Managing Spotted Wing Drosophila in Blueberry and Cherry Orchards in California

Jhalendra Rijal, Ph.D., IPM Advisor

UCCE Merced, Stanislaus, and San Joaquin Counties & Statewide IPM Program

jrijal@ucanr.edu; @IPMAdvisorNSJV

Background

Spotted wing drosophila (SWD), Drosophila suzukii, is an invasive pest that was introduced in California in 2008 and attacks multiple fruit crops. SWD has spread throughout the United States within a short period. SWD is a small fruit fly species, and the fruit feeding larval stage causes damage to several fruits, especially small berry fruits such as raspberry, blueberry, blackberry, strawberry, and a few thin-skinned fruits such as cherry. SWD adults (Fig. 1) look similar to other flies of the genus Drosophila, including the common vinegar fly. SWD adults are about 1/8 inch in size and have prominent red eyes, pale brown thorax and abdomen with black bands on the abdomen. The male fly has two distinguishing characteristics: 1) black spot on the tip of each wing, and 2) dark band encircling the forelegs. The female fly has a heavily serrated egg-laying apparatus known as an ovipositor (Fig. 2).

Life cycle and the nature of the damage

Out of ~1500 Drosophila species worldwide, SWD is one of two species capable of causing severe damage to healthy and ripening fruits. This is because unlike other vinegar flies, the SWD female is equipped with a serrated (i.e., saw-like) ovipositor that can incise the fruit skin and deposit eggs inside healthy fruit. The ‘oviposition scar’ can be visible on the fruit skin in which the white egg tube (filament) is often evident (Fig. 3). Eggs hatch in a few days and the tiny white maggots (i.e., legless larvae) feed inside the fruit and pupate. SWD can complete a life cycle in as little as one week and produce several generations per season. In infested fruits such as cherry and blueberry, direct damage by internal-feeding larvae leads to tissue damage and ultimately, the fruits collapse (Fig. 4). Fruits injured by SWD egg-laying and feeding activities become an easy target for several other secondary pests such as vinegar flies and even fungal infections (Fig. 5), which are not otherwise a problem for healthy fruits. Due to direct and indirect damage, there is a high risk that SWD infested fruit lots will be rejected at initial processing/handling facilities and during the export process. Therefore, the economic threshold for SWD in fruits is practically ‘zero’.

Monitoring for SWD in orchards

Because of the broad host range, unique egg-laying behavior, high fecundity, and the large number of generations per season, damage caused by SWD in soft and susceptible fruits such as cherry and blueberry become severe very quickly. Therefore, it is very important to pay attention to SWD adult activities in the orchard throughout the season, as well as any other signs of fruit damage in the orchard on a regular basis. SWD traps can help determine the presence and seasonal activity of SWD in orchards because the population and fruit susceptibility vary within the season. Both cherry and blueberry fruits become susceptible to SWD attack as soon as the fruit color begins to change—from green to pink/blue (in blueberry) or green to red (in cherry)—and the susceptibility continues until harvest. There are several types of commercially available traps and attractants(Fig. 6). Some traps are a ‘jar’ type, and others are a ‘sticky’ type. Also, there are two liquid-based baits as well as synthetic lures. Alternatively, traps can be constructed locally, using apple cider vinegar or other kinds of baits. Some local trap designs are described here (https://bit.ly/2UvUNTW). Although some traps are more specific than others in catching SWD, none of the pitfalls to date have shown complete specificity to SWD, as they catch other small insects and lookalike vinegar flies. In this context, identification of SWD from other flies (mostly vinegar flies) is crucial. Here is an excellent id-kit (https://bit.ly/2UuasmZ). It is recommended to hang traps on shady sides of the tree as SWD prefers the areas that are moist and under shade. The height of traps should be within reach or at eye-level. In addition to placing traps for monitoring, it is highly advisable to be vigilant for signs of SWD damage on the fruits still in trees as well as those dropped onto the orchard floor. Here is excellent information for recognizing fruit damage caused by SWD (https://bit.ly/2QVqILm)

Managing SWD in orchards

Non-insecticidal methods. Although the use of insecticide has been the primary way to control SWD in crops, several cultural and other practices help reduce the orchard’s population overall and help manage SWD flies. SWD likes damp and shady areas; therefore, cultural practices that open up the orchard canopy for greater sunlight penetration and promote proper water/drainage management that avoids oversaturation would create an environment that is less favorable to SWD. If the orchard is surrounded by wild host plants (https://bit.ly/3auagcN) which can be removed, that helps to reduce SWD incidence. Timely harvest and orchard sanitation by removing the overripe and dropped fruits are effective ways to minimize the SWD population build-up within the orchard. Recently, the use of exclusion netting (80-mm fabric) to exclude SWD adults movement into the orchard has been adopted in other states especially for blueberry, raspberry, etc. For this purpose, high tunnel or bird-netting structures can be repurposed and modified (Fig. 7). Not all practices fit into all crops, but it is important to use appropriate additional or alternative practices wherever possible to minimize the long-term damage and other secondary consequences of SWD insecticide sprays. These practices are especially relevant for organic production systems where very few insecticides are available.

Insecticidal methods. Insecticide use is the primary way to control SWD in California orchards. Several groups of insecticides, particularly a few pyrethroids (e.g., Warrior II and Delegate), malathion, and spinosyn (e.g., Success, Entrust and Delegate) products showed efficacy against SWD (Table 1). Although some neonicotinoids showed good mortality of egg and larvae in trials, their performance is weak against SWD adults, and therefore not recommended to prevent damage. It is recommended to spray insecticides 2-3 times when fruits begin to color using 7-10 days intervals until harvest for cherry and blueberry orchards in California. When selecting an insecticide, major consideration should be given to the efficacy of the pesticide against adults, presence of other pests, the preharvest interval (PHI; i.e., wait time between a pesticide application and crop harvest), the re-entry interval (REI), and the fitness within the overall integrated pest management (IPM) program. Additionally, the selection of a particular type of insecticide is dictated by the maximum residual limit (MRL; i.e., the highest level of a pesticide residue that is legally tolerated in food at harvest) which varies among different countries for the export market.

In organic production, spinosad (Entrust), pyrethrin (Pyganic and Evergreen EC60-6), biological insecticides such as Grandevo (fermentation product of bacterium, Chromobacterium subtsugae strain PRAA4-1) have shown efficacy against SWD across different states. However, these products need to be applied in rotation and on a timely basis (i.e., at least 7-day intervals) to achieve satisfactory control.

Since multiple applications of insecticide are required for effective SWD management, selection and rotation of active ingredients within the season is critical to minimize the potential risks of secondary pest outbreaks, adverse impacts to natural enemies, and avoiding pest resistance. Recently, there have been some anecdotal reports of recommended insecticide programs not controlling the SWD population in cherry in the San Joaquin Valley. A recent study reported the first evidence of SWD flies resistant to spinosad (Success) insecticide in coastal California in cranberries (Gress and Zalom 2018). The authors found the LC50 (i.e., the concentration needed to kill 50% of the test population) of spinosad on SWD flies collected from the treated fields was up to 7.7-fold higher than those collected from the unsprayed fields. Therefore, the judicial use of available insecticides and other monitoring and management options is crucial for sustainable management of SWD.

Table 1. Efficacy of various insecticides to use against spotted wing drosophila (SWD) in cherries (Table Credit: Robert Van Steenwyk, UC Berkeley). (Note: The insecticide information here may not be up to date. Always check the label and legality of the pesticide prior to use.)

References:

Beers et al., 2010. http://jenny.tfrec.wsu.edu/opm/displayspecies.php?pn=165

Walse et al., 2010. J. Integ. Pest Mngmt. 2(1): 2011; DOI: 10.1603/IPM1001

Dreves and Langellotto-Rhodaback 2011. http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/20883/em9026.pdf

Hauser 2011. Pest Manag. Sci. 2011; 67: 1352–1357; DOI 10.1002/ps.2265

Lee et al., 2013. Environ. Entomol. 42(6):1348-1355. http://dx.doi.org/10.1603/EN13148

Lee et al., 2015. Ann. Entomol. Soc. Am. 108(2):117-129. http://dx.doi.org/10.1093/aesa/sau014

UC IPM guidelines, http://www.ipm.ucdavis.edu/EXOTIC/drosophila.html

Gress, B.E., and F. G. Zalom 2018; Pest Manag Sci.75: 1270–1276, https://www.ncbi.nlm.nih.gov/pubmed/30324771