First line of defence

May 04, 2010
Written by Wendy Romero, Cynthia Scott-Dupree, Graeme Murphy, Theo Blom, Ron Harris
Western Flower Thrips (WFT) (Frankiniella occidentalis (Pergande)) are, without a doubt, a major concern in Ontario floriculture greenhouses (Fig. 1. a and b).

FIGURE 1. a and b. Western Flower Thrips are a major concern for growers; once established in the greenhouse, they’re difficult to control. (Photos by Dave Cheung, U of Guelph)

Once they have become established in the greenhouse, WFT are extremely difficult to control. They can develop resistance easily to pesticides, and as a result, chemical efforts are often ineffective. Even if they are effective, these chemicals can have a negative impact on biological control agents used in many Ontario greenhouses.

Currently, attention has been focused on controlling thrips pests that may be coming in on imported cuttings. There are several concerns:

Firstly, the influx of what could potentially be large numbers of thrips every week places constant pressure on any control program.

Secondly, if these thrips are insecticide-resistant, it removes one control option that growers have available to them.

And finally, new thrips species may be introduced in this way.
Using reduced risk control methods would permit growers to establish thrips-free and insecticide residue-free cuttings from the outset, thus ensuring that ongoing greenhouse biological control programs are not affected negatively.

This study is being conducted by Wendy Romero (M.Sc. candidate) at the School of Environmental Sciences at the University of Guelph, Ontario, to identify reduced risk methods to suppress WFT on chrysanthemum cuttings. Hot water, and reduced risk insecticides – Safer’s® Insecticidal Soap (potassium salts of fatty acids 2.0 per cent, Woodstream Canada Corp) or Landscape® Horticultural Oil (mineral oil 99 per cent, Plant Products Co. Ltd.) were evaluated as immersion treatments. They are considered reduced risk because of their low toxicity, low residual effect on biological control agents and modes of action that WFT are unlikely to develop resistance to. Insecticidal soap and mineral oil are control products that have a high possibility of being registered as immersion treatments for WFT control in Canadian floriculture greenhouses.

First, it was determined whether treatments were phytotoxic to cuttings. Chrysanthemum cuttings (cv ‘Sunny Shasta’) were immersed in hot water at various temperatures (35-43°C) for different immersion periods (5-60 minutes) and transferred to a cooling bath (21°C) immediately after for one minute. For the reduced risk insecticides, cuttings were immersed for one minute in different concentrations of insecticidal soap (5-40 ml/L) or horticultural oil (5-60 ml/L). After immersion, cuttings were stuck in Oasis® for rooting using overhead misting. Phytotoxicity was evaluated 20 days after immersion based on survival, number of roots per plant and height of cuttings. Hot water treatments at 35°C for 15, 30, 45 and 60 minutes; 39°C for 15 and 30 minutes; and, 41°C for 5 and 15 minutes were not phytotoxic to cuttings. Insecticidal soap treatments (5, 10, 20 and 40 ml/L) and oil treatments (5, 10, 20 and 30 m/L also were not phytotoxic to cuttings.

Second, treatments that were not phytotoxic treatments were tested for their effectiveness in controlling WFT. Larvae and adults of WFT were immersed in hot water, insecticidal soap or horticultural oil inside a vented Petri dish. Mortality was assessed 24 hours after immersion. In addition, spinosad (Success® 480 SC, Dow AgroSciences Canada) was tested as an immersion treatment at the registered label rate of 0.05ml/L (50 ml/1,000 L) for foliar application against WFT in greenhouse ornamentals. Treatments needed to provide >80 per cent mortality to be considered feasible for use in an IPM program.

Hot water: High mortality (>80 per cent) of second-instar larvae and adult WFT was obtained at 39°C for 30 minutes or 41°C for 15 minutes for hot water immersion treatments (Fig. 2). Furthermore, a significant reduction in number of eggs was achieved with both treatments compared to the control (Fig. 3).

FIGURE 2. Mean corrected per cent mortality (± SE) of WFT immature (first- and second-instar larvae) and adults 24 hours following hot water immersions. Means with the same letter are not significantly different.

FIGURE 3. Mean corrected number (± SE) of WFT eggs hatched four days following hot water immersion. Means with the same letter at 0.05 are not significantly different.

Insecticidal soap, spinosad: Insecticidal soap treatments and spinosad at the recommended rate resulted in <30 per cent mortality of larvae and adult WFT (Fig. 4). The 40 ml/L treatment of insecticidal soap provided the highest mortality of adult WFT of all the treatments at approximately 75 per cent (Fig. 4). The spinosad treatment resulted in approximately 58 per cent adult WFT mortality (Fig. 4). The results indicate that neither insecticidal soap nor spinosad provide the required level of control of WFT on chrysanthemum cuttings in an IPM program.

FIGURE 4. Mean corrected per cent mortality (± SE) of adults and immature (first- and second-instar larvae combined) WFT after immersion in insecticidal soap. Means with the same letter are not significantly different.

Horticultural oil: All horticultural oil treatments resulted in 100 per cent mortality of WFT adults and 64-82 per cent mortality of first- and second-instar larvae (Fig. 5). Currently, horticultural oil is not registered for use in the greenhouse (is currently registered for use against scale insects, mealybugs, whitefly larvae and other nursery and landscape pests), but based on these results, it looks to be a promising reduced risk insecticide for control for larvae and adult WFT on chrysanthemum cuttings.
FIGURE 5. Mean corrected per cent mortality (± SE) of WFT adults and larvae after immersion in horticultural oil. Means with the same letter are not significantly different.

Efficacious hot water and horticultural oil immersion treatments show potential for inclusion in an IPM program directed towards preventing or reducing WFT on chrysanthemum cuttings. Although these are promising techniques, they are not the silver bullet in the battle against thrips. Total mortality is not achieved when immersion treatments are used and some thrips remain alive. However, the advantage of these techniques is that overall WFT numbers will be reduced so that physical controls (e.g., yellow sticky cards, sticky tape, plant traps) and biological control agents can work more effectively.

Growers must be aware that the results presented apply only to chrysanthemum cuttings and that other plant material could have different responses to these treatments. Additional research needs to be conducted on other plant material to test for phytotoxicity.

As part of the same study, two biopesticides – Beauveria bassiana (fungus) and parasitic nematodes – will also be evaluated for their efficacy against WFT on chrysanthemum cuttings.

In addition, hot water, insecticidal soap, horticultural oil and B. bassiana will be tested for their efficacy in controlling silverleaf whitefly (Bemisia tabaci biotype “B”) on poinsettia cuttings.

Funding for this research was provided by Flowers Canada Ontario (FCO), the Canadian Greenhouse Conference, Plant Products Co. Ltd. and a NSERC Industrial Postgraduate Scholarship to W. Romero sponsored by FCO. Thanks to the growers (SVS; Spring Valley Gardens and Kuyvenhoven Greenhouses) who provided plant material.
• Wendy Romero, University of Guelph, This e-mail address is being protected from spambots. You need JavaScript enabled to view it
• Cynthia Scott-Dupree, U of G, This e-mail address is being protected from spambots. You need JavaScript enabled to view it
• Graeme Murphy, OMAFRA, This e-mail address is being protected from spambots. You need JavaScript enabled to view it
• Theo Blom, U of G, This e-mail address is being protected from spambots. You need JavaScript enabled to view it
• Ron Harris, U of G, This e-mail address is being protected from spambots. You need JavaScript enabled to view it

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