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Guardian plant trials

March 11, 2014  By Suzanna McCloskey

The American Floral Endowment (AFE) is still up to murder, sex and greed
(the association’s current fundraising theme!), this time with a new
research report.

The American Floral Endowment (AFE) is still up to murder, sex and greed (the association’s current fundraising theme!), this time with a new research report.

Marigolds are ideal guardian  
Marigolds are ideal guardian plants in thrips control programs. PHOTOS COURTESY AFE



In the study, scientists from the Entomology Research Laboratory at the University of Vermont (UVM) describe how they lured and then murdered western flower thrips through an integrated non-chemical pesticide system that is easy to use.

Thrips are deadly for ornamentals, causing substantial economic loss by damaging flowers and leaves or transmitting plant viruses, and most chemical pesticides have proven to be ineffective at controlling thrips. Many pesticides need to come in contact with the pest to be effective, but thrips evade impending doom by pupating in the soil, increasing management difficulties.

“Report #216: Formulations of Insect-Killing Fungi in Combination with Plant-Mediated Integrated Pest Management (IPM) Systems for Thrips” describes an innovative strategy that uses marigolds in combination with other biological control agents to manage this persistent pest.


“It’s a brave new world these days for pest management,” notes UVM research professor and extension entomologist Margaret Skinner. “Our results with the marigold system are very encouraging and show how easy and effective they can be.

“They offer new opportunities for growers to combat their pests proactively without chemical pesticides.”

There are several types of plant-mediated IPM systems that serve different purposes.

For example, “indicator plants” improve early pest detection; “trap plants” attract pests from the crop where they can be targeted with a treatment or removed entirely; and “banker and habitat plants” provide sites with hosts and shelter to produce biological control agents.


“Guardian plants” combine all of these functions together into one multi-faceted system.

A thrips lure on a sticky card  
A thrips lure on a sticky card. PHOTOS COURTESY AFE


UVM researchers had great success using marigolds as a guardian plant when testing the system over the past two years at six commercial greenhouse sites across Vermont and New Hampshire.

They released predatory mites (Amblyseius cucumeris) on a flowering marigold, mixed a granular formulation of an insect-killing fungus into the upper surface of the potting mix, and positioned a thrips pheromone lure in the foliage.

Researchers found that a high percentage of adult thrips were attracted out of the crop to the marigolds, where their populations were maintained at manageable levels.

Immature thrips are prey for the predatory mites on the flowers and foliage. If there are no thrips, the mites feed on pollen produced by the marigold.

Thrips that escape predation by the mites drop to the soil to pupate, where they become infected with the insect-killing fungus. The fungus is a granular formation, which enables it to colonize the potting mix, effectively eliminating the need for reapplication.


The combination of the predatory mites, the infectious fungus in the potting mix and the lure yielded promising results, maintaining thrips populations at low levels for up to 12 weeks, while untreated marigolds suffered significantly more damage.

There were notably more thrips on the guardian plants than on the adjacent crop plants, demonstrating the attractiveness of marigolds and the potential of guardian plants to protect the crop.

Using this guardian plant system yields plants of higher quality and value using few or no pesticides, which can make for a positive marketing plan.

Growers who adopt this type of chemical pesticide-free management approach could benefit from promoting their efforts to use “green” management tactics, as consumers are becoming more aware of concerns surrounding exposure to chemical insecticides.

One of the growers participating in this project even received an environmental award recently for his efforts to reduce pesticide use.


The third and final year of these trials will begin this spring. “Our test results so far show all the signs that this system can work effectively under real world greenhouse conditions,” Skinner said.

Another year of trialling is planned.  
Another year of trialling is planned.


“One more year of testing will further demonstrate its effectiveness. There are still many questions to be answered, but this work should pave the way for a bright future for marigold guardian plants.”

Much more information is available in the full report on AFE’s website, along with more than 150 additional research reports on a variety of topics.

Suzanna McCloskey is a communications specialist with American Floral Endowment.

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