Progress report for GS22-269
Project Information
This is an updated annual progress report, and the final report will be submitted at the end of the project.
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. We also built a follow-up project based on these outcomes. On this project, we have recently started the classical release of an exotic parasitoid Ganaspis brasilensis (Hymenoptera: Figitidae) in the surveyed locations. The newly introduced parasitoid G. brasilensis were collected in the original habitat of the host (SWD) in Asia, where they co-evolved with the host. We are closely monitoring the establishment of this classical release in these locations.
The specific objective of this project is to:
- To explore, collect and identify the parasitoids of SWD and quantify their abundance, diversity, and distribution concerning temporal and phenological variations of the blueberry production system in Georgia.
To evaluate the parasitoids’ by rearing them and testing for their efficiency to parasitize the SWD
Cooperators
- (Educator and Researcher)
- (Educator and Researcher)
- (Educator and Researcher)
Research
Parasitoids of SWD were field surveyed in eight blueberry growing sites located in Bacon Co., Pierce Co., and Appling Co. of southeastern Georgia. Among the selected locations, five sites were conventionally managed farms, and three others were organically managed farms. The presence of SWD at all sites was 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 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:
The parasitoid abundance and diversity across the fields over time were analyzed using descriptive statistics. Individual fields were treated as replicates and the seasonal trends in abundance, relationship with phenological stages of the host plant, and spatial distribution across locations were analyzed using ANOVA. Post HOC means separation was done using Tukey’s HSD test. The variations between cropping practice and habitat type were analyzed using students’ T-test, and statistical differences between each means were separated using each pair of t-statistics. All the 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 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. The abundance of L. boulardi was numerically higher during the same period but the results were not significantly different (F = 0.97; df = 5,122; p = 0.43).
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:
During this study, we found that L. boulardi species was the most captured species of parasitoid from the sentinel traps placed in associated areas. In similar studies, L. boulardi is one of the most common species of Drosophila spp. parasitoids collected previously in the SWD-invaded regions of 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 is 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 (Gabarra et al. 2015: Rossi Stacconi et al. 2013), North America (Miller et al. 2015; Wang et al. 2016), and many other locations all around the world (Wang et al 2020). Although in 2021, the larval parasitoid L. boulardi was more abundantly captured trapping than the pupal parasitoid P. vindemiae, 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 appears to be quite common in most similar studies (Daane et al 2016: Fleury et al., 2009: 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 (Lee et al., 2019; 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 that follow these Drosophila (Desneux et al., 2007).
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 Extention
- 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 on progress:
Assessing native parasitoids of the invasive pest Drosophila suzukii in the southeastern U.S.
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
This information will be submitted with the final report at the end of this project.
This information will be submitted with the final report at the end of this project.
Information Products
- Aspects of Spotted-wing Drosophila Biological Control Efforts in Georgia (Display)
- Aspects of Spotted-Wing Drosophila Biological Control Efforts in Georgia, USA (Conference/Presentation Material)