Spatio-temporal distribution and management of Drosophila suzukii in Louisiana mayhaw

Objective 1: Monitor for SWD in commercial mayhaw orchards (cooperating farms) and wild mayhaw in Louisiana

Selection of field sites

Studies will be performed during the 2023 and 2024 mayhaw growing seasons at three commercial orchards and wild or homegrown mayhaws in Louisiana. Three participating farms in our project represent three different mayhaw production systems found in Louisiana.

Conventional: Lynnberry Farm LLC in Jackson, LA is a conventional mayhaw farm that uses conventional techniques and controls pests and diseases using conventional pesticides. All mayhaw varieties are relatively new grafted varieties. This farm is a perfect example of a traditional commercial mayhaw farm.

Organic: The commercial organic mayhaw orchard in Jackson, LA which is participating in this research has all new grafted mayhaw cultivars like Spectacular, Maxine, and Royal Star similar to another participating conventional farm. The grower controls any pest and disease problems with OMRI-listed chemicals.

Poorly maintained: The cooperator in Denham Spring, LA owns a small mayhaw orchard and the mayhaw trees are more than twenty years old. All mayhaw cultivars are native, not any grafted popular cultivars like the two other cooperating farms. The orchard is maintained by cultural control and no chemical is used. This farm is a perfect representation of native small-scale mayhaw orchard systems in Louisiana.

Research in these cooperating farms will help us to do a comparative study between SWD occurrence in conventional, organic, and homegrown (poorly maintained) mayhaw systems.

Monitoring study

Studies will be performed during the 2023 and 2024 mayhaw growing seasons at three different locations in Louisiana. Scentry traps (Scentry Biologicals, Billing, MT) will be deployed from early April into four blocks, with four traps per block. Within a block, traps will be deployed along a transect with each transect running along a gradient from the edge of the woods to the middle of the mayhaw orchard. Transects will be spaced approximately 10 m apart. The distance of the traps from the edge of the woods to the middle of the orchard will represent four individual treatments: a) 0 m (woods edge), b) 10 m (field edge), c) 20 m (within-field), and d) 40 m (within-field). Scentry traps are deli-cup-style traps with a red paper band and small entry holes around the perimeter of the trap. Each trap will contain one Scentry SWD lure and will be filled with approximately 250 mL tap water with two to three drops of unscented dish soap (Seventh Generation, Inc., Burlington, VT) as a drowning solution. Each trap will be placed at fruit height in the canopy of mayhaw trees. The drowning solution will be collected and replaced weekly, and lures will be replaced every four weeks. To record the temperature and humidity, a HOBO data logger will be placed in the lower canopy of a mayhaw tree in the center of the orchard. Treatments, trap orientation, and sample collection methodology will be identical in both years.

Incubation study

Every week 100 mayhaw fruits will be collected randomly from each replication and brought back to the lab for incubation. Collected fruits will be kept in a plastic container (9.60 x 9.60 x 3.50 inch) and placed in an incubator at 23°C, 60 ± 5% RH, and 14:10 L:D cycle. The lid of the container will be cut and replaced with a mesh net (150 mm, The Cary Company, Addison, IL) to make it breathable. Two to three weeks after the number of adult SWD emerged from the fruits will be counted.

Statistical Analysis

Separate analyses will be performed for conventional and organic sites. The effects of treatment, week, and treatment-by-week interaction on adult trap catch and adult emergence will be analyzed using a multivariate analysis of variance (MANOVA) with repeated measures. All trap catch and emergence data will be transformed if needed to achieve normality. Means will be separated using Tukey’s Honest Significant Difference (HSD) test. All analyses were performed using SAAS.

Objective 2: Construct SWD life tables on mayhaw fruits.

Insects

A colony of SWD will be established from the wild flies collected from mayhaw orchard. Flies will be reared on an instant, potato-based diet (Formula 4-24, Carolina Biological Supply, Burlington, NC) in 0.25-L polypropylene bottles (Applied Scientific, Kalamazoo, MI) and closed with foam plugs (Jaece, North Tonawanda, NY). The colony will be housed in a growth chamber at 23°C, 60 ± 5% RH, and 14:10 L:D cycle.

Mayhaw fruits

Organic mayhaw fruits will be collected weekly from one of the participating farms in Denham Springs, LA. Only fully ripe mayhaws without any insect and disease damage will be used for the life table study. Fruits will be sterilized by immersion in 70% alcohol for 5 seconds followed by washing with autoclaved double distilled water. Excess water from the surface of the fruits will be wiped off with a paper towel.

Life table construction

Seven-day-old SWD pairs will be separated from colonies on artificial diet and placed in 16-oz plastic containers with lids. Subsequently, five fresh and cleaned mayhaw fruits will be placed at the bottom of the container for obtaining eggs. After 24 h, the adults will be removed, and the number of eggs on fruits will be counted. The fruits will be kept inside the container for 15 days. After inoculation, containers will be placed in incubators, set to the respective constant temperatures (15, 20, 25, and 30 °C) at 60 ± 10% RH and 14:10 L:D photoperiod. The experimental design will be completely randomized with 20 replicates per temperature, where each replicate will compose at least 10 SWD eggs, totaling 200 eggs per treatment. We will evaluate egg-to-adult development time (days) and the egg-to-adult development survival percentage following Schlesener et al. 2020. Emerged male and female SWD will be counted separately to calculate the sex ratio. Generally, male flies have a dark spot along the first vein near the tip of each wing while females do not have any spots on wings (Fig. 1. A & B). Female SWDs are identified by their saw-like ovipositor with two rows of dark-colored teeth on it (Fig. 1. C).

Fertility life table at constant temperatures

After emergence, 24-hour-old pairs of SWD will be taken from each temperature (15, 20, 25, and 30 °C) and placed in a 1-gallon square plastic jar. One side of the plastic jar will be cut open and covered with a piece of fine mesh. Fresh ripe mayhaw fruits (obtained from an organic farm) will be washed and 10 fruits will be placed in a 9 cm Petri dish. Petri dishes containing fruits will be offered as an oviposition substrate and food for each SWD pair. The fruits will be checked every day to record the number of eggs and any fruits with eggs will be removed and replaced with new ones. Twice a week the Petri dish will be changed. Spotted wing drosophila pairs will be kept until they die. The experimental design will be completely randomized with 10 replicates per temperature. Pre-oviposition time (days), oviposition time (days), post-oviposition time (days), total fecundity (number of eggs per female per lifetime), daily fecundity (number of eggs per female per day), adult longevity (days), and larval emergence (%) will be evaluated (Schlesener et al. 2020). For evaluation of egg viability or larval emergence, up to 30 eggs per treatment will be chosen randomly and placed in a Petri dish (9- cm diameter, 1.5 cm high) lined with a moistened filter paper to prevent dehydration. The Petri dish will be incubated for 5 days at the same temperature from which they were collected. An egg is considered viable if it is ruptured and has an empty chorion whereas eggs that remained turgid after the incubation period will be considered nonviable (Schlesener et al. 2020).

Statistical analysis

The age-specific life table will be constructed using Southwood and Henderson (2000). Age-specific survival (lx) and fecundity (mx), intrinsic rate of increase (rm), (Σe–rmlxmx = 1), finite rate of increase (λF = erm), mean generation time [TG = lnRo/rm], and doubling time (DT = ln(2)/rm) will be calculated following Souza and Davis (2020).

Objective 3: Validate SPLAT for managing SWDs in mayhaws through laboratory and on-farm trials.

Field testing

The aim of this study is to use an attract-and-kill approach (SPLAT) to create an improved and environmentally sound SWD IPM program in Louisiana mayhaws. During the 2024 field season, the field study will be conducted at two different locations, one in Denham Spring and the other in Jackson in LA. SPLAT will be applied at recommended rates and the same OMRI-listed insecticide used by the grower will be combined as a killing agent with the attractant. There will be four treatments: application of SPLAT i) once every 7 days, ii) once every 14 days, iii) plots treated with grower’s standard (positive control), iv) untreated plot (negative control). Plots will be arranged in a randomized design and will contain four replications of each treatment. SPLAT will be applied to the base or lower canopies of mayhaw plants using diaphragm pump backpack sprayers. At least two applications will be made during the field season. In both years and sites, adult SWD abundance will be evaluated using Scentry traps and lures (Scentry Biologicals, Inc., Billings, MT). Lures will be replaced on the fourth week of the trial. Tap water (250 mL) with a few drops of unscented dish soap will be used as a drowning solution in each trap. Four traps will be deployed randomly in each plot and the contents from each trap will be emptied weekly into a collection jar. The drowning solution will be replaced weekly, and the number of SWD per trap will be counted under a dissecting microscope.

Mayhaws will be harvested from plants around each trap to measure adult emergence, a proxy for larval infestation. Around 100 fruits (ripe or ripening) will be collected around each trap and the weight of each sample will be measured. Each sample will be incubated in a modified breathable plastic container (as described in objective 1) in an environmental chamber at 23°C, 60 ± 5% RH, and 14:10 L:D cycle for three weeks. The number of SWD adults that will emerge from each sample will be counted by sex under a dissecting microscope.

Residue Testing

In a laboratory experiment, the residual effects of SPLAT on SWD will be evaluated. There will be four treatments: SPLAT aged for i) 0, ii) 7, iii) 14, days, and iv) control. Each treatment will have 4 replications and will be repeated 5 times. Two ml of SPLAT will be applied on a 47 mm Petri dish and aged accordingly before starting the experiment. Mayhaw shoots containing 10 leaves and 5 ripe fruits will be cut off the trees and placed individually inside a 1 L clear plastic container (Gordon Food Service, Wyoming, MI). To minimize moisture buildup, lids will be modified by cutting a 5 cm diameter section of each container and a fine mesh (150 mm, The Cary Company, Addison, IL) will be affixed over the hole using hot glue. Ten adult SWD (5 male, 5 female) that are between 2 and 5 days old will be gently removed from a laboratory colony, anesthetized with CO2, and added to the container. To limit fly mortality, a 4 cm long piece of dental wick moistened with distilled water will be placed inside the container. Spotted wing drosophila mortality will be checked for 7 days and the number of SWD eggs and larvae on mayhaw fruits will be counted.

Data analysis

All trap catch and emergence data from field testing will be transformed if needed to achieve normality. Means will be separated using Tukey’s Honest Significant Difference (HSD) test. The mortality data and oviposition data from residue testing will be analyzed using Analysis of Variance, followed by Tukey’s HSD (Honest Signifcant Diference) test for pairwise comparison in PROC GLIMMIX (SAS Institute Inc 2013).

Objective 4: Present data at the annual Commercial Fruit and Vegetable Field Day at the LSU AgCenter Botanical Gardens and publish extension brochures and peer-reviewed articles.

We will work closely with growers, researchers, and extension agents in Louisiana. Information and findings from our study will be shared with Louisiana mayhaw growers and other mayhaw-growing states like Georgia, Mississippi, and Florida. Dr. Ferguson who is an Associate Extension Agent – Horticulture at LSU AgCenter will assist us to reach and connect with mayhaw and other small fruit growers in Louisiana.

We will present our experimental results at the annual Commercial Fruit and Vegetable Field Day at the LSU AgCenter Botanical Gardens and at regional conferences. The LSU Fruit and Vegetable Growers Field Day invites commercial growers as well as newcomers who want to start growing commercially, even if it's on a small scale.

We will prepare extension articles and fact sheets on SWD biology, damage, and IPM techniques to control this pest. Results will also be published in one peer-reviewed journal article.

Social media is a strong platform to disseminate scientific fining and knowledge. We will use available social media to share our research findings with growers and other stakeholders.