Onion thrips (Thrips tabaci (Thysanoptera: Thripidae)) in cabbage on Prince Edward Island: observations on planting date and variety choice
© Blatt et al. 2015
Received: 20 June 2015
Accepted: 6 August 2015
Published: 19 August 2015
Onion thrips (Thrips tabaci Lindeman (Thysanoptera: Thripidae)) can be a pest in organic onion production on Prince Edward Island. This study was to examine the effect of planting time and variety on infestation levels and damage by onion thrips on cabbage (Brassicae oleracea capitala (L.)). A field site was planted with 2 main and 8 lesser varieties of cabbage over 4 planting dates. Some varieties were short season and harvested on July 31 with longer season varieties harvested on September 2. Blue sticky traps were used to capture thrips migrating into the field site from July 22–September 2. Traps were counted weekly and cabbage heads within the field site were visually surveyed for thrips. At harvest, heads were weighed and measured, thrips damage was assessed then the head was dissected and thrips counted on the first four layers of the head. Thrips exhibited a preference for Lennox over Bronco throughout the season although thrips populations were not high enough to effect economic damage in 2014. Planting date influenced cabbage head weight and size with later plantings yielding the largest heads. Use of planting date and variety to avoid thrips populations is discussed.
KeywordsOnion thrips Cabbage Prince Edward Island Variety preference Planting time
Onion thrips represent a serious threat to cabbage production in cabbage growing regions of the world (Garamvölgyi et al. 2004; Shelton et al. 2008). Damage can appear as a bronze discoloration and/or a rough texture on leaves within the head (North and Shelton 1986b) causing injury up to 20 layers deep (Trdan et al. 2005; Respondek and Zvalo 2008) and reducing marketable production. For Prince Edward Island (PEI), thrips can cause significant loss to the cabbage crop in organic fields. Control is often recommended at the early head formation stage (7.5 cm leaf ball) (PEI Department of Agriculture, Fisheries and Aquaculture 2005), with pesticides available for conventional producers (Shelton et al. 1998; Trdan et al. 2007) but no viable products available for organic production. The decision to spray is currently based on a presence/absence determination of thrips. Avoiding thrips populations through selection of planting date and less susceptible varieties are two potential options available to organic growers. Varietal preference and susceptibility or resistance to thrips has been documented in Europe and the United States. Some of the traits investigated to explain this preference include leaf wax thickness, reflectance or color of the leaves around the head. In Hungary, 52 white cabbage varieties were evaluated for resistance to thrips damage. None of the varieties was completely resistant although a handful of varieties showed decreased susceptibility (Garamvölgyi et al. 2004). Other studies have found Thrips tabaci to exhibit a variety preference using locally available commercial varieties (Trdan et al. 2005; Shelton et al. 1998). Shelton et al. (2008) found planting date and variety to significantly influence the extent of thrips damage in the top 10 layers of cabbage head leaves. Cabbage planted later in the season (July 25 vs. July 3) experienced less damage for specific varieties. It is speculated that these later planted cabbage heads experienced lower thrips pressure due to reduced immigration into the field (North and Shelton 1986b; Shelton and North 1986), or increased mortality due to high rainfall during the latter part of the season. Of the varieties commonly planted in PEI, it is unknown whether thrips show a preference, or if selecting early developing varieties and planting early in the season will serve to avoid the thrips populations.
The objectives of this work were to evaluate the influence of planting date and cultivar on the incidence of T. tabaci within a commercial organic cabbage planting, and determine if monitoring with sticky traps can predict thrips damage.
Cumulated weekly trap counts, mean (SE), for thrips near a cabbage field in North Wiltshire, PEI, from July 22 to September 2, 2014 showing impact of location on thrips abundance throughout the season
1.5 (3,11), 0.26
4.2 (3,13), 0.03
13.2 (3,13), 3.05 × 10−4
37.4 (3,12), 2.28 × 10−6
31.1 (3,12), 6.10 × 10−6
16.2 (3,12), 1.62 × 10−4
4.52 (3,12), 0.02
9.7 (3109), 1.03 × 10−5
ANOVA table for number of thrips found on cabbage heads during season and at harvest and proportion of layers showing damage for Bronco and Lennox varieties
Variety (Fdf, P)
Plant date (Fdf, P)
Harvest date (Fdf, P)
Thrips on heads over season
12.313,55, 2.86 × 10−6
33.411,55, 3.64 × 10−7
Thrips on heads at harvest
7.131,55, 9.99 × 10−3
2.273,55, 8.99 × 10−2
10.751,55, 1.82 × 10−3
Proportion of layers showing damage
5.921,55, 1.82 × 10−2
ANOVA table for cabbage weight and size showing significance of variety and planting date, F (P value)
22.07 (1.85 × 10−5)
7.56 (2.61 × 10−4)
26.18 (4.08 × 10−6)
3.78 (1.54 × 10−2)
28.72 (1.70 × 10−6)
3.94 (1.28 × 10−2)
Thrips tabaci were readily captured on unbaited sticky traps throughout the growing season. The population moved from surrounding vegetation into the crop and was more abundant on the south and eastern sides of the plot. This was unexpected as the east side of the field site contained the main roadway access to the field and therefore was considered non-favorable to thrips survival. Weed and plant species present around the north, east and south sides of the field did not differ markedly in composition containing a mixture of Plantago spp., Poa spp., Taraxacum spp., Aster spp., Erigeron spp. and Lactuca spp. Plants along the eastern side of the field were frequently crushed by tractors, cultivators and other farm equipment. Given the lower plant quality and the frequent disturbances, higher thrips along this side seems counter-intuitive. Along the south edge of the field, traps located 3 m off the field edge had the highest captures. To the south of these traps, the adjacent field contained fall rye. It is conceivable that cabbage is more attractive to T. tabaci than fall rye and this caused a general migration away from that field (planted in lettuce in 2013) and into the cabbage field. Thrips are known to use lettuce as a host (Workman et al. 2007) and could have built up populations during 2013, then moved towards the cabbage in 2014. A similar argument could exist for the field across the roadway which was also planted in fall rye during 2013. Rye has been noted as a host for T. tabasci but is not recorded as a major pest of this crop. It is quite likely that the planting scheme chosen focused the thrips to move toward the cabbage planting in 2014. Additionally, consideration of the general wind direction may have contributed to the pattern of trap catches observed. From July 28 through August 14, the bulk of the population was captured on sticky traps located along the south and east edges (facing away from the cabbage field). During that time, according to Environment Canada records, the wind direction was predominantly from the North–East and South which would have blown the thrips onto the South and East traps more so than those located on the North edge of the cabbage plot. It is clear that distribution within the field is influenced by factors other than those examined during this study. A broader examination of the landscape and correlation with thrips number may provide insight into the likelihood of thrips survival in natural habitats. Temperature was found to be the main factor influencing thrips populations (Morsello et al. 2008). In this study the population gradually increased then decreased by early September coinciding with both a decrease in daily temperatures and shorter day length. Should climate change result in warmer temperatures for this region, then the population could have multiple generations in a year (Bergant et al. 2005) creating the potential for damage later in the season. A further factor to consider is haplotype. Nault et al. 2014 found haplotype populations of thrips on cabbage to change significantly during the growing season in New York during 2005 and 2007. It is known that different haplotypes have different reproductive rates and ability to transmit viruses. It is therefore possible that if the haplotypes in cabbage in PEI change, that this could affect the extent of damage observed on cabbage towards the end of the growing season.
Despite the large numbers of thrips captured on the traps, thrips observed on the cabbage heads over the season were very low. This suggests that either the thrips were not able to effectively colonize the heads in the field or that visual surveys were unable to adequately detect a representative portion of the population present on the head. Correlation between thrips observed during the last cabbage head survey did not correlate well with thrips found during cabbage head damage evaluation for cabbages harvested later in the season. The correlation was higher for cabbage heads harvested earlier. This suggests that head surveys are not indicative of the population contained within the head once the bulk of the population has moved into the field. Alternative means to quantify the thrips present in the plot itself will need to be evaluated. Damage from thrips was negligible at harvest and none of the observed damage was significant enough to render the cabbage heads unmarketable.
Thrips tabaci showed a varietal preference for Lennox over Bronco throughout this study. The variety Bronco is considered an earlier variety requiring fewer days to harvest than Lennox. In this study, Bronco was harvested after 76 and 82 days (May 15 and May 9 planting dates, respectively). This is longer than the stated days to harvest of 64–68 days indicated for this variety. In North Dakota, Greenland et al. (2000) found Bronco to require 116–131 days to mature, depending upon year. The same study found temperature to correlate with yield. Lennox was harvested after 83, 93 and 107 days in the field (June 9, May 29 and May 14, planting dates, respectively). This is shorter than the 105 days to maturity more typical in other areas.
Variety preference for thrips has been studied in cabbage for some time. While the varieties used in this study were rarely included in these studies, they could provide future directions for inquiry. Voorrips et al. (2008, 2010) evaluated accessions of cabbage known to be susceptible and resistant to thrips attack. Over the years of the study, there were several of the traits which explained the variation in damage observed, however Brix and leaf wax layer late in the season (just before harvest) were the two traits that were consistent over varieties and years. Trdan et al. 2004 studied 6 varieties and found wax to differ among varieties in accordance with thrips damage. Study of plant compounds in cabbages identified as resistant to T. tabaci showed no correlation with damage (Trdan et al. 2008a). The strongest factor influencing thrips damage was wax content of the cabbage leaves. Fail et al. (2008) studied light reflectance from cabbage leaves and found the outer head leaves to differ. The color of the outer head-forming leaves did correlate with the number of thrips adults found in the cabbage heads. A follow up study by Fail et al. (2013) demonstrated that light reflectance can correlate with thrips presence in some years but not others, suggesting that other variables may influence thrips’ response to spectral cues. Bálint et al. (2013a) evaluated epidermal thickness between resistant and susceptible cabbage varieties. Thickness did not differ between varieties and did not correlate with thrips damage. Bálint et al. (2013b) studied leaf reflectance of UV-A, and visible light wavelengths. Their findings confirmed a negative relationship between the UV-A light reflection and onion thrips during the early stages of cabbage head formation. It is suggested that more intensive reflection of leaves may deter onion thrips from establishing on these varieties. As this was the first examination of varietal preference of thrips for cabbage on Prince Edward Island, it was unclear if a preference would be observed. Future studies on Prince Edward Island should endeavour to investigate the mechanism of the varietal preference observed.
Earlier planting and harvest of Bronco did result in avoidance of the majority of the invading thrips. By July 31, the date of harvest for Bronco, only 20 % of the immigrating thrips were captured on the sticky traps. The remaining 80 % of the migrating thrips were captured over the month of August. Even with the bulk of the population occurring after the harvest of Bronco, thrips populations were not high enough to cause economic losses to cabbages harvested in September of 2014. Head weight and size was larger for the cultivar Lennox and the later planting, but no correlation between head weight and thrips presence on the cabbages was observed. Lennox cabbage heads at harvest (September 2) were largest from the later planting date of May 29. This differs from the literature where Kleinhenz and Wszelaki (2003) found planting date and cultivar to be significant for head weight and size with earlier plantings producing significantly heavier heads from later plantings. Bronco was one of these varieties and performed reasonably well with 4 of the 10 traits examined affected by planting date. These included weight, volume, and polar and equatorial dimensions.
Organic cabbage producers on Prince Edward Island have few options with respect to thrips management. Varietal resistance, selection of short season varieties and timing of planting have promise in managing the naturally occurring thrips populations. Other options that organic growers may explore in future include intercropping or mulching. Intercropping of cabbage with a clover (Teunissen et al. 1995) or encouraging clover in the grassways around the fields may serve to increase natural enemies (Pobožniak and Wiech 2005). Another option may be mulching which Trdan et al. (2008b) found to show potential for increasing early cabbage production. While not examined in this study, the reproductive modes of T. tabaci may offer an opportunity for control. Li et al. (2014) found thelytokous T. tabaci to be better adapted to survive on cabbage while arrhenotokous T. tabaci were better adapted for onion. These reproductive modes influence longevity and fecundity which may, in turn, provide opportunities when deciding pest management strategies.
The cabbage variety Bronco was less preferred by thrips. The more preferred variety Lennox when planted and harvested earlier in the growing season harbored fewer thrips and experienced lower damage than Lennox planted and harvested later in the season. Planting varieties with tolerance to thrips damage may serve to protect cabbages without use of insecticides. For organic growers on PE Island, there are currently no available chemical products to protect their crop. Thrips have demonstrated a preference for certain cabbage varieties in other parts of the world and these have been used successfully to reduce thrips damage. Timing of planting and harvest may also play a role in avoiding damage from thrips. This study demonstrates that commercially available varieties planted and harvested early in the season on PE Island exhibit reduced damage from thrips.
Cabbage planting dates, cultivars and data collected
Details of plot sizes, number of plants/plot, planting and harvest dates and cultivars established in the trial on PEI, 2014
20 × 6
1, 6, 11, 16
20 × 4
2, 7, 12, 17
20 × 6
3, 8, 13, 18
20 × 8
4, 9, 14, 19
20 × 3.5
20 × 3.5
Grand Vantage, Bronco, Prime Vantage/Grand Vantage, Lennox
5, 10, 15, 20
20 × 3.5
20 × 3.5
Bruno/Grand Vantage, Excalibur, Lennox
5, 10, 15 and 20
Onion thrips monitoring
Thrips were surveyed in two ways: blue sticky traps and visual surveys. Traps were placed around the outside perimeter to monitor the thrips population migrating into the field. Visual surveys were used to quantify thrips numbers inside the plots as these would represent actual attack of the cabbage heads. Blue sticky traps measuring 10.2 cm × 12.7 cm from Solida (Montreal, Quebec) were clamped onto wooden slats and positioned approximately 60 cm above the ground. This height put them just above the developing cabbage head and foliage. Traps were established in the field on July 16, 2014 as the heads were beginning to form, consistent with the movement of thrips into the crop. Traps were placed along three sides of the field margin (North, East and South) outside each quadrat (see Fig. 8) with a second line of traps placed 3 m farther south of the southern edge. The second line was to determine if trapping could occur farther away from the field edge thereby avoiding damage from machinery. All traps were monitored twice each week beginning July 22 through to harvest in early September. Traps were changed each time and thrips identified in the laboratory using a microscope. On the same days as the traps were monitored, four cabbage heads from within each quadrat were surveyed for presence of thrips by visual surveying of the outer layer and under the 1st leaf layer of the cabbage head.
All analyses were done using unbalanced ANOVA in R unless indicated otherwise (R-Core Team 2012). To examine the thrips population around the study site over time, thrips captured on sticky traps were pooled across location and compared across capture dates. The effect of trap location was evaluated by pooling traps within each location (i.e. north, east, south-edge and south-outer) and comparing across capture dates. Thrips entering the field site were compared with thrips within the field site by correlating cabbage head thrips counts with trap catches throughout the season by date. The influence of variety and planting date on total thrips observed on cabbage heads over the growing season was analysed. Location of thrips within the cabbage head at harvest, cabbage head weight and cabbage size was analysed for the two main varieties, Bronco and Lennox, and planting date. Percentage of cabbage heads showing thrips damage were analysed by location with the cabbage head and correlated with number of thrips present at harvest and the total thrips present on the heads over the season. And finally, correlation between trap catches and head weight and cabbage size was examined using linear models. Cabbage head weight and size (circumference along polar and equator dimensions) were compared for variety, planting date and harvest date. Thrips counts were transformed by sin−1(sqrt(x + 0.5)) where necessary prior to ANOVA where homogeneity of variance was violated.
SB, JD, AR and SA conceived and designed the experiments. JD, AR and SA conducted the experiments. SB analyzed the data and wrote the manuscript. All authors read and approved the final manuscript.
The authors would like to thank Brookfield Gardens Inc. for the use of their field and their contribution of cabbages for study, Caitlin vanKeymeulen for technical assistance and Christine Noronha (AAFC-Charlottetown), Steve Javorek (AAFC-Kentville) and two anonymous reviewers for thoughtful comments on the manuscript.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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