control of the cyclamen mite Phytonemus pallidus (Acari:
Tarsonemidae) in Eastern Canada strawberry fields
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Résumé
Le tarsonème du fraisier Phytonemus pallidus Banks (Acari : Tarsonemidae) est un important ravageur en fraisière. L’utilisation d’acariens prédateurs (Acari : Phytoseiidae) est un moyen de lutte prometteur contre le tarsonème, mais l’efficacité en champ a été peu étudiée sous le climat de l’Est du Canada. Le but de cette étude de deux ans était d’évaluer l’efficacité de l’acarien le plus recommandé contre P. pallidus, Neoseiulus cucumeris Oudemans, à contrôler le tarsonème du fraisier en contexte de fraisière commerciale au Québec. Les populations de P. pallidus ont également été suivies au champ pendant deux années consécutives. Nos résultats démontrent qu’à fortes densité d’introduction, N. cucumeris est efficace, mais est trop dispendieux pour une utilisation commerciale. De plus, il est très sensible au froid, ce qui limite son efficacité tôt et tard en saison, de même que l’année suivante. Le suivi des populations de ce ravageur au champ a permis la confirmation de faits connus, comme sa grande prolificité, tout en permettant l’acquisition de connaissances nouvelles et essentielles sur sa phénologie, telle que sa présence très tardive au champ. La problématique du tarsonème du fraisier demeure très complexe et d’autres recherches sont nécessaires pour développer des stratégies de lutte efficaces.
Mots-clés : Acariens phytoséiides, Tarsonemidae, Neoseiulus cucumeris, Fraise (Fragaria X
ananassa Duch.)
Abstract
The cyclamen mite Phytonemus pallidus Banks (Acari: Tarsonemidae) is a major pest in strawberry production. Biological control with predatory mites (Acari: Phytoseiidae) is promising but its effectiveness under Eastern Canada’s field conditions has been the subject of little research. The aim of this two-year study was to evaluate the potential of the predatory mite most recommended to control cyclamen mites in commercial strawberry fields in the province of Quebec, P. pallidus, Neoseiulus cucumeris Oudemans. Field populations of P. pallidus were also monitored for two consecutive years. In 2016, N. cucumeris was able to effectively suppress cyclamen mites but was too expensive for commercial use, appeared to be too cold-sensitive to maintain adequate control throughout the season and was unable to prevent a P. pallidus population outbreak the following year. Monitoring cyclamen mite populations in the field has confirmed some well-known facts on this pest, such as its high prolificity, and has also revealed new essential knowledge on its phenology, such as its presence late in the season. Control of the cyclamen mite remains a complex issue that requires further research.
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Key words: Strawberry mite, Tarsonemidae, Neoseiulus cucumeris, strawberry (Fragaria X
ananassa Duch.)
Introduction
Strawberry is an important and valuable crop worldwide. With more than 20 000 T harvested annually, Canada is the world’s 30th largest producer (FAOSTAT 2016). More than 50 % of this
production is grown in the province of Quebec, the third largest producer in North America, after the states of California and Florida (ISQ and MAPAQ 2016). Pest and disease issues are constant challenges for strawberry producers. Among these, the cyclamen mite (Phytonemus pallidus Banks; Acari: Tarsonemidae) has been an increasing source of concern around the world in recent years. This major pest, sometimes known as the strawberry mite, is a phytophagous mite that feeds on young leaves before migrating to the plants’ flowers and fruits later in the season (Schaefers 1963). Infested leaves become distorted, wrinkled and stunted. Under severe infestation, plants become dwarfed with brownish brittle leaves (Alford 2007). Fruits first turn brown under the sepals, then become small and dry as infestation progresses (Hoy 2011; Jeppson et al. 1975), which leads to widespread fruit rejection and yield reduction. In fact, a population density of 35 adults per leaflet has been shown to result in a 23 % loss in yield (Stenseth and Nordby 1976).
Control of this pest is very difficult and complex. Because cyclamen mites are not visible to the naked eye, early detection is difficult, and they are often discovered only when symptoms appear. Mites also avoid light and require between 80 and 90 % relative humidity (Garman 1917). As a result, they live inside young unfolded leaves and flower buds, making them very difficult to reach with most control methods, including non-systemic acaricide (Fitzgerald et al. 2008; Hoy 2011; Zhang 2003). Moreover, females are highly prolific, as this species is mainly parthenogenetic, and can lay an average of two eggs per day (Alford 2007; Easterbrook et al. 2003). In addition, several generations overlap during the season, and adult females overwinter on site, deep in the crown of the plant (Alford 2007; Jeppson et al. 1975). Consequently, infestations become exponential in multi-year strawberry production systems such as the matted rows common in the province of Quebec. Historically, producers have achieved effective control of this pest with endosulfan sprays (Łabanowska 1992; Schaefers 1963; Stenseth and Nordby 1976; Tanigoshi and Bergen 2005). But in recent years, this acaricide has been withdrawn in many countries due to its deleterious effects on the environment and human health. It is also banned in Canada as of December 31, 2016 (Gouvernement du Canada 2011). Since then, much research has aimed to identify another effective acaricide, but results are variable and only provide partial control (Fountain et al. 2010; Gobin and Bangels 2008; Łabanowska et al.
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2015; Lafontaine et al. 2011; Raudonis 2006; Zalom et al. 2009). Therefore, finding an alternative to chemical control is essential.
Biological control with predatory mites (Acari: Phytoseiidae) is a promising solution. In the last 20 years, many indigenous and introduced species have been studied as biological control agents against the cyclamen mite. The phytoseiid mite Neoseiulus cucumeris Oudemans, a type III generalist predator, is one of the predators most recommended against P. pallidus by biological control supply companies and the most studied (McMurtry and Croft 1997). Indeed, its effectiveness against this pest has been known for more than 60 years (Huffaker et Kennett 1953). Despite great variability, its effectiveness against P. pallidus has been demonstrated in many studies (Berglund et al. 2007; Croft et al. 1998; Easterbrook et al. 2001; Petrova et al. 2008; Svensson 2008; Tuovinen and Lindqvist 2010). However, as N. cucumeris is known to be sensitive to cold temperatures (Gillespie and Ramey 1988; Jones et al. 2005; Svensson 2008) and most studies originate from Europe where this predatory mite often overwinters, its efficacy and survival under the Canadian climate remain unknown. Previous experiments have mostly been performed under laboratory conditions or in protected crop production, such as greenhouses, and no in-field study has been performed in a commercial context under Canada’s temperate climate. Thus, the aim of this two-year study was to evaluate the effectiveness of the predatory mite N. cucumeris to control P. pallidus under commercial strawberry field conditions in the province of Quebec. We also monitored cyclamen mite population dynamics in the field for two consecutive years for a better understanding of its phenology in Eastern Canada.
Materials and Methods
Field experiment
Experimental set up and treatments
The experiment was conducted on a commercial strawberry farm located in Sainte-Foy (46°46’06.7” N 71°23’56.5” W), Quebec, Canada, and planted in matted rows. Three treatments were compared: (1) Introductions of N. cucumeris; (2) Acaricide (abamectin, Agri-Mek® SC) applications and (3) Untreated control. Treatments were replicated six times in a randomized complete block design, for a total of 18 plots. The experimental unit was a 12 m2 plot, consisting of three rows of June-bearing
strawberry in first year of production. To prevent plot contamination, a 20 m buffer zone was established on the perimeter of each plot. Temperature and humidity were recorded using two HOBO data loggers (Onset, Bourne, MA). The field was managed by the producer according to their usual
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practices. If insecticide treatment had to be applied in the field (ex: against the tarnished plant bug, Lygus lineolaris), all plots were covered with a waterproof plastic tarpaulin during spraying to prevent potential deleterious effects on phytoseiid mites.
Predator release
Curative rates were based on the biological control supplier’s recommendations. Introduction frequency and rates were adjusted as necessary, according to weekly counts. Thus, predators were introduced every one or two weeks from June 21 to August 10. Two releases of 500 N. cucumeris/m2
and four of 1000 N. cucumeris/m2 were carried out. The number of predators required per plot to
achieve high curative rates was obtained by fractioning the vermiculite volume contained in the purchased bottles. Predatory mites were released in the field in the late afternoon for optimal conditions, within 24 h after delivery by the supplier. They were distributed evenly within the plot, on foliage. Predatory mites were generously provided free of charge by Koppert Biological Systems, Inc. Canada (Berkel en Rodenrijs, The Netherlands).
Acaricide application
The acaricide treatment consisted of abamectin (Agri-Mek® SC, Syngenta Crop Protection Canada, inc., Guelph, ON, Canada), sprayed after foliage mowing during post-harvest operations. This compound was selected because of its efficacy against P. pallidus and common use among Quebec producers. Since abamectin has no effect on eggs, two applications were performed, on August 9 and 22,at a rate of 225 mL of Agri-Mek® SC/ha with the addition of 0.2 % non-ionic adjuvant Agral® (Syngenta Crop Protection Canada, Inc., Guelph, ON, Canada). Total spray volume (2000 L/ha) was applied by producers using a conventional mounted field sprayer equipped with #8010 spray nozzles regulated at a pressure of 60 psi.
Sampling
A total of 10 young unfolded leaves per plot were randomly sampled every week from June 21 to September 6 and then every two weeks until October 17. Leaves were placed individually in plastic bags and immediately stored in a cooler. In the laboratory, samples were kept at 4°C until counting. Both sides of leaflets were examined under a stereomicroscope, and motile stages (female, male, larvae) and eggs of P. pallidus, T. urticae, thrips and phytoseiid mites were counted. Phytoseiid mites were collected with a fine brush and kept in a 70 % ethanol solution until mounted on a microscope slide in PVA mounting medium (BioQuip Products, Rancho Dominguez, CA, USA) for taxonomic identification by the Laboratoire d’expertise et de diagnostic en phytoprotection of Quebec’s Ministry
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of Agriculture (MAPAQ). When necessary, identification of phytoseiid specimens was validated by Frederic Beaulieu from the Agriculture and Agri-Food Canada National Identification Service in Ottawa. Not all phytoseiid mites counted could be identified, either because they could not be captured during the count, were too damaged to be mounted on a slide or identification was impossible (e.g. characteristic features not visible).
Second-year follow-up
In the fall of 2016, half of the plots were destroyed by the producer due to plant disease. However, to observe evolution of treatments over time, the nine remaining plots (three repetitions of each treatment) were sampled weekly until November 29, 2016, and from May 4 to July 10, 2017, using the method described above. As the aim of the follow-up was to monitor the treatment effect over time, no acaricide or predatory mite treatments were carried out in these plots in 2017.
Fruit yield could not be measured in 2016 but was assessed in 2017 on the nine remaining plots in Sainte-Foy. Harvest area consisted of a 1 m2 subplot within each plot. Strawberries were harvested
twice a week from June 26 to July 10. Fruits were classified as marketable or unmarketable. Unmarketable fruits were either smaller than 6 g or damaged by disease, pests or abiotic factors. Fruits in each category were subsequently counted and weighed. Average fruit weight was obtained by dividing total weight by the number of fruits.
Statistical analysis
To evaluate the treatment effect, two-way analysis of variance (ANOVA) with repeated measures was adjusted to data using either the MIXED procedure of SAS (SAS Institute Inc., Cary, NC) or the lme function in the nlme package of R software (R Development Core Team 2013). In each model, the correlation structure that best fit the data was chosen based on the AIC criteria. The variable P. pallidus was generally square-root transformed to meet the normality assumptions, except for Sainte-Foy in the 2016 experiment, for which the log-transformation was used. No statistical treatment could be performed on predatory mites due to the high proportion of zeros. The fruit yield in the second-year follow-up was analyzed with the same ANOVA model, but on the raw data. Following a significant effect in any of the ANOVA tables, the protected LSD method was used to identify pairs of treatment that showed differences. All analyses were performed at a significance level of 5% and all the least square means were reported on the scale of the raw data.
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Phenology of the cyclamen mite
Experimental set up
To better understand the dynamics of P. pallidus in the field under Quebec’s climatic conditions, three untreated plots were followed for two growing seasons on the selected strawberry farm, located in Sainte Foy (46°46’06.7” N 71°23’56.5” W), QC, Canada. Plots consisted of June-bearing strawberry plants in matted rows in their first production year in 2016 and second year in 2017. Two HOBO data loggers were placed in the field to record temperature and humidity.
Sampling
Ten young unfolded leaves were randomly sampled once a week from June 14 to November 29 in 2016, and from straw mulch removal (May 4) until the end of the harvest (July 10) in 2017. Leaves were stored and counted in the laboratory following the method described above. For this descriptive analysis, the number of eggs, larvae, females and males P. pallidus were noted.
Results
Field experiment
In the Sainte-Foy field, N. cucumeris failed to cause a decrease in the P. pallidus population at the beginning of the season. Abnormally cold nights (2.4°C on June 24) at the end of June might have affected survival and efficacy of the first batches introduced. The effects of this sensitivity to cold may have persisted for some time, as the phytoseiid population was still very low at the end of July, despite another release. Thus, as of July 19, introduction rates were doubled, and carried out weekly instead of every other week. Afterwards, the number of cyclamen mites decreased in the N. cucumeris treatment and these plots became distinct from the control. The interaction treatment X period (sampling dates) was significant (F28,210=5.34, p < 0.001) for P. pallidus motile forms (Fig. 9a). At
the beginning of the season and up until August 2, cyclamen mite densities were not different among treatments. On this date and on August 9, the N. cucumeris treatment had fewer motile forms per leaf than the two other treatments (F2,210=46.89, p < 0.001). Following the first acaricide application on
August 9, the number of P. pallidus decreased in the treated plots. The acaricide and N. cucumeris treatments were statistically comparable for two weeks, but different from the control (F2,210=21.56,
p < 0.001). Following the second acaricide application on August 22, the acaricide treatment became statistically different from the two others (F2,210=14.67, p < 0.001) and this difference remained until
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of September 6, while those in the N. cucumeris treatment increased. Consequently, these two treatments did not differ until the end of the experiment. The same pattern was observed for P. pallidus eggs (data not shown).
Fig 9. Average numbers (±SE) of P. pallidus motile forms (Fig. a) and phytoseiid mite motile forms (Fig. b) per young
leaf for the acaricide, N. cucumeris and control treatments on each sampling date for the 2016 field experiment in Sainte Foy, Quebec. Dark-grey arrows show N. cucumeris release dates and light-grey arrows, the acaricide applications. Columns with the same letters are not significantly different (protected LSD test, p < 0.05). No statistical test could be performed on the phytoseiid mite dataset due to the high proportion of zeros.
Early season phytoseiid mite density was low in the N. cucumeris treatment (Fig. 9b). On average, density was below 0.12 motile forms per leaf until July 19, even though predatory mites had already been released twice in these plots. The number of phytoseiid mites started to increase as of July 26 and remained stable until August 15, averaging around 0.8 motile forms per leaf. However, density decreased rapidly as of August 22 until the end of the season. Few phytoseiid mites were found in other treatments during the experiment, with a maximum of 0.28 motile forms per leaf (Acaricide,
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August 9), and consisted mainly of N. fallacis. After the first acaricide application, no phytoseiid mite was found in this treatment until the fourth week (October 3), when only a few individuals (0.07 ± 0.04) were present. The Phytoseiid mite species that were identified are presented in Table 2. Table 2. Total number of phytoseiid mite species identified in the 2016 field experiment
Phytoseiid mite species Experiment Treatment A. andersoni N. sp. nr callunae N. cucumeris N.
fallacis Other Unidentified Total
Sainte-Foy 2016 Acaricide 0 1 1 6 1 44 53 N. cucumeris 0 0 53 5 3 208 269 Control 7 0 1 18 1 50 77 Second-year follow-up
To observe treatment effect over time, the nine remaining plots in Sainte-Foy were monitored in the fall of 2016 and in 2017 (Fig. 10). The interaction of treatment X date was significant for the two years combined (F58,174=4.72, p < 0.001). From October 17 until the end of the 2016 growing season,
the N. cucumeris treatment was not different from the control, except on November 23 (F2,174=9.14,
p = 0.001), when more P. pallidus were counted in the control. On the last sampling date in 2016, the acaricide treatment had significantly fewer P. pallidus motile forms than the control, but the N. cucumeris treatment had an intermediate level and was statistically similar to the two other treatments.
At the beginning of 2017, the number of P. pallidus motile forms was low, and remained similar among treatments until May 29. A week later, populations increased rapidly for all treatments, with a more marked effect for the control, whose population multiplied by 10. Between June 20 and 26, the number of P. pallidus exploded in the N. cucumeris treatment, with averages increasing from 10.8 ± 5.4 to 48.8 ± 7.1 motile forms per leaf. It even surpassed the control on June 26 and became statistically different on July 10 (F2,174=22.92, p < 0.001), with an average 2.5 times higher than the control. Density in the acaricide treatment remained low, at under 11 motile forms per leaf from August 15, 2016 to July 10, 2017. On the last sampling date, motile numbers differed among treatments. At this date, the acaricide treatment had the lowest number of motile forms per leaf (10.4 ± 3.2), followed by the control (25.0 ± 5.6) and finally by N. cucumeris (60.5 ± 0.7). The number of P. pallidus eggs followed similar tendencies (data not shown). No phytoseiid mites were found in the 2017 samplings.
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Fig. 10. Average numbers of P. pallidus motile forms per young leaf (±SE in shaded area) for the acaricide, N. cucumeris
and control treatments on each sampling date for the 2016 field experiment and follow-up in 2017 in Sainte Foy, Quebec. Dark-grey arrows show N. cucumeris release dates and light-grey arrows represent acaricide application dates in 2016. No predatory mite release or acaricide application was carried out in 2017.
Follow-up fruit yield
Fruit yield differed among treatments in the second-year follow-up of the field in Sainte-Foy (Fig. 11). Marketable yield was different for all treatments (F2,4=22.05, p = 0.007) and decreased in
the following order: acaricide, N. cucumeris and control. The acaricide and N. cucumeris treatments produced similar total yields (t4=1.8, p=0.150), which were higher than that of the control (F2,4=12.12,
p = 0.020). Fruit size also differed among treatments (marketable: F2,4=9.94, p = 0.028;
unmarketable: F2,4=19.16, p = 0.009). The average fruit weight for marketable strawberries was
higher for the acaricide treatment than the control, whereas the N. cucumeris treatment was intermediate and not different from the two others. Total number of fruits harvested was 962 for the acaricide, 987 for N. cucumeris and 777 for the control.
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Fig. 11. Strawberry yield for the three treatments (Acaricide, N. cucumeris, and Control) at the second-year follow-up (2017)
of the 2016 field experiment in Sainte Foy, Quebec. Total yield (± SE) and average individual fruit weight (± SE) of marketable, unmarketable and total strawberries are shown in Fig. a and b, respectively. Columns with the same letters are not significantly different (protected LSD test, p < 0.05).
Phenology of the cyclamen mite
The evolution in the number of P. pallidus females, larvae, males and eggs monitored in the field for two consecutive growing seasons is presented in Figure 12. In 2016, sampling began on June 14, when strawberry plants were blooming. At that time, only an average of 0.5 females, 0.03 larvae, 0.1 males and 0.38 eggs per leaf were observed. Over the following weeks, the population increased continually and peaked on August 30, with an average of 36 females, 11 larvae, 1 male and 97 eggs per leaf. Afterwards, the P. pallidus population decreased for all stages, especially the number of eggs. Field sampling stopped on November 29, as strawberry plants were covered with ice and snow.