Thrips are deadly for ornamentals and other crops, causing substantial economic loss by damaging flowers and leaves or transmitting plant viruses, and most chemical insecticides have proven to be ineffective at controlling thrips. 

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 suppression tactics to manage this persistent pest.

"It's a Brave New World these days for pest management! Our results with the marigold system are very encouraging and show how easy and effective they can be," said UVM Research Professor and Extension Entomologist, Margaret Skinner. "They offer new opportunities for growers to combat their pests proactively without chemical pesticides." 

Plant-mediated IPM systems can improve early pest detection, attract pests from the crop where they can be targeted with a treatment or removed entirely and/or provide habitat, food and shelter for biological control agents. ‘Guardian plants’ combine all of these functions together into one multi-faceted system.

Researchers had great success using marigolds as a guardian plant when testing the system in commercial greenhouses in Vermont and New Hampshire. They released predatory mites on a flowering marigold, mixed an insect-killing fungus into the upper surface of the potting mix and positioned a thrips pheromone lure in the foliage. Adult thrips were attracted out of the crop to the marigolds.

The combination of predatory mites, the 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. 

Growers who adopt this type of 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.