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First line of defence

Western Flower Thrips (WFT) (Frankiniella occidentalis (Pergande)) are, without a doubt, a major concern in Ontario floriculture greenhouses (Fig. 1. a and b).

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:

EVALUATING REDUCED RISK METHODS TO CONTROL WFT ON CUTTINGS
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.

ARE THESE METHODS SAFE FOR PLANTS?
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.

HOW GOOD ARE THEY AT KILLING THRIPS?
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.

WHAT IS NEXT?
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, wromero@uoguelph.ca
• Cynthia Scott-Dupree, U of G, cscottdu@uogueph.ca
• Graeme Murphy, OMAFRA, graeme.murphy@ontario.ca
• Theo Blom, U of G, tblom@uoguelph.ca
• Ron Harris, U of G, vwarren@uoguelph.ca