Final report for GS24-314
Project Information
Flower thrips (Frankliniella tritici) are important pests of blueberries, and managing them during bloom can be challenging because insecticide applications may affect pollinators and other beneficial insects. This project investigated the spatio-temporal distribution and alternative hosts of flower thrips and characterized the relationship between thrips density and injury to blueberry flowers and fruit. Frankliniella tritici was the dominant species found in Georgia blueberry fields, with peak populations occurring during peak bloom and greater aggregation near surrounding wild habitats early in the season. Several weed species, particularly fetterbush, wild radish, and dewberry, supported thrips populations and may serve as alternative hosts. Greenhouse experiments further showed that flower injury increased with thrips density, and that responses varied among blueberry cultivars. These findings improve our understanding of flower thrips ecology and injury, and provide information to support monitoring and the development of more sustainable thrips management strategies in blueberry production.
The overall goal of this study is to investigate the spatio-temporal distribution patterns and alternative hosts and characterize the blueberry flower thrips injury in Georgia to ultimately help blueberry farmers make informed decisions to control flower thrips more sustainably.
Specific objectives
- Determine the spatio-temporal distribution patterns and identify the alternative hosts of blueberry flower thrips in Georgia.
- Characterize flower thrips injury in blueberry crops.
Cooperators
Research
Specific objective 1: Determine the spatio-temporal distribution pattern and alternative host of blueberry flower thrips in Georgia
Spatio-temporal distribution:
2.1 Study area and survey design
Flower thrips were collected from three southern highbush and three rabbiteye blueberry fields across three counties: Bacon, Appling, and Pierce. All six blueberry fields were located on commercial farms and were well-maintained by the growers, following the necessary agronomic practices. No insecticide applications on these farms were applied specifically for thrips management. Twenty sampling spots were selected in each field. The distance between the sampling spots was approximately 15-20 meters. Sampling spots were selected based on their distance from the field edge closer to the wild habitat, with four spots at the edge of the field and the remaining spots located at 15-meter, 30-meter, 45-meter, and 60-meter distances from the field edge towards the center of the field. GPS coordinates of each sampling point were recorded using iPhone 11. A double-sided, 3" x 5" yellow sticky card (Catchmaster brand) was placed in the bushes at each sampling location to monitor thrips populations. Frankliniella tritici was most attracted to yellow compared with blue or white traps in tomatoes (Cho et al. 1995). However, the yellow sticky card alone is not suitable for determining the flower thrips' population density. Therefore, flower samples were also collected randomly from each spot. 10 flower clusters were collected to determine the thrips abundance. Adult flower thrips were gathered from flower samples as well as yellow sticky cards. However, when it came to counting the population of larvae, only flower thrips samples were considered. Flower clusters from each spot was placed in the zip lock bags, marked with location, types of blueberries (southern highbush and rabbiteye), date, and spot number, and transported to the fruit entomology lab, Athens, Georgia, for processing. Similarly, weekly monitoring of thrips was done using yellow sticky cards during the blueberry season. Every week, yellow sticky cards were replaced with new yellow sticky cards.
2.2 Sample processing
The samples were brought to the lab where they were placed in a liter size plastic cup with 75% ethanol for 30 minutes to dislodge various life stages of thrips. Then, the flowers were taken out of the cup, leaving thrips in alcohol. The contents in alcohol were sieved using a 250-μm grating, USA Standard Testing Sieve (W. S. Tyler, Inc.). The residue in the sieve was washed off with 75% alcohol into a Petri dish and checked under a dissecting microscope at 12X to count the number of various thrips species. Subsamples of thrips were sent to Dr. Cheryle A. O’Donnell at the Systematic Entomology Laboratory (USDA APHIS PPQ in Beltsville) for accurate species identification.
Alternative host identification:
Identification of alternative host plants for flower thrips were recorded in adjacent wild habitats surrounding the six study sites, which included four commercial blueberry fields and two wild habitats in Bacon County, Georgia. Flower samples from dominant weed species were collected monthly from November to March. Sampling locations were systematically spaced 20 meters apart to ensure representative coverage and reduce spatial bias across the wild habitat. During the spring seasons of both years, flowers from the dominant weed species were specifically targeted to identify their role as alternative hosts for overwintering thrips. The selected weed species included wild radish (Raphanus raphanistrum, Brassicaceae), blue toadflax (Nuttallanthus canadensis, Plantaginaceae), dewberry (Rubus trivialis, Rosaceae), fetterbush (Lyonia lucida, Ericaceae), four-petal St. John’s wort (Hypericum tetrapetalum, Hypericaceae), heartwing sorrel (Rumex hastatulus, Polygonaceae), orange milkwort (Polygala lutea, Polygalaceae), spiderwort (Tradescantia ohiensis, Commelinaceae), wild blueberry (Vaccinium angustifolium, Ericaceae), and sand blueberry (Vaccinium myrsinites, Ericaceae). The sample size for each species was determined based on the size and availability of flowers from individual plants to ensure adequate representation for thrips assessment. Samples were carefully collected in containers filled with 70% ethanol to preserve the specimens. In the laboratory, each sample was examined under a stereo microscope, and all the thrips presents were identified, counted, and recorded. This study aimed to determine the role of common weed species in supporting overwintering thrips populations, offering valuable insights into potential alternative host plants that could serve as reservoirs for thrips during periods when blueberry flowers were not available.
Statistical analysis:
For spatio-temporal distribution, seasonal variation of F. tritici population including both adult and larva, role of blueberry types on thrips population and regression analysis to determine the role of maximum and minimum temperature on thrips population were analyzed and graphs were prepared using JMP pro software (SAS Institute Inc., Cary, NC, 1989-2023). In this study, the Inverse Distance Weighted (IDW) method was employed for interpolating the local aggregation indices, aiming to identify the clustering and uniform distribution patterns of flower thrips within blueberry fields. The analysis, using the SADIE method, allowed for the calculation of aggregation (cluster) and gap indices. SADIE analysis was conducted using the "epiphy" package in R version 3.6.3 (R Core Team 2020). Through SADIE analysis, local indices of dispersion were generated for each sampling point, indicating either a positive cluster index (Vi) for counts above the average or a negative gap index (Vj) for counts below the average.
To spatially interpolate the local aggregation or association indices, the inverse distance weighted (IDW) method was employed in ArcGIS Pro version 2.9. This method uses the inverse of the distances between sample points to assign weights for estimating values at unsampled locations. By applying the IDW method, a continuous surface representing the spatial distribution of the aggregation or association indices was created in ArcGIS Pro.
For alternative host analysis, differences in the mean number of adults recovered from weed species were analyzed using one-way ANOVA (P> 0.05; Tukey’s HSD test).
Specific objective 2: Characterization of flower thrips injury in blueberry
Experiments in Greenhouse:
This study was conducted in a greenhouse to quantify and characterize the injury caused by Frankliniella tritici to blueberry flowers and assess its effects on fruit development and yield. Four blueberry cultivars were used: two southern highbush (SHB) varieties (Farthing’ and ‘Suziblue’) and two rabbiteye (REB) varieties (Brightwell’ and ‘Climax’). Each plant was individually placed in a thrips-proof cage with a mesh size ≤ 150 µm (0.15 mm) to prevent thrips escape. For each plant, a single flower cluster with six open flowers was selected and tagged for monitoring.
The experiment followed a completely randomized design with five thrips density treatments: 0, 4, 8, 16, and 32 adult F. tritici per flower cluster. Each treatment was replicated ten times per variety, resulting in a total of 50 cages per variety. Thrips were collected from nearby flowering blueberry fields one day prior to infestation using a beating sheet and aspirator. Only adult female thrips were selected for use in the experiment. Prior to infestation, selected flowers were hand-pollinated to ensure fruit set. Thrips were then artificially introduced into treatment cages at the specified densities, while control cages received no thrips.
Data were collected at three developmental stages: flower stage, fruit set stage, and harvest stage. At the flower stage (72 hours post-infestation), the number of dropped flowers was recorded to calculate flower drop rate, and flower injury severity was rated on a 0–3 ordinal scale based on visible damage symptoms. At the fruit set stage, the number of successfully developed fruits was recorded per flower to determine the fruit set rate and bloom-to-fruit ratio. At harvest, fruits were evaluated for injury severity on a 0–3 scale and measured for weight (using a precision scale in grams) and diameter (using digital calipers in millimeters).
Objective 1:
Spatio-temporal distribution:
Species identification:
Based on species identification of the subsamples sent for analysis, it was determined that F. tritici is the overwhelmingly dominant thrips species in Georgia blueberries, comprising over 95% of the total population. Given its overwhelming prevalence, we have chosen to focus our study on F. tritici, as understanding its spatio-temporal distribution in blueberries.
Seasonal abundance:
The activity of F. tritici was particularly prominent during the blueberry blooming period. During the flowering period of blueberries, the first two weeks (week 1 and week 2) accounted for approximately 50-60% of the total blooming, indicating a significant proportion of flowers in bloom. Weeks 3 to 5 represented the peak blooming period, characterized by the highest concentration of flowers. However, by week 6, the stage known as the "flower drop" stage, only 10-20% of the flowers remained on the plants. This decline in flower presence marks the later stage of the flowering period. The peaks of flower thrips activity were observed between weeks 3 and 5 across all fields. Additionally, yellow sticky card data correlated with flower samples data, showing a clear peak of adult population density late in the season across all the fields.
Spatial distribution:
The spatial analysis of samples collected from six fields of both southern highbush and rabbiteye blueberries, using flower samples and sticky cards, revealed a significant aggregation index. This indicates a spatial aggregation or grouping of flower thrips, including both adults and larvae. In this study, aggregation of thrips populations was more prominent near wild habitats early in the season, suggesting that these areas may serve as sources for thrips dispersal into blueberry fields. This indicated the movement of thrips from their natural hosts in the wild habitat into the blueberry fields. Wild habitats surrounding blueberry fields contain a wide range of alternative host plants, including flowering weeds, shrubs, or grasses, especially before blueberries start blooming.
Alternative host identification
The analysis revealed significant differences in thrips abundance among the weed species for both adults (F (9,49) = 4.93, P = 0.0002) and larvae (F (9,49) = 4.01, P = 0.001). For adult thrips, higher numbers (125.5 ± 33.63) were recorded in fetterbush, significantly differing from several other species. Dewberry (88 ± 30.61) and wild radish (88.67 ± 51.97) also supported relatively high numbers of adults. Conversely, orange milkwort (2 ± 0.00) and blue toad flax (5.5 ± 2.91) had the lowest adult thrips densities. For larval thrips, in wild radish the highest abundance (76 ± 45.07) of larval thrips were recorded. Dewberry also supports relatively high larval densities (47.34 ± 19.12). In contrast, fetterbush (2.50 ± 0.95) and orange milkwort (1 ± 0.00) supported the lowest larval densities (Table 3). The results highlight that fetterbush and wild radish play key roles as alternative hosts for adult and larval flower thrips, respectively, while species such as dewberry also contribute to maintaining thrips populations. Conversely, weeds like orange milkwort appear to be less suitable hosts for flower thrips. These findings provide critical insights into the ecology of flower thrips and emphasize the importance of targeted weed management strategies as part of integrated pest management (IPM) in blueberry production systems.
Table: Mean (± SE) abundance of adult and larval F. tritici on different weed species collected near blueberry plantings during the flowering period.
|
Weed species |
Adults (Mean ± SE) |
Larvae (Mean ± SE) |
|
Blue toad flax |
5.50 ± 2.91 b |
3.12 ± 1.34 b |
|
Dewberry |
88 ± 30.61 ab |
47.34 ± 19.12 ab |
|
Fetterbush |
125.5 ± 33.63 a |
2.50 ± 0.95 b |
|
Four petal St. John’s wort |
14.5 ± 10.5 ab |
3.50 ± 2.5 b |
|
Wild radish |
88.67 ± 51.97 ab |
76 ± 45.07 a |
|
Heartwing sorrel |
10.85 ± 2.36 b |
8.35 ± 4.97 b |
|
Orange milkwort |
2 ± 0.00 b |
1 ± 0.00 b |
|
Spider wort |
59.5 ± 30.73 ab |
6.25 ± 3.61 b |
|
Wild blueberry |
40 ± 24.89 ab |
12.50 ± 7.92 b |
|
Sand blueberry |
54.14 ± 13.28 ab |
3.85 ± 1.05 b |
|
F |
4.93 |
4.01 |
|
DF |
9, 49 |
9, 49 |
|
P |
0.0002* |
0.001* |
Means (± SE) followed by different letters within a column are significantly different according to Tukey's HSD test (P < 0.05). Weed species were sampled during the flowering period of blueberries in nearby weed populations.
Objective 2:
Increasing densities of Frankliniella tritici caused greater flower injury and flower drop across the four blueberry cultivars evaluated. Flower injury increased consistently with thrips density, while the effect on flower retention and subsequent fruit development varied among cultivars. ‘Suziblue’ and ‘Climax’ showed greater flower drop at high thrips densities, whereas ‘Brightwell’ showed substantial flower injury but relatively limited effects on fruit development. In the southern highbush cultivars, particularly ‘Farthing’ and ‘Suziblue’, higher thrips densities also reduced fruit development and fruit weight, indicating that feeding during bloom can affect both flower survival and subsequent fruit quality.
Overall, the results demonstrate that the impact of flower thrips is density- and cultivar-dependent. The strong relationship between cumulative thrips feeding (thrips-days) and flower injury across all cultivars indicates that increasing feeding pressure results in greater floral damage. Cultivar sensitivity became more evident at moderate to high densities, particularly around 8-16 thrips per flower cluster, suggesting that this range may be important for developing practical management thresholds. These findings emphasize the importance of monitoring flower thrips during bloom and considering cultivar susceptibility when making management decisions.
Educational & Outreach Activities
Participation summary:
ESA Annual Meeting 2024: Delivered a 10-minute oral presentation on the overwintering biology, seasonal trends, and alternative hosts of flower thrips in blueberries to an audience of over 30 agricultural professionals and entomologists.
Annual Blueberry Update Field Day: Presented a research poster on Seasonal Abundance and Alternative Host Identification of Flower Thrips in Blueberries to an audience of over 100 participants, primarily blueberry growers.
Project Outcomes
This project contributes meaningfully to the long-term profitability of blueberry production through several dimensions. Economically, by improving our understanding of the relationship between thrips density and blueberry flower injury, this project provides information to inform the development of more informed management thresholds and help reduce unnecessary insecticide applications and associated input costs. Identifying alternative weed hosts for flower thrips offers actionable insights into habitat management strategies, such as selective weed suppression, that can reduce thrips influx without compromising pollinator or beneficial insect habitats.
Throughout the course of this project, our understanding of the complex interactions between pest ecology and sustainable blueberry production has increased. By investigating the spatio-temporal distribution of flower thrips, identifying key alternative hosts in wild habitats, and characterizing thrips injury to blueberries, we developed a more nuanced appreciation of how surrounding vegetation impacts pest dynamics in agricultural systems. Our attitudes evolved to place greater value on ecological monitoring and preventative strategies, moving beyond reactive insecticide use.