B. N., and D. G. Pfeiffer. 2019. Brown rice vinegar as an olfactory field attractant for Drosophila suzukii (Matsumura) and Zaprionus indianus Gupta (Diptera: Drosophilidae) in cherimoya in Maui, Hawaii, with implications for attractant specificity between species and estimation of relative abundance. Insects 10: 80 (18 p).

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