Final report for GS22-269
Project Information
Drosophila suzukii (Matsumura) (Diptera Drosophilidae) commonly known as spotted-wing drosophila (SWD) is an invasive insect pest threatening many small fruit industries in the Americas and Europe for more than a decade. While many control approaches are being utilized in the invaded regions to manage this pest, biological control using parasitoids is one of the promising strategies for the sustainable management of SWD. We conducted a two-season-long field exploration for native parasitoids of SWD during 2021 and 2022 around major blueberry-producing locations in Georgia, United States. Fruit-baited sentinel traps infested with SWD eggs were placed around eight commercial blueberry orchards in Southeast Georgia. A total of 371 Drosophila-related parasitoids were collected and classified into three families: Figitidae, Pteromalidae, and Diapriidae. Among them, Leptopilina boulardi (Hymenoptera: Figitidae) and Pachycrepoideus vindemmiae (Hymenoptera: Pteromalidae) were the most abundant species in the collection. Previously, only P. vindemiae was known to parasitize successfully on SWD. In subsequent laboratory testing, L. boulardi collected from Georgia also parasitized less than 7% of the SWD but failed to eclose as adults from the infested larvae. The number of parasitoids captured was higher during the peak blueberry ripening to harvest season compared to the flowering, fruiting, or after the fruits were harvested. Parasitoids were most abundant in the wild locations compared to cropped fields, but no difference was observed when organic fields and conventional planting systems were compared. This is the first account of any Drosophila-related native parasitoid survey conducted in Georgia. A follow-up project based on these outcomes was built for the classical release of an exotic parasitoid Ganaspis brasilensis (Hymenoptera: Figitidae) around the surveyed locations. A total of 14,164 G. brasilensis (8493 females and 5671 males) were released around these locations and are being closely monitored.
This project will focus on exploring, collecting, identifying, and testing the efficiency of key parasitoids of the SWD in the newly invaded habitats, particularly in fruit-growing regions of the southeastern U.S. The specific objective of this project will be:
(1) To explore and collect the larval and pupal parasitoids of the SWD in the blueberry orchards, other fruit crop farms, and alternative wild habitats. The project focused on exploration and collection of parasitoids on many blueberry orchards, other small fruit orchards, and the nearby wild habitats of those orchards located mainly in Georgia, USA.
(2) To study the abundance, distribution, and habitat selection of the native parasitoids of SWD in Georgia. This objective targeted the understanding of the relationship of the parasitoids with their habitat composition, diversity, and their association at a temporal and spatial scale.
(3) Evaluate the biology, life history, and host selection of native parasitoids concerning their SWD population in these habitats. The collected parasitoids were reared in the laboratory and were tested for their efficiency to parasitize the D. Suzukii. If a new species parasitoid was discovered that was not previously reported in either Georgia or even in the U.S., they were further tested and evaluated for their association to SWD parasitism.
Cooperators
- (Educator and Researcher)
- (Educator and Researcher)
- (Educator and Researcher)
Research
Parasitoids of SWD were explored in eight blueberry growing sites located in Bacon Co., Pierce Co., and Appling Co. of southeastern Georgia. Among the selected sites, five locations were conventionally managed farms, and three others were organically managed farms based on pesticide and fertilizer application practices. The presence of SWD in all sites were confirmed in 2015.
Sentinel traps and sampling procedure
Sentinel traps were made of a 946 mL clear plastic container (Fabri-Kal®, Kalamazoo, MI) clear plastic cups with eight 0.8 cm diameter holes punctured around the cup to allow the entry of flies and parasitoids into the fruit bait. The fruit bait (approx. 50 gm mixed fruits) was infested with SWD eggs 1-2 days old before the installation in the field. The bait was placed above the center of an agar-based media placed in a 2 oz plastic cup, placed inside the sentinel trap. Each cup was filled with 1.5 cm thick media, above which fruit bait was placed. The agar media maintained the humidity inside the traps to prevent early desiccation of fruit bait inside the trap. A thick cardboard tarp roof was placed at the top of each sentinel trap to protect the bait from exposure to direct sunlight. To prevent ants from entering the trap, a sticky ring tanglefoot (Tanglefoot Company, Grand Rapids, Michigan, USA) was installed above the cardboard roof. The traps were hung about 1-2 m above ground in the host plant using a metal wire attached to the trap. The SWD-infested fruit-baited traps were retained in each location for 10 ± 2 days in 2021 and 12 ± 2 days to allow parasitism in the larvae and pupae in SWD-infested fruit baits. The traps were removed from the field after retention and brought back to the lab. All containers were incubated for four weeks under ambient laboratory conditions of 21 °C and 65% relative humidity. All the samples collected during the process were kept under observation for six weeks for the emergence of any adult flies and parasitoids. Any emerged parasitoids were collected into small cages for identification, sorting, and further rearing.
Identification of the collected parasitoids
The traps collected from the field were examined twice a week for parasitoids emerging from host pupae. After all the non-parasitized immatures of the flies inside the fruits matured, they were removed. The traps were kept under observation until any parasitoids of the present cease to emerge after maturing. Containers were examined for 6-8 weeks to ensure the recording of all emerged parasitoids. At first, similar species based on visual identification were sorted and classified. For each species, both males and females were kept in the same vial and facilitated mating. Honey water was always provided to the parasitoids as a source of food. For the consultations on identification, voucher samples were sent to the Systematic Entomology Laboratory in Beltsville, MD during the first year. During the second year, the parasitoids were identified based on the reference from the first year, and previously published documents (Gibson et al 1997: Lue et al 2016). After the identification, all the iso-female lines reserved from the field sample were merged and parasitoid colonies were established.
Assessment of parasitism and success rate of parasitism:
When the parasitoids collected from the sample started to emerge in the lab, the assessment of their efficiency of parasitism was done by using the degree of infestation (DI) as described by Wang et. al (2016). DI was calculated to measure the proportion of hosts that are successfully parasitized per female. For this, 100 SWD immatures (2nd instar larvae) were subjected to parasitize by L. boulardi for 48 hours and the test was replicated 4 times in the laboratory (based on the frequency of successful capture). Each time, live parasitoids were captured from each location and kept together in the same container to facilitate mating. After each test, the proportion of SWD larvae that were successfully parasitized was recorded. A similar experiment was repeated to test the efficiency of parasitism of P. vindemiae, subjecting each mated female to 1–2-day old SWD pupae for 48 hours, and the test was replicated 10 times.
The success rate of parasitism (SP) was estimated as pi/ (T-di) (where pi = the number of emerged parasitoids). The SP was measured by calculating the probability that a parasitized host will give rise to an adult wasp and estimated as (T- di)/T (where T= the number of emerged flies in the absence of the parasitoids, di = the number of emerged flies in the presence of parasitoids). We put the parasitized SWD larvae under observation for 1.5 months to see if any adult parasitoids emerged to calculate the SP.
Statistical analysis:
Seasonal trends in parasitoid abundance were categorized based on sampling dates and their relationship with relative phenological stages of the blueberry were analyzed using ANOVA for each year. Differences in the the mean parasitoids trapped in per sampling date were separated using least mean squares. Differences in mean parasitoid abundance in different habitats were analyzed using students’ T-test. All data were analyzed in JMP Pro 16 ® (SAS Institute, Cary, NC).
Total parasitoids capture
Data sets for the total number of parasitoids collected during the study were pooled together to distinguish the proportion of parasitoids captured during the study. A total of 371 adult parasitoids emerged from the host pupae collected from sentinel traps placed in 8 different locations. The taxonomic identification revealed that most of the parasitoids that emerged were cosmopolitan generalist drosophilid parasitoid species with Leptopilina boulardi (Barbotin, Carton et Keiner-Pillault) (Hymenoptera Figitidae) capturing the most (242, 65%), followed by P. vindemiae Pteromalidae, (123, 33%). Two other genera of drosophila-related parasitoids: Trichopriya sp. (3, 1%) and Spalangia sp. (1, 0.2%) were also captured during this study. Some parasitoid specimens that remained unidentified (2, 1%) were labeled as ‘others.
Seasonal differences
The mean seasonal occurrence of the parasitoids collected from flowering to the harvest of blueberries was analyzed separately for each year to discern if any seasonal pattern arises across years. We observed a consistent temporal pattern of parasitoid occurrence during both years. For both parasitoids collected, the first parasitoid occurrence was recorded during the early fruiting season of the blueberries. As the season progressed, the occurrence of both L. boulardi and P. vindemiae was most abundant during the late-ripening season until the fruits were completely harvested in the field. After the harvest season came to an end, the parasitoid collection started to decline and ceased completely when the blueberry bushes were pruned. This pattern was consistent for both drosophilid parasitoids for either of the sampling seasons. During 2021, the abundance of L. boulardi was significantly different during the late-ripening (F = 2.53; df = 5,119; p = 0.03) and during the end of the harvest season (F = 2.50; df = 5,119; p = 0.02) compared to parasitoids captured during any other crop stages. During 2022, the abundance of P. vindemiae was significantly different during the late-ripening season (F = 2.68; df = 5,122; p = 0.02) compared to any other crop stages.
Parasitism test
The degree of infestation (DI) of the parasitoids, which is the proportion of the SWD immatures that were successfully parasitized by L. boulardi on SWD larvae was very low based on our laboratory observation. On four total observations involving 100 SWD immatures on each, we found that only 6.67 ± 0.81 of the larvae were parasitized by L. boulardi female. We quantified the success rate of parasitism (SP), which is the measurement of the probability that a parasitized host will give rise to an adult wasp. We observed that none of the parasitoids successfully emerged from the infested immatures of SWD. Therefore, based on this study the SP for L. boulardi collected in Georgia on SWD larvae was determined as zero. Similarly, the assessment of parasitism and success rate of parasitism for P. vindemiae were also evaluated. On 10 total observations, 43.5 ± 2.52 of the larvae were parasitized per female of P. vindemiae. The success rate of parasitism (SP) for P. vindemiae captured at different times in Georgia was 27.3 ± 1.77. Since P. vindemiae are known to successfully parasitize SWD, we have outlined a more detailed study to test their efficiency and success of parasitism on various conditions in a separate laboratory study that will be conducted in the future which is not discussed in this literature.
Discussion:
In this study, we found that L. boulardi species was the most captured species of parasitoid collected from the sentinel traps. In other similar surveys, this was one of the most common species of Drosophila spp. parasitoids collected previously in the USA (Miller et. al 2015; Wang et al. 2016), Europe (Miller et. al 2015), Asia (Girod et al 2018), and Mexico (González-Cabrera et al 2020). The standout abundance of L. boulardi in the field was due to their capacity to readily attack other common Drosophila spp. larvae in the field (Buffington and Forshage, 2016).
Similarly, P. vindemiae was another generalist pupal parasitoid of drosophila that was collected in our study. Many other similar studies have reported collecting the P. vindemiae from the SWD-invaded regions in Europe (Rossi Stacconi et al. 2013), North America (Miller et al. 2015; Wang et al. 2016), and other locations all around the world (Wang et al 2020). In 2021, the larval parasitoid L. boulardi was more abundantly captured in our traps than the pupal parasitoid P. vindemiae. However, in 2022 both of their abundances were nearly equal. The lower number of pupal parasitoids in 2021 could be an underestimation because of the experimental procedures. In 2021 the infested larvae in the fruit bait got a lower time of exposure (10 ± 2d) in the field compared to exposure to the SWD-infested fruit traps in 2022 (12 ± 2d). Two more days of field exposure might have allowed the SWD-infested fruits to produce more pupae in the later days of exposure, giving a relatively equal chance for the pupal parasitoid to infest into the pupae. Underestimation of pupal parasitoids compared to larval parasitoids are quite common in similar studies (Daane et al. 2016, Miller et al. 2015) where a lower number of pupal parasitoids were correlated with a lower period of exposure to pupa for parasitism. This is because longer exposure of the SWD-infested bait in the field, especially during the fruiting season would otherwise lead to the unintentional release of SWD in the field.
We also observed a temporal trend in the composition and distribution of the parasitoids collected between the sampling dates during the blueberry fruiting season during both years. We had a low number of parasitoids captured during the flowering and early fruiting season and the number gradually increased until the fruits were harvested. This trend might have existed because of a few reasons, one being the frequent application of insecticides during the flowering and early fruiting season in the blueberry production system to minimize SWD infestations (Van Timmeren & Isaacs 2013). Extensive use of insecticides during the flowering and fruiting season can reduce the fecundity, longevity, and development rates of parasitoids (Desneux et al. 2007). Hence, the parasitoid number escalated at peak fruit season or after the harvest, when the use of chemicals ceased completely. Another reason that might have contributed to the higher incidence of parasitoids near/after harvesting is due to higher success of capture during and after the fruit harvest. This is likely because of the rotten fruits that may have attracted more Drosophila flies on the fallen fruits during the ripening period and right after harvest. This could affect the success of parasitoid capture in the sentinel trap placed nearby and explain such a trend of the parasitoids.
Educational & Outreach Activities
Participation Summary:
Workshop/field days
- Poster: Aspects of Spotted-wing Drosophila Biological Control Efforts in Georgia. Statewide Annual Blueberry Growers Update, January 2022, UGA Extension
- Poster: Aspects of Spotted-wing Drosophila Biological Control Efforts in Georgia. Entomological Society of America, Annual Meeting, November 2023
Webinars, talks, and presentations
- Symposium Presentation (Invited): Aspects of Spotted-wing Drosophila Biological Control Efforts in Georgia: Recent Advancements Toward Developing Sustainable IPM for Spotted-Wing Drosophila, Entomological Society of America, Southeastern Branch Entomology Meeting, March 2023
- Presentation: Exploration and Study of the Native Parasitoids for Biological Control of Spotted-Wing Drosophila, Georgia Entomological Society Meeting, April 2022
- Presentation: Spotted-wing drosophila biological control efforts in Georgia, USA, Entomological Society of America Joint Annual Meeting, November 2022
- Proceedings: Spotted-Wing Drosophila Biological Control Efforts in Georgia, USA, North American SWD Biological Control Working Group Proceedings, March 2022
Journal article:
Neupane, S.B., Schmidt J. M., Snyder W.E., Hudson W.G., Wang X, Daane K.M., Buffington M., and Sial. A.A. (2024). Assessing native parasitoids of the invasive pest Drosophila suzukii in the southeastern US. Environmental Entomology (Accepted)
Newsletter submitted:
Controlling Spotted Wing Drosophila using tiny wasps in Georgia blueberry farms. Dixie Blueberry News - The Southern Region Small Fruit Consortium, 2023
Project Outcomes
Economic benefits:
By identifying and accessing the native parasitoids for the biocontrol of SWD, we endorsed the strategies of promoting these biocontrol agents with the growers through our publication, outreach efforts and public interaction. Increasing public awareness and encouraging the blueberry growers to maintain a positive attitude towards the native parasitoids will help to preserve and augment the parasitoids naturally through the practice of reduced pesticide use of pesticides. Similarly, integrating a successful biological control program with the existing SWD management tactics also helps to reduce pesticide use and cut the pest management cost in the long term.
Environmental benefits:
This project supports the adoption of biocontrol methods and promotes advocacy on the reduced use of chemical pesticides to control the pest. Such reduction in use of pesticides helps to mitigate the negative impact of those chemicals on the environmental health, contamination of soil and water, and harm to the non-target species that includes the beneficial insects like beneficial insects and pollinators. Similarly, the use of the native parasitoids, careful release, and management of exotic species like G. brasiliensis can lead to more sustainable pest control practice by supporting the natural pest regulation services, reducing the need of chemical pesticides, and risk of pesticide resistance.
Social benefits:
The educational and outreach activities conducted through this project have informed the grower about the existence of natural enemies of SWD within their farm. This information enables them to make rational decisions about their future pest control strategies which contributes to their overall success and future wellbeing. Through the workshops, presentations and publications, we were able to connect with the growers and exchange our experience and knowledge about SWD control. This collaborative approach has helped to build networks of wide range of fruit growers, researchers, and stakeholders who are working within the agricultural niche affected by SWD. This will further help us to strengthen the relationship between the growers and researchers.
Contribution towards sustainability:
The findings of this research were essential in determining the need for the release of exotic parasitoids and to promote the existing parasitoids to gain ecological and economic benefit from them. Beyond that, this study also helps to make policies that are essential in importing and releasing the new parasitoids. For making such decisions we were able to provide the baseline data and experience that was needed to make the essential policies and decisions regarding the release of exotic parasitoids. This was done by documenting the efficiency of biocontrol methods of the existing natural enemies like parasitoids, directing future studies to make essential manipulation in the system so that the biocontrol agents can adapt easily. Similar experiences may be required to navigate the pest management decision in other pest control projects in the future. This study also provided insights to the policy makers and funding agencies for making more informed decisions based on immediate priority and long-term sustainability of existing agricultural system affected by SWD.
This project expanded our understanding on the prospect of utilizing the native species parasitoids in controlling an invasive pest like SWD. Similarly, it also provided us with the opportunity to understand the prospect of releasing an exotic parasitoid G. brasiliensis to control SWD. Our understanding increased on the ecology, lifecycle, seasonal dynamics and the behavior of the native parasitoids associated to SWD collected from the Southeastern United States.
Through this project, we have an elevated appreciation for biological control efforts of SWD and learning from this experience, we are further committed to finding more sustainable SWD management solutions in the future. We have also improved our skills on rearing, researching parasitoids, and classically releasing the parasitoids to help them adapt and thrive in the natural conditions where SWD has been previously established. We also have an improved sense of awareness in our understanding about sustainable pest management practices from the experiences we gained during this project.
We have increased public awareness among the growers and collaborators through the outreach efforts associated with this project. Considering this project as the baseline survey before releasing an exotic specialist natural enemy of SWD like G. brasiliensis, we came to know about the environmental, scientific and legal hurdles of releasing the exotic natural enemies in a classical biocontrol program appropriate and the measures to tackle those issues. These experiences will be valuable for us in navigating through other similar projects in future.
For the future we recommend the monitoring of both the native parasitoids and the classically released parasitoids to ensure their establishment and effectiveness over time. There also must be an evaluation of IPM practices which integrate these parasitoids with the existing pest management tactics such as cultural practice, use of resistant varieties and economic viability of crop production after integrating these biocontrol agents. Finally, we also emphasize cross regional studies to preserve the natural parasitoids and promote the classical release of exotic parasitoid so that areawide/national pest management goals can be achieved against the SWD management in future. This will help to achieve both the current SWD management goals and improve the future pest control strategies by increasing the collaboration and knowledge sharing among the collaborators and stakeholders.
References:
Buffington, M. L., & Forshage, M. (2016). Redescription of Ganaspis brasiliensis (Ihering, 1905), new combination, (Hymenoptera: Figitidae) a natural enemy of the invasive Drosophila suzukii (Matsumura, 1931) (Diptera: Drosophilidae). Proceedings of the Entomological Society of Washington, 118(1), 1-13.
Daane, K. M., Wang, X. G., Biondi, A., Miller, B., Miller, J. C., Riedl, H., & Walton, V. M. (2016). First exploration of parasitoids of Drosophila suzukii in South Korea as potential classical biological agents. Journal of pest science, 89, 823-835.
Desneux, N., Decourtye, A., & Delpuech, J. M. (2007). The sublethal effects of pesticides on beneficial arthropods. Annual Review of Entomology., 52(1), 81-106.
Girod, P., Borowiec, N., Buffington, M., Chen, G., Fang, Y., Kimura, M. T., & Kenis, M. (2018). The parasitoid complex of D. suzukii and other fruit feeding Drosophila species in Asia. Scientific Reports, 8(1), 11839.
Miller, B., Anfora, G., Buffington, M., Daane, K. M., Dalton, D. T., Hoelmer, K. M., & Walton, V. M. (2015). Seasonal occurrence of resident parasitoids associated with Drosophila suzukii in two small fruit production regions of Italy and the USA. Bulletin of Insectology, 68(2).
Stacconi, M. R., Grassi, A. L. B. E. R. T. O., Dalton, D. T., Miller, B., Ouantar, M., Loni, A., & Anfora, G. (2013). First field records of Pachycrepoideus vindemiae as a parasitoid of Drosophila suzukii in European and Oregon small fruit production areas. Entomologia, 1(1), e3-e3.
Van Timmeren, S., & Isaacs, R. (2013). Control of spotted wing drosophila, Drosophila suzukii, by specific insecticides and by conventional and organic crop protection programs. Crop protection, 54, 126-133.
Wang, X. G., Stewart, T. J., Biondi, A., Chavez, B. A., Ingels, C., Caprile, J., & Daane, K. M. (2016). Population dynamics and ecology of Drosophila suzukii in Central California. Journal of Pest Science, 89, 701-712.
Wang, X., Lee, J. C., Daane, K. M., Buffington, M. L., & Hoelmer, K. A. (2020). Biological control of Drosophila suzukii. CABI Reviews, (2020).