Final report for GS16-163
The spotted wing drosophila, Drosophila suzukii, has become recognized as an invasive species of concern not only in Georgia, but across the United States. The host plants of spotted wing drosophila (SWD) include berry fruits, such as blueberries. SWD is able to deposit its eggs into the ripening fruit causing a dramatic decrease in yield, while also allowing the larvae to survive various insecticide applications within the berry. As a recently introduced species there is little research available to provide an integrated pest management (IPM) program in efforts of protecting fruit industries. Information regarding native predators of SWD located in the Southeastern United States will facilitate the implementation of effective control programs. This will be completed through a series of field studies. Nine experimental sites will be selected that represent different management strategies including conventional agricultural fields, organic fields, and untreated fields. Within each agricultural system, the field margin, field interior, and associated border habitats with non-crop hosts will be evaluated. Experiments will determine the presence of native parasitoids, SWD activity, and predator activity. Molecular gut-content analysis will be performed in order to determine prey consumption of the native predators across habitats. The impacts on local population dynamics will also be examined through spatial analysis. This research has the potential to protect the blueberry industry across the nation, as well as improve biological control strategies.
- To determine how different management strategies and alternative SWD host plants affect populations of natural enemies and populations of SWD in blueberry production systems. Performance target for this objective is to determine where natural enemies in blueberry agricultural landscapes are most abundant and diverse, and identify ways to feasibly improve the current biocontrol services.
- Estimate predation and parasitism levels on SWD and determine which identified biocontrol species have the potential to enhance control of SWD in the Southeast. Performance target for this objective is to determine if natural enemies observed are providing biocontrol services for SWD and other pests of blueberries. This will be accomplished using molecular gut content analysis and bate stations to estimate parasitoid activity in blueberry fields in relation to management and location within field.
- Effects of management strategies on natural enemies and D. suzukii
In collaboration with growers and UGA extension in Georgia, we located 10 commercial blueberry orchards for the 2016 study (Whitehouse et al. 2018) and 20 commercial blueberry orchards for the 2017 study. The selected blueberry orchards consisted of Vaccinium ashei (rabbiteye blueberry) and Vaccinium corymbosum (southern highbush blueberry). We compared three different management systems among blueberry orchards: conventional, certified organic, and unmanaged. Management classifications were made based on intensity and type of insecticides applied. The conventional sites utilized broad-spectrum synthetic insecticides including primarily organophosphates and pyrethroids, and in some cases a spinosyn (i.e. delegate) with herbicides applied between the blueberry rows for weed management. The organic sites utilized reduced-risk organically certified (OMRI listed) insecticides and mowed vegetation between blueberry rows, but did not apply herbicides. The unmanaged sites did not utilize pesticides and varied from abandoned orchards to small scale harvesting with vegetation present between orchard rows mowed infrequently. Non-crop habitats surrounding sites consisted mostly of coniferous forests with dense shrubbery and blackberry (Rubus spp). The presence of vegetation between blueberry crop rows and herbicide application was determined by visual observation and discussions with each grower.
Each orchard was sampled along a transect containing three stations. Transects include 25 m into the crop (orchard interior), along the orchard margin or first crop row (orchard border), and 15 m within the bordering, non-crop forested habitat (forest). At each station, two sampling techniques were conducted: suction sampling for predatory arthropods and yeast bait-traps for D. suzukii (following Landolt et al. 2012). Suction sampling was conducted with a modified reversed-flow leaf blower (SH 86 C-E; Stihl, Waiblingen, Germany) containing a mesh bag over the intake was used to collect natural enemy communities. One entire blueberry plant was suction sampled at each station (approximately 2 m2), and one non-crop area of 2 m2 was suction sampled in the adjacent forest border. The samples were transferred to plastic bags and placed on ice until permanent storage at -20°C in the laboratory.
Individual specimens were separated from samples, identified to taxonomical level (family or order), and transferred to individual 1.5ml microcentrifuge tubes containing in 95% ethanol and returned to a -20°C freezer for storage. To characterize the structure of predator communities found in and around blueberry orchards, natural enemy counts were grouped by feeding habits and taxonomically as either Arachnids, insect predators, or parasitoids. Most arachnids were identified to family with the exception of immature spiders lacking key features. All insect predators were identified to family, and parasitoids Hymenoptera were grouped as parasitic Apocrita.
- Estimating predation and parasitism levels on D. suzukii
In order to document biological control activity on D. suzukii, we attempted to rear parasitoids from D. suzukii infested fruit placed in traps, and predators captured from transect suction samples (see above) were screened for the presence of D. suzukii DNA in their guts. For determining parasitism levels, infested blueberries were placed in a plastic container with holes in the side to allow for parasitoids to enter. These containers were placed within a Delta trap and were positioned 1-2 m above the ground in the lower canopy of the blueberry crops. This was thought to prevent desiccation and also provide shelter for the D. suzukii infested traps against abiotic factors. To attempt this method, traps were deployed at nine sites, each with three locations and three replicates for a total of 81 traps. Locations include transects of 25 m into the crop field, along the field margin or first crop row, and within the non-crop habitat. Each replicate within the blueberry fields were separated by 6 crop rows. Traps were collected each week within 72 hours following deployment to the field (following Guerrieri et al. 2016).
The second approach was to identify the collected parasitoids and predators through molecular analysis. The collected specimens were immediately transferred to 70% ethanol and placed on ice. These were stored at -20ºC in separate vials containing pre-chilled 70% ethanol. Sequencing with COI primers were used to confirm and document parasitoids. To document predators of D. suzukii, predators collected were screened for the presence of D. suzukii DNA using molecular-gut content analysis with species specific primers for D. suzukii (following Wolf et al. 2018).
During the 2016 study, we collected a total of 1,113 natural enemies from suction sampling, including: 857 arachnids, 128 insect predators, and 128 parasitoids. During the 2017 study, we collected a total of 2,292 natural enemies from a total of 420 suction samples including: 1,896 arachnids, 197 insect predators, and 199 parasitoids. A total of 3,036 spotted wing drosophila (Drosophila suzukii) individuals were successfully recovered from 387 sugar-yeast bait traps. We lost some traps due to tractor damage and to a large storm. Arachnids were the numerically dominant predatory arthropod taxa with twelve families followed by insect predators, composed of nine families, and Hymenopteran parasitoids were made up of parasitic Apocrita. The distribution of Arachnids as the dominate predator taxa was consistent across all three management practices.
Unmanaged blueberry sites contained the highest mean natural enemy populations 9.92 (±0.94) followed by organic 5.16 (±0.71), and conventional 2.14 (±0.26). In conventional orchards, the highest natural enemy counts were in forest transects compared to the interior blueberry orchard. Whereas, in organic orchards, border transects had the highest natural enemy populations. In unmanaged orchards, natural enemies were observed at similar abundance across the three transects. In addition, we found that conventional systems with vegetation between blueberry rows had a significantly higher abundance of natural enemies (2.688 ± 0.398) compared to bare ground between blueberry rows (1.519 ± 0.239). Vegetation did not appear to influence the abundance of D. suzukii (3.355 ± 0.995) and bare (2.955 ± 0.830). Therefore, researching ways to integrate vegetation should be considered for future research and with more fields, the interaction between within field vegetation and landscape context.
During the 2016 study we also evaluated the parasitoid activity and found the emergence of D. suzukii at 5.533 individual adults per trap on average with 0 parasitoid adults emerged throughout the 8 week study. The traps experienced desiccation, mold, ant infestations, and other abiotic disruption. Modifications to trapping protocol were used including infested blueberries and infested yeast based medium with natural or artificial inoculation of D. suzukii larvae and pupae, as well as 48 hour or 72 hour treatments in the field. Despite this effort, there were 0 parasitoid adults observed after deploying 648 traps. In addition, all parasitoids were sent for sequencing the Folmer region of COI, and results are pending for these identifications from the suction samples. Once complete, the sequencing data will provide identifications of parasitoids present in these blueberry landscapes.
Molecular gut content analysis was used to estimate predation on spotted wing drosophila and other secondary pests from collected generalist predators. Of the nearly 4,000 predators collected, DNA extractions were completed on 1,328 samples. Screening for spotted winged drosophila was completed for 1,128 samples and 200 were sequenced for the presence of pest DNA in their guts using Illumina MiSeq. From the initial data we found 0.4% of predators positive for SWD. The Illumina sequencing data is very promising and shows incidence of predation on multiple blueberry pests including D. suzukii (Fig. 1). Further processing will determine which predators are the most important to conserve for improving natural pest control on blueberry pests.
Fig. 1. Predator families collected in blueberry landscapes (right black bars; n=188). Prey families observed in predator samples (left bars) determined by Illumina MiSeq of COI. Thickness of gray lines connecting predators to prey, represents a greater number of reads (prey sequences) recovered from predator families. Green bars for prey indicate known pest families in blueberry systems.
Educational & Outreach Activities
Twenty growers involved in research and five presentations.
Whitehouse, T.S. and J.M. Schmidt. 2017. Evaluation of natural enemies in blueberry agroecosystems: implications of management practice and surrounding landscape. M.S. Thesis. Graduation Date: December 15, 2017
Whitehouse T.S., A.A. Sial, and J.M. Schmidt. 2018. Natural Enemy Abundance in Southeastern Blueberry Agroecosystems: Distance to Edge and Impact of Management Practices, Environmental Entomology 47: 32–38, https://doi.org/10.1093/ee/nvx188
Whitehouse, S. Evaluation of natural enemies in blueberry agroecosystems: implications of
management practice and surrounding landscape. Thesis seminar delivered at UGA
Entomology, Athens, GA. Nov 2017.
Whitehouse, S., J.M. Schmidt, and A.A. Sial. Natural enemies in blueberry agroecosystems:
implications of management practice and non-crop habitat. Oral presentation delivered at
the Entomological Society of America Conference, Denver, CO. Nov 2017.
Whitehouse, S. and J.M. Schmidt. Predator community along blueberry agroecosystem
gradients: implications of management and non-crop habitat. Oral presentation delivered
at the Georgia Entomological Society Meeting, Jekyll Island, GA. Apr 2017.
Whitehouse, S. and J.M. Schmidt. Challenges for organic blueberry biocontrol: are there any
natural enemies out there? Poster presentation delivered at the Georgia Organics
Conference, Atlanta, GA. Feb. 2017.
Whitehouse, S. Conservation biological control of spotted wing drosophila (Drosophila suzukii).
UGA Graduate School 3MT Competition, Athens, GA. Mar 2016.
Our findings suggest that the blueberry agroecosystem located in south Georgia contains a diverse community of natural enemies, mostly made up of spiders. Our findings show that although conventional practices may remove these beneficial arthropods from the orchards, habitat management may alleviate those negative effects. The results suggest that further research is needed to provide solutions for blueberry growers and types of habitat management that will improve natural enemies and reduce pests within blueberry systems.
The current blueberry agroecosystem investigated is an excellent model system to evaluate the patterns of natural enemy and pest abundances. Blueberry production in the southeastern U.S. is undergoing agricultural intensification, which merits the need for alternative pest control options to alleviate both economic and environmental costs endured. Our study suggests that conventional management practices negatively impact natural enemy abundance and alter natural enemy distribution. For example, we consistently found a higher abundance of natural enemies distributed towards the adjacent forested habitat in conventional systems and a more even distribution in the less intensive, unmanaged systems. We could then conclude that a combination of insecticide exposure and availability of non-crop habitat influenced the spatial distribution of natural enemies within the respective blueberry orchards. Our results suggest that the presence of vegetation within fields increases natural enemy abundances, while having neutral effects on D. suzukii. This provides support for the implementation of habitat management to incorporate local vegetation without an unwanted increase of D. suzukii. Further analysis of molecular data will characterize the interactions between predators and key pests to elucidate the structure of these communities and trophic interactions.
Cultural practices to enhance habitat within orchards requires further calibration in this system. Our research shows higher populations of natural enemies in orchards with some vegetation between rows. How much vegetation, what types of vegetation, or frequency of mowing may help make further decisions to facilitate biological control and promote biodiversity. Further evaluation of the feeding linkages between predator taxa and key pests will provide vital information to the food web structure of this system and ways to enhance biological control.