Cyclamen mite in strawberry: recent research towards new management strategies

Cyclamen mite continues to be a difficult-to-manage and prevalent pest in strawberry, but research conducted in recent years in Ontario has resulted in promising new sampling and control methods. Successful control methods will form the basis of an integrated management strategy that will reduce the impact of this important strawberry pest

All life stages of cyclamen mite (eggs, nymphs, adults) live primarily in the plant crowns, newly emerged, tightly wrapped leaves and fruit buds. Feeding injury by cyclamen mite causes yellowish-brown, wrinkled, toughened and small leaves with shortened petioles. Significant injury to fruit buds causes fruit to be small, hardened, seedy and generally unmarketable.

It is recommended to scout for cyclamen mite by looking for early signs of leaf damage and to open wrapped leaves and examine them for mites with a hand lens. However, this method is time consuming (adult mites are just 0.2–0.3 mm long), especially when mite populations are low and below treatment thresholds (5–10 per leaf). Agri-Mek SC (abamectin) is the only registered miticide for cyclamen mite in strawberry, with a maximum of two applications per year. Magister SC (fenazaquin) is now registered in strawberry for mites, with a pending label expansion to include cyclamen mite.

Areas of previous and ongoing research:

1. Developing a molecular method for detecting cyclamen mite on plants, particularly on transplants before they are planted.
2. Determining an optimal steam treatment protocol for cyclamen mite elimination from transplants without compromising plant health and production.
3. Testing miticides and biopesticides for efficacy against cyclamen mite, with a focus on Magister SC.
4. Determining whether there is tolerance or resistance to cyclamen mite among commonly grown cultivars.
5. Determining the effect of UV-C on cyclamen mite.

Results-to-date:

1) The upstream requirements for developing a molecular method for detecting cyclamen mite have been completed. We sequenced DNA from cyclamen mites collected in Ontario and Quebec and developed primers that are sensitive (can detect very small quantities of cyclamen mite DNA) and specific (do not amplify DNA of other potential pests on strawberry). The next step is to develop a sampling protocol that rapidly, accurately and affordably detects cyclamen mite on leaves. The advantage of testing for cyclamen mite on transplant leaves is to implement control strategies before planting (steam treatment) or soon after planting (miticides) if cyclamen mite DNA is detected.

2) Three (A, B, C) steam treatment experiments have been completed, starting in 2019. Two on-farm steam treatment experiments were conducted in 2023 and results are pending.

• A) Bareroot ‘Jewel’ and ‘Annapolis’ transplants were acquired from a nursery, and some were infested with cyclamen mite from our colony. Transplants were steam treated in a converted growth chamber at AAFC in Vineland at 44 °C for 1 h or 48 °C for 0.5 h following a 37 °C for 1 h pretreatment. Transplants were set on the same or the following day that they were steam treated. Both 44 °C for 1 h and 48 °C for 0.5 h significantly reduced the number of cyclamen mite compared to the infested control (Figure 1). However, treatment at 48 °C for 0.5 h resulted in 30–40% plant survival for both cultivars, whereas control and 44 °C for 1 h plant survival was about 90%.
• B) Bareroot ‘Jewel’ transplants were separated into large (10–13 mm diameter) and small (6–8 mm diameter) crowns, infested with cyclamen mite from our colony and steam treated at 44 °C for 1 or 4 h. Steam treatment significantly reduced the number of cyclamen mite compared to the control (Figure 2). However, 44 °C at 4 h reduced survival of small-crowned plants to 40% and large crowned plants to 80%. Survival of untreated and 44 °C for 1 h treated transplants was 90–95%.
• C) Bareroot transplants of 12 commonly grown or newer June-bearing cultivars (see Figure 3) were steam treated at 44 °C for 2 h and set in research plots in 2022. Transplants were not infested with cyclamen mite prior to treatment. Over all cultivars, the average survival of steam treated plants was 93.8% and for control plants was 93.9%. A few cultivars had slightly lower survival when steam treated: Cabot, Mira and particularly St. Laurent (Figure 3). Harvest data from 2023 is currently being analyzed.

3) In 2019, we began conducting research plot and greenhouse experiments with miticides and biopesticides for cyclamen mite. We compared field applications of Magister SC to Agri-Mek SC and found that both miticides controlled cyclamen mite equally well. All tested biopesticides (Grandevo, Venerate, Bb Protec, Ecotrol, Botanigard ES, JMS Stylet Oil and Vegol) did not provide high levels of control. More recently, we conducted efficacy trials comparing Magister SC and Agri-Mek SC to 2 other miticides registered in strawberry, Nexter (pyridaben) and Oberon (spiromesifen) and to Pylon (chlorfenapyr). In potted strawberry plants in the greenhouse in 2021 and 2022, all products provided at least some mite control and all products controlled the mites in the new leaves better than in the plant crowns. Percent reductions in mite numbers compared to the untreated control were 92–98% in the new leaves with Agri-Mek SC, Magister SC, and Nexter and 50–75% with Pylon and Oberon.

In a research plot experiment, the same miticides tested in the greenhouse were applied 1 week after mowing at label rates in a spray volume of approximately 1000 L/ha. Mite counts from leaf samples indicated excellent control by all products, with Nexter and Pylon resulting the lowest mite counts at 6 weeks after spraying (Figure 4). Spring 2023 mite counts and harvest data are currently being analyzed. The next steps are to evaluate season-long miticide programs in June-bearing and day-neutral growing systems and at farm levels

4) Two greenhouse experiments were conducted in 2021 and 2022 evaluating the susceptibility of common June-bearing cultivars to cyclamen mite. Potted strawberry transplants were infested with cyclamen mite from our colony, and the number of mites per plant was counted 4–5 weeks later. In the first experiment ‘Malwina’ had more mites than ‘Evelyn’ and ‘Jewel’. In the second greenhouse experiment there were no significant differences between the number of mites on the 7 cultivars included in the experiment.

The same cultivars were established in research plots and infested with cyclamen mite. One year later, ‘St. Laurent’ had more mites than ‘Mira’, ‘Valley Sunset’, ‘Jewel’, ‘Wendy’, ‘Lila’, and ‘Evelyn’. The plots were harvested and berries were graded for cyclamen mite injury. ‘St. Laurent’ and ‘Wendy’ had more damaged berries than ‘Valley Sunset’ and ‘Jewel’. Although no cultivars included in these experiments were resistant to cyclamen mite, there appears to be some cultivars that are less susceptible to cyclamen mite.

5) Ultraviolet-C light (UV-C) (254 nm) is a promising method to control plant diseases, such as powdery mildew, and twospotted spider mite in strawberry. Since UV-C needs to be applied at night, autonomous vehicles with light arrays have been developed and are being used in some strawberry-growing regions. We tested the effectiveness of UV-C against cyclamen mite in potted strawberry plants using light arrays in a growth chamber. However, UV-C produced no discernable, direct mortality of cyclamen mite in 3 separate experiments where we altered parameters such as plant size, plant cultivar, light angle, duration of exposure, and the time between infesting plants with mites and the first exposure to UV-C. Since cyclamen mite primarily live concealed in new, wrapped leaves and the plant crown, it is likely not exposed to UV-C, as UV-C is a surface sterilant and does not penetrate leaf tissue.

Conclusions:

The best way to manage cyclamen mite is to start with “clean”, mite-free transplants. Steam treatment at 44 °C for 1 or 2 h effectively reduces mite numbers to very low levels without damaging bare-root transplants. Treatment at 44 °C for 4 h, which is recommended for control of some diseases, may have detrimental effects on some plants.

Diligent scouting for cyclamen mite should precede a decision to implement control measures, such as miticides. Once a sampling method is developed, molecular testing will be available to augment current scouting practices. If mites are found and plant injury is observed, Magister SC is an effective alternative to Agri-Mek SC. Miticides should be applied at high water volumes and slow tractor speeds, and 1 week after mowing is the best time to apply miticides in June-bearing cultivars. More research is needed to determine the best timings for 2^nd and 3^rd applications in a season (eg., early or late spring or fall). Certain June-bearing cultivars, such as ‘Evelyn’ and ‘Jewel’ appear to be less susceptible to cyclamen mite, and choosing less susceptible cultivars could contribute to cyclamen mite management. 

We plan to continue our research on cyclamen mite management in strawberry in future years, incorporating other aspects of control, such as use of predatory mites or entomopathogenic fungus. We welcome questions, ideas, and feedback from you, the growers and industry, to help us provide results that are timely and useful.

Funding for this research was provided by the Ontario Agri-Food Innovation Alliance Tier 1 Research Program, and the Berry Growers of Ontario.

The research was led by Justin Renkema (AAFC Vineland) and Rebecca Hallett (University of Guelph). Marlee-Ann Lyle (MSc student, University of Guelph) conducted research on molecular tools for sampling and pesticide efficacy. Erica Pate (MSc student, University of Guelph & OMAFRA) conducted research on steam treatment and cultivar susceptibility. Angela Gradish (University of Guelph) coordinated research activities and reporting, and managed research on molecular tools for sampling. Collaborators were Amanda Naum, Valerie Fournier (Université Laval), and Fumi Takeda (USDA). Technical assistance was provided by Lori Bittner, Tara Preston, summer students and farm staff (AAFC Vineland), and Avery Hewson-Dunbar (University of Guelph).