Developing an integrated pest management program for a newly introduced pest in Florida blueberries: the spotted wing drosophila, Drosophila suzukii
A survey study conducted in southern highbush blueberries in 2013 showed that spotted wing drosophila is present in all of the seven major blueberry-producing counties surveyed in Florida. We conducted a trapping study that showed the basic plastic cup trap (without color or yellow sticky card modifications) proved an effective trap for SWD when baited with apple cider vinegar. Bait evaluations showed however, that yeast-based baits were more attractive to SWD than vinegar-based baits. Yeast-based baits remain non-specific to SWD and attract many other fly species, making identification more labor intensive.
Obj. 1: To survey SWD in various blueberry growing regions and counties throughout Florida – completion date 2013
Obj. 2: To develop an effective monitoring system for SWD
a) To determine the most attractive bait to SWD – completion date 2013
b) To determine the most effective trap design for capturing SWD – completion date 2013
Obj. 3: To evaluate female SWD oviposition behavior
a) To determine viability of the most grown blueberry species in Florida, Rabbiteye and Southern Highbush, for larval development – completion date 2012
b) To determine susceptibility of different berry maturity stages to SWD oviposition – completion date 2012
Obj. 4: To identify reduced-risk insecticidal tools to control populations of SWD – completion date 2012
What work has been accomplished to date and what work is left to do?
Obj. 1. The survey study was conducted during year two on 14 farms in eight Florida counties. Traps were made with 0.95 L clear plastic cups with lids with eight to ten-1/4 inch holes along the sides of the cup. Traps were secured to the middle of blueberry bushes with twist-ties. Each trap was baited with 150 ml of apple cider vinegar (ACV) and a drop of odorless dish detergent. The detergent reduces the surface tension of the bait and is thought to reduce fly escape from the trap. Each location had between four and seven traps placed along the perimeter of the blueberry field and in the center. Traps were collected weekly for the duration of blueberry season until harvest was completed. Number of weeks at each location varied based on the length of the season. Male and female SWD were identified and counted at the UF Small Fruit and Vegetable IPM Laboratory in Gainesville, Florida. The mean number of SWD per trap was calculated for each county.
- 2013 Results: We found that SWD was present in all counties, which confirmed our findings from 2012 (Fig. 1). DeSoto County did not have any captures during the 2012 season and therefore was not included in the 2013 survey. Similar to 2012, we found a general trend showing more female SWD than males in all counties, with four counties showing significant differences (P ≤ 0.05, Fig. 2). It is possible that the ratio of males to females in the field is skewed towards females, however unlikely. Male and females flies are searching for different things in the field; males are attempting to locate females whereas females are searching for oviposition sites. The volatiles released by the ACV may be more similar to volatiles representing preferred oviposition locations, therefore attracting more females to the traps. The reason for the skewed sex ratio is unknown but warrants further investigation.
- 2-Year Comparison: The mean SWD captured in 2012 was significantly higher than in 2013 (t = 3.95; df = 1000.9; P < 0.0001, Table 1). There were a total of 844 and 498 SWD captured in 2012 and 2013, respectively. There were significant differences in SWD captures among counties in 2012 (F = 22.02; df = 8, 584; P < 0.0001) and 2013 (F = 2.52; df = 7, 645; P < 0.0001, Table 3-4). Citrus County had the highest mean captures in 2012 (4.81 ± 0.31). Marion County (1.44 ± 0.37) and Alachua County (1.34 ± 0.20) were significantly higher than Orange (0.53 ± 0.17) and Suwannee County (0.05 ± 0.49) in 2013. Small differences were found when evaluating differences between years for each county. Citrus County had significantly higher SWD captured in 2012 (t = 7.46; df = 144.3; P < 0.0001) compared with 2013. Alternatively, in Marion County there were significantly more SWD captured in 2013 compared with 2012 (t = 2.81; df = 44.3; P = 0.0074). All other counties had similar captures between the two years in the study.
- Differences between SWD captures in 2012 and 2013 may be due to temperature differences between the years. Weekly mean temperatures averaged below 20°C until the first week in May 2013; conversely during the early part of 2012 temperatures averaged between 20 and 23°C. The SWD has shown decreased activity at temperatures below 20°C and above 25°C (Kanzawa 1935, Kimura 2004). Additionally, growers had become aware of the risk posed by SWD after the survey in 2012 and were proactive in implementing management programs for this pest in 2013. Further statewide surveys will help to answer whether the current management programs are effective at reducing SWD populations to the next season.
- Spotted wing drosophila captures varied by county. The highest mean captures were in the central (Citrus Co.) and north-central (Alachua Co.) parts of the state. Citrus County has a large number of organic blueberry growers, who are limited in the available control options available for this fly. Secondly, several growers in Citrus County grow strawberries adjacent to blueberries. Strawberries are a known host of SWD and the crop seasons overlap to a limited extent in the field. If SWD is established in strawberries the pressure on blueberries is likely to be greater. Alachua County has the highest acreage of blueberries in high tunnels in Florida. The tunnels act as greenhouses, creating artificial warm environments that could promote SWD development early in the season. It is unknown whether the risk to blueberries in high tunnels is greater than field-grown blueberries but this warrants further investigation.
Objective 2a (bait) and 2b (trap) are excerpts from Iglesias et al. 2014 (submitted for publication to J. Econ. Entomol.
- The effectiveness of baits used to monitor SWD was evaluated in a commercial organic southern highbush blueberry farm in Citrus County. The experiment was conducted from 8 May to 27 May 2013. Four monitoring traps each baited with different bait treatments (discussed below) were placed at 20-m intervals in a randomized experimental design with four replicates. Traps were placed within the center of the field. The traps were hung in the shaded areas of the shrub canopies. Four bait treatments were evaluated: 1) apple cider vinegar (ACV), 2) yeast + sugar mixture, 3) yeast + flour mixture (yeast, whole wheat flour, ACV, sugar, water, detergent, and 4) wine + vinegar [red grape wine, rice vinegar, and detergent] (Fig. 3). Our wine + vinegar bait was based on a 60:40 mixture of Merlot red wine and rice vinegar with the addition of detergent (Landolt et al. 2012b). Our bait consisted of concentrated rice vinegar (25% acetic acid) whereas Landolt et al. (2012b) used diluted vinegar (4%), resulting in a bait with lower acetic acid concentration. All baits were evaluated using the basic plastic cup trap as described in the survey study.Samples were collected weekly. Male and female SWD were identified and counted for each sample collected (Triplehorn and Johnson 2005, Markow and O’Grady 2006, Vlach 2010). Data were analyzed using a two-way repeated measure analysis of variance (ANOVA) (SAS Institute, 2002; JMP, SAS Institute 2013). Counts of SWD were log transformed to ln(x+0.5) to normalize the distribution and homogenize the variances before analysis. The factor interaction (bait*time) tested was not significant so was omitted in the final analyses. Significant mean differences were separated with Tukey’s HSD test and considered significant when P ≤ 0.05.
- Results from the blueberry bait study showed significant differences among the bait treatments [F = 13.76; df = 3, 11.99; P = 0.0003] (Fig. 4). Both yeast + flour and yeast + sugar baits had significantly greater mean SWD captures than ACV and the wine + vinegar mix. Captures of male and female flies did not differ significantly in any of the treatments.
- The baits also attracted other dipteran species, specifically the recent invasive Zaprionus indianus Gupta (Drosophilidae) and other non-target flies in the family Drosophilidae (excluding SWD and Z. indianus). The number of Z. indianus captured was significantly greater in the yeast + flour mix than all other bait treatments [F = 8.85; df = 3, 11.93, P = 0.0023] (Fig. 5). Significantly more non-target drosophilids were captured in the yeast + flour mix than in the trap with ACV or wine + vinegar (F = 13.42; df = 3, 12.23, P = 0.0004). The yeast + sugar bait was not significantly different than any of the other baits.
- Discussion: The results showed that the yeast baits (yeast + sugar and yeast + flour) were more attractive than the vinegar baits (ACV and wine + vinegar) which are similar to other studies (Dreves et al. 2012, Walsh et al. 2011). However, the yeast-based baits were highly attractive to other drosophilids, making identification more difficult and time consuming. The vinegar and wine baits on the other hand, are generally clear or can be easily rinsed from specimens and permit quick identification in the field or laboratory. The vinegar and wine baits also act as excellent preservatives for collected specimens. The big disadvantage is that SWD captures may be slightly lower than yeast-based baits and may not be effective where populations are low. Unless a specific bait for SWD is developed other vinegar flies and small non-targets will likely be found in SWD traps.
- Treatment had a significant effect on SWD captures in the high tunnel (F = 6.18; df = 3, 12; P = 0.0088) and the organically managed field [F = 6.490; df = 3, 12; P = 0.0074] (Fig. 7). In the high tunnel, the yeast-baited cup + yellow sticky card inside captured significantly greater number of SWD than the ACV-baited cup + yellow stimulus and the basic cup trap. However, the ACV-baited cup + yellow sticky card was not significantly different than any of the other trap treatments. In the organically managed field, the yeast-baited trap captured significantly more SWD than the ACV-baited cup + yellow sticky card and the basic cup trap whereas the ACV-baited cup + yellow stimulus was not significantly different than the other traps. No significant differences in number of females and males captured were observed in any of the traps in 2013.
- Discussion: “Our findings indicate that neither the yellow visual stimulus nor detergent increases SWD captures when using the basic cup trap (0.95-liter clear plastic deli containers with lids) baited with apple cider vinegar (ACV). The modified ACV-baited traps (yellow visual stimulus band, yellow sticky card, and detergent) were not significantly different from the basic unmodified ACV-baited trap in either year. The result is consistent with previous findings that the color yellow does not increase ACV-baited trap captures of SWD (Lee et al. 2011 and Basoalto et al.2013). In our 2013 trap comparison study, the trap with the yellow sticky card and the yeast + sugar captured significantly more SWD than the same trap baited with ACV in the organically managed blueberry farm. These results suggest that the type of bait is a more important factor in attracting SWD flies than the trap design modifications tested. The yellow sticky card may catch a few SWD; however, this tends to complicate matters since identification of SWD on the card can be difficult. We noticed that specimens captured on the cards tend to desiccate rapidly, requiring immediate identification, whereas flies captured in the ACV drowning solution maintained their color and overall appearance. This is an important finding since additional time would be required by growers to insert yellow foamboards or yellow sticky cards with the hope that this tactic will increase the effectiveness of the trap in detecting SWD. The addition of detergent is also a common practice thought to improve SWD captures. However, our results indicate that the detergent could be omitted and the efficacy of the trap system would remain unaffected, reducing the time and cost of constructing the traps.”
- Figure 2: Female and male SWD captured in 2012 (left) and 2013 (right) blueberry-growing seasons. Asterisks (*) indicate significant differences at P = 0.05.
- Figure 6: Trap treatments for Objective 2b in year 2013. From left to right: 1) basic cup trap (described above), 2) cup + yellow stimulus band + detergent, 3) cup + yellow sticky card inside, and 4) cup + yellow sticky card inside baited with a 150 ml yeast + sugar mixture.
- Figure 7: Mean SWD captured per trap in the high tunnel and organic field experiments. Different letters indicate significant differences at P = 0.05.
- Table 1. Mean number of SWD captured in each county in the 2012 and 2013 survey study.
- Figure 1: Mean SWD captured per trap in each county for 2012 and 2013 blueberry-growing seasons. Asterisks (*) indicate significant differences at P = 0.05.
- Figure 3: Bait treatments for objective 2a in year 2013. From left to right: 1) apple cider vinegar (ACV), 2) yeast + sugar mixture, 3) yeast + flour mixture (yeast, whole wheat flour, ACV, sugar, water, detergent, and 4) wine + vinegar (red grape wine, rice vinegar, and detergent).
- Figure 4: Mean SWD captured per trap per week in the 2013 bait experiment. Different letters indicate significant differences at P = 0.05.
- Figure 5: Mean Drosophilidae and Zaprionus indianus captured per trap per week in the 2013 bait experiment. Different letters indicate significant differences at P = 0.05.
Impacts and Contributions/Outcomes
Identifying appropriate traps and baits will allow us and the growers to detect the first appearance of SWD in fields and locate low populations that will guide management action. Moreover, growers will be able to focus their control activities in areas that have high populations of SWD. Our recommendation to blueberry growers is to use the basic cup trap for monitoring for SWD in their fields, based on the results of our trapping study in years one and two. The yeast-based baits evaluated performed better than the vinegar-based baits in our study. However, both types of bait mixtures have their advantages and disadvantages. Vinegar-based baits are clear and preserve specimens in the field longer than yeast baits. Yeast baits are more attractive to SWD but also to other insects, making identification more time consuming. We recommend the yeast + sugar bait especially when populations are low but note that more time will be required for identification. Monitoring should begin as berries begin to change from green to pink.
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