Optimizing Thresholds and Reduced-Risk Management Strategies for the Control of SWD in Maine's Wild Blueberries

Progress report for GNE21-260

Project Type: Graduate Student
Funds awarded in 2021: $10,528.00
Projected End Date: 12/05/2022
Grant Recipient: University of Maine
Region: Northeast
State: Maine
Graduate Student:
Faculty Advisor:
Dr. Philip Fanning
University of Maine
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Project Information

Project Objectives:

1) Optimization of trap captures and threshold modeling.

Objective (1) will entail testing the efficiency of six different styles of SWD monitoring traps, and the incorporation of these different style trap captures into a risk-based action threshold model to help growers determine the ideal timing for management strategy implementation.

 

2) Exploration of reduced-risk management strategies. 

Objective (2) will involve assessment of the efficacy of insecticide border applications in large scale field trials, compare the efficacy of multiple behavioral control and push-pull tactics in field cages, and a field and semi-field bioassay assessment of a phagostimulatory adjuvant.

Introduction:

Spotted-wing Drosophila (SWD) Drosophila suzukii (Matsumura), is a major pest of concern for fruit growers in the United States. SWD is an invasive vinegar fly native to southeast Asia. First found in the continental U.S. in 2008 [1], it rapidly expanded across the rest of the country in the next four years [2]. Unlike most other drosophila, SWD prefer ripening as opposed to overripe fruit [3]. SWD are polyphagous with a broad host range of thin-skinned crops, cultivated and uncultivated [4]. Female SWD utilize a hardened serrated ovipositor to lay eggs in ripening fruit while still in the field, leading to direct damage and fruit decay from larval feeding, as well as opening the fruit up to secondary infection by other insects and pathogens [5, 6]. SWD was first found in Maine in October 2011 [7]. During the 2012 growing season, SWD caused an estimated $1.38 million crop loss in Maine wild blueberries, with actual losses likely exceeding that number [8].  Wild blueberry (Vaccinium angustifolium Aiton) is a native and economically important Maine crop. Maine produces approximately 12% of all blueberries in North America, contributing over $250 million to the state's economy each year; in 2017, Maine was the third largest state in domestic blueberry production in terms of volume, producing over 30,000 metric tons of wild blueberry [9]. Since its establishment in Maine, SWD has become one of the major pests for wild blueberry growers, with economic impacts having the potential to exceed $6.8 million per year under worst-case scenarios [10].

A zero-tolerance policy regarding SWD larvae in the fresh and processed berry market has led to insecticide usage [11].  Insecticide applications have doubled in some crops since the SWD invasion. In highbush blueberry, these increases in sprays for SWD have reversed a trend of reducing the usage of broad-spectrum insecticides, with a 45% increase in the percentage area treated between 2011 and 2019 (Johnson and Fanning, unpublished data).  Reliance on insecticides is dangerous as it will lead to insecticide resistance, currently seen with spinosad in SWD populations in California [12].  Managing SWD has increased farm labor and input costs and reduced profitability [2]. Insecticide costs have increased by $480-$1,200 per acre in CA raspberries [13] (Farnsworth et al. 2017). In Maine wild blueberries, the difference between an economical and uneconomical management program is one spray. Specialty crops employ 68% of all agricultural workers in the USA, therefore increased insecticide use for SWD management increases the potential for human exposure to toxic insecticides. One study investigated biological, behavioral, and preventative management specifically in Maine wild blueberries, exploring insect exclusion netting, entomopathogenic fungi, and mass trap deployment to attract and kill [14]. However, none of these strategies resulted in sufficient pest control to justify the replacement of insecticidal treatments.

To reduce insecticide inputs, environmental impacts, health risks associated with insecticides use and cost by growers associated with the management of this species, greater knowledge of SWD’s biology, attractants for monitoring and appropriate traps, and reduced-risk strategies are needed.

Research

Materials and methods:

Experiment 1.1: Comparative Evaluation of Baited Monitoring Traps for SWD -

This trial will entail a field comparison of six different SWD monitoring traps. The six traps are as follows:

 

1)    PHEROCON® PEEL-PAK Broad Spectrum Lure + Cup Trap

2)    PHEROCON® PEEL-PAK Broad Spectrum Lure + SWD STKY Trap

3)    Unbaited PHEROCON® SWD STKY Trap

4)    Scentry trap + Lure

5)    Red Solo Yeast + Sugar Water Trap

6)    Spinosad Treated Red Solo Yeast + Sugar Water Trap

 

There will be four replicates, with trap locations being randomized within each replicate. All traps will be placed along field edges, approximately 10m apart, throughout the treatment plots. Lures for traps 1, 2, and 4 will be replaced every four weeks, while the yeast/sugar water mixture for traps 5 and 6 will be replaced weekly. Traps will be checked with SWD captures being recorded weekly. SWD STKY traps, and drowning solutions for traps 1, 4, 5, and 6, will all be replaced weekly. New Spinosad treated traps will also be set weekly, so residues never age more than seven days.

 

Experiment 1.2: Threshold Modeling - Three traps (identical to trap 5 in Experiment 1) per site will be placed along the borders of 10 wild blueberry fields. Trapping will continue until the fields are harvested. Traps will be swapped weekly, and a 6oz fruit sample will be taken from each trap's location. Adult male and female SWD captures per trap will be counted, and fruit samples will be monitored for larval infestation through the salt extraction method [16] (15). Data collected here will be incorporated into the existing action threshold model.

 

Objective 2: Exploration of Reduced Risk Management Strategies

 

Experiment 2.1: Efficacy of Border Insecticide Applications - Select nine crop year wild blueberry fields and lay out three 165ft long transects spaced 100ft from each other and the field’s edges. Three fields will receive an insecticide application covering the first 20ft into the field. Three fields will receive a full field insecticide application. The remaining three fields will serve as an untreated control. Fruit samples (ca. 6oz / sample) will be collected from five locations along each transect (10ft, 20ft, 40ft, 80ft, and 165ft) once a week until harvest, with the salt extraction method [16] being used to determine larval infestation levels.

 

Experiment 2.2: Cage Trial Comparison of Behavioral Control Tactics - This experiment will be conducted in 20ft X 10ft outdoor field cages. Randomized block design with six treatments, each treatment being replicated four times. Treatments are as follows:

 

  1. ACTTRA SWD ® + Delegate WG ® (ACTTRA SWD ® mixed with Delegate WG ® at 0.25% a.i. (v/v), applied at a rate of 1. 5 L of formulation/acre1. (6.90 mL formulation per cage of 200 sq. ft).
  2. Combi-Protec ® + Delegate WG ® as a full cover spray, full rate (14 fl. oz. combi-protec, 50 gal water and 6 oz. Delegate WG ® per acre).
  3. Combi-Protec ® + Delegate WG ® as full cover spray, half rate (14 fl. oz. combi-protec, 50 gal water and 3 oz. Delegate WG ® per acre)
  4. Delegate WG ® + Induce spray at the highest field recommended rate for SWD control in blueberries (6 oz/acre of Delegate WG ® in 50-gal water/acre + 0.12% Induce as surfactant)
  5. Untreated control

 

Environmental conditions will be recorded with a HOBO data logger inside and outside the cages. 100 flies (1:1 Male:Female ratio) of 5-7 days old will be released into the center of each cage around 5-6pm. Fruit will be sampled (ca. 6oz / sample) from four different points per cage at two different time points: before the SWD release to assess baseline infestation rates, and seven days post SWD release to determine treatment efficacy. Fruit samples will be evaluated for larval infestation using the salt extraction method [16].

 

Experiment 2.3: Evaluate Push-Pull Strategies in Small Size Field Cages - This experiment will be conducted in small size (10ft X 5ft) outdoor field cages, spaced 10m from each other. There will be four treatments, with each treatment being replicated six times. Treatments are as follows:

 

1) SWD ACTTRA ® + Delegate WG ® (+surfactant) (pull alone)  

2) 2-Pentylfuran (push alone)

3) SWD ACTTRA ® + Delegate WG ® (+surfactant) + 2-Pentylfuran (push-pull)

4) Untreated Control

 

Treatments will be randomly assigned to cages. In treatments 1 and 3, SWD ACTTRA ® + Delegate WG ® will be applied in a 2ml dollop to a leaf located approximately 2.5ft from three cage walls 8 hours before the flies are added to the cages. In treatments 2 and 3, two sets of 2 polyethylene sachets releasing 2-pentylfuran (15mg/h) will be placed on the other side of the cage approximately 2.5ft from three cage walls. A total of 50 flies (1:1 male:female ratio) aged 5-7 days old will be released per cage around 5-6pm. After 24 hours, berry samples (3 sets of ca. 6oz from each half of the cage [6 samples per cage]) will be collected. Fruit samples will be evaluated for larval infestation using the salt extraction method [16].

 

Experiment 2.4a: Field Evaluation of Efficacy of Combi-protec - This experiment will be conducted at the experimental Blueberry Hill Farm. There will be four treatments as follows:

 

1)    Insecticide only: Delegate WG ® 6oz/ac + Induce spray on 7 day intervals

2)    Delegate WG ® + Combi-Protec ® (14 oz/50 gal)

3)    50% rate of the same insecticides + Combi-Protec ® (14 oz/50 gal)

4)    Combi-Protec ® only, control treatment (14 oz/50 gal)

 

Treatments will be applied to 20ft by 30ft plots in a complete randomized block design. Each treatment will be replicated four times. For each plot, four 6oz berry samples will be taken weekly from a transect across the center. Fruit samples will be evaluated for larval infestation using the salt extraction method [16].

 

Experiment 2.4b: Semi-field Bioassay of Combi-protec - For this experiment, sprays will be applied at Blueberry Hill Farm, with cuttings then being taken and used in a semi-field bioassay held on campus, at the University of Maine. There will be 11 treatments as follows:

 

1)    Delegate WG ®

2)    Delegate WG ® + Combi-Protec ® (14 oz/50 gal)

3)    Delegate WG ® (50% rate) + Combi-Protec ® (14 oz/50 gal)

4)    Assail 70WP ®

5)    Assail 70WP ® + Combi-Protec ® (14 oz/50 gal)

6)    Assail 70WP ® (50% rate) + Combi-Protec ® (14 oz/50 gal)

7)    Verdepryn 100SL ®

8)    Verdepryn 100SL ® + Combi-Protec ® (14 oz/50 gal)

9)    Verdepryn 100SL ® (50% rate) + Combi-Protec ® (14 oz/50 gal)

10) Combi-Protec ® only, control treatment (14 oz/50 gal)

11) Untreated control

 

Treatments will be applied in the field, and stems with 3-4 leaves will be clipped and collected in a full water pick stuck inside a 32oz deli cup. Each assay container will have 13 loose un-infested berries taken from their respective treatment placed in the bottom. 10 adult flies (1:1 male:female ratio) aged 5-7 days old will be added to the assay containers 1- and 3-days post treatment to assess residual activity. After flies are added, the assay containers will be capped with a ventilated lid and placed in an environmental chamber (22°C; 70% RH) for 6 days. Adult fly mortality will be assessed at 24 hours after exposure to treated foliage. On day 6 berries will be removed and larval infestation will be assessed through the salt extraction method [16].

Research results and discussion:

Experiment 1.1: Comparative Evaluation of Baited Monitoring Traps for SWD

In preliminary results from 2021, first trap captures occurred during the week of 28 June. Data collected from 28 June to 9 August were used for the analysis; SWD populations after 9 August were high enough to overpower any difference in trap style. Due to non-normality data were analyzed using a Kruskal-Wallis test. All baited traps performed significantly better than an unbaited red sticky trap (Fig. 1). The Pherocon PeelPak lure + red sticky trap outperformed all the other styles of traps tested (Fig. 1). When looking at cumulative adult capture (Fig. 2) and cumulative adult male capture (Fig. 3) the Pherocon PeelPak Lure + red sticky trap reached threshold levels of SWD quicker than any of the other trap styles.

A baited red sticky trap shows promise for use in SWD monitoring. During summer 2021 it outperformed the other styles of traps tested. With lower SWD populations the red sticky trap was also easiest to assess, especially if only looking for males as the current threshold model calls for.

This trial will be repeated during summer 2022.

Experiment 1.2: Threshold Modeling -

This work is currently ongoing with no preliminary results.

Experiment 2.1: Efficacy of Border Insecticide Applications -

This work will be done in summer 2022.

Experiment 2.2: Cage Trial Comparison of Behavioral Control Tactics -

Splitting the replicates between two different dates displayed no effect on larval infestation, so all replicates were grouped for analysis. Data were analyzed using a Kruskal-Wallis test. All treatments displayed a significant reduction in SWD larvae per gram of fruit sampled compared to the untreated control cages (P = 0.0005) (Fig. 4). Also, the Combi-Protec with full rate Delegate resulted in significantly lowered infestation than the full rate Delegate treatment (P = 0.0117)(Fig. 4).

In this trial, both ACTTRA TD and Combi-Protec combined with reduced amounts of active ingredient provided the same control as a full-rate spray of Delegate.

This trial will be repeated during Summer 2022.

Experiment 2.3: Evaluate Push-Pull Strategies in Small Size Field Cages -

This work will be completed during summer 2022.

Experiment 2.4a: Field Evaluation of Efficacy of Combi-protec -

Due to no or low infestation, the first two sampling dates (9 and 16 August) were not included in any analyses. Due to non-normality data were analyzed using a Kruskal-Wallis test. Data were adjusted for sample weight prior to analysis. Of the fruit sampled on 24 August, the only difference was between the Combi-Protec (CP) only and Delegate WG + CP treatments (P = 0.0304) (Fig. 5). For fruit collected on 31 August there was a significant difference when comparing Assail 70WP to the untreated control, Delegate WG 50% + CP, and Delegate WG + CP (P = 0.0304) (Fig. 5).

It currently appears that Combi-Protec mixed with a half-rate of some insecticides has the potential to provide adequate control for SWD. However, we did not include a treatment of just half-rate insecticide without Combi-Protec. This leaves open the possibility that differences seen might not be a result of the phagostimulant and would have been observed with just the half-rate insecticide.

This trial will be repeated during Summer 2022.

Experiment 2.4b: Semi-field Bioassay of Combi-protec -

At the 1 D.A.T. time point Delegate WG, Delegate WG mixed with Combi-Protec, and Delegate WG 50% mixed with Combi-Protec all had significantly higher mortality than the untreated control (P < 0.05) (Fig. 6). There was no significant difference in mortality between any of the treatments at the 3 D.A.T. time point (Fig. 6). When it came to emergence of adult flies from berries in the bioassay containers, none of the treatments were significantly different from the untreated control (Fig. 7).

This trial will be repeated during Summer 2022.

Figures:

 

Figure 1. Average adult SWD captures from first adult capture on 28 June to 9 August (± S.E.). Columns topped with the same letter are not significantly different.
Figure 2. Cumulative adult SWD capture (± S.E.) for five weeks of trapping. 22 June had no adult captures whereas by 19 July almost all trap styles were over 20 cumulative adult SWD.
Figure 3. Cumulative adult male SWD captures (± S.E.).
Figure 4. Average (± S.E.) number of SWD larvae per gram of blueberry. Columns topped with the same letter(s) did not display significant differences.
Figure 5. Average (±S.E.) number of SWD larvae per gram of fruit collected. Columns topped with the same letter(s) within the same sampling date did not display significant differences.
Figure 6. Average (±S.E.) percentage mortality in semi-field bioassays. Darker bars are for fruit and foliage collected 1 day after treatment application and lighter bars are for fruit and foliage collected 3 days after treatment. Columns topped with the same letter(s) within the same color bars did not display significant differences.
Figure 7. Average (±S.E.) adult SWD emergence from exposed fruit in the semi-field bioassays. Columns topped with the same letter(s) within the same color bars did not display significant differences.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

Growers need affordable, environmentally friendly, and effective management options for SWD control.  To reach Maine’s wild blueberry growers I will present at extension events and maintain contact with growers whose fields are being used for trials.  In-person extension events will include informational flyers detailing the work, results, and recommended management strategies based on the results.  Each experiment will also be written up as a short report and included in the 2022 Wild Blueberry Research and Extension Reports from the University of Maine Cooperative Extension and will also be available online. Finally the results will also be incorporated to update the current factsheet UMaine Cooperative extension fact sheet regarding the management of SWD.

To reach the scientific community, I will present at the 2022 national Entomological Society of America (ESA) meeting and regionally at the ESA Eastern branch meeting. I will also publish my data on SWD thresholds and sustainable management practices in peer-reviewed journals.  This project on SWD management will also be published as part of my Master’s thesis. Data for the project could also be incorporated into regional and national collaborations on SWD management and contribute insights and future publications. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.