# Investigating Major Infestation Routes of Several Key Thrips Species (Thysanoptera: Thripidae) in Greenhouse-Grown Chrysanthemums in Ontario, Canada

## TL;DR

This study identifies how two types of thrips enter greenhouses in Ontario, showing that western flower thrips come with imported cuttings while onion thrips enter from outside, and highlights the need for accurate monitoring methods.

## Contribution

The study identifies distinct infestation routes for western flower thrips and onion thrips in greenhouses and highlights the limitations of using sticky cards for monitoring.

## Key findings

- Western flower thrips primarily enter greenhouses via imported plant cuttings.
- Onion thrips are likely entering greenhouses from outside sources, as shown by a strong correlation between outdoor and indoor populations.
- Sticky cards poorly reflect actual thrips populations in crops, underrepresenting western flower thrips and overrepresenting eastern flower thrips.

## Abstract

For decades, western flower thrips (WFT) was considered the primary pest of greenhouse flower crops in Ontario, Canada. Recently, onion thrips (OT), typically a pest of outdoor crops, has become an increasing problem inside greenhouses. This study investigated if these two pests enter chrysanthemum greenhouses through different routes, as understanding how pests come in can improve pest management strategies. Our study found that WFT primarily enters from plant material (cuttings) that is imported from outside of Canada. We found that OT likely invades from outside sources, as there was a significant correlation between the number of OT caught on outside trap cards and the number found inside the greenhouse over time. We also found a third species, eastern flower thrips (EFT), was abundant on sticky cards, but few were found in the crop itself. These findings suggest that WFT infestations can be reduced by treating incoming cuttings, while OT infestations could be reduced by blocking their entry from outside using mass trapping cards or thrips-proof screening. Additionally, we found that sticky cards did not accurately reflect thrips populations in the crop itself. This highlights the importance of crop inspections for accurate monitoring and informing pest management decisions.

Western flower thrips (WFT; Frankliniella occidentalis (Pergande)) is a major pest of greenhouse ornamental crops in Ontario, Canada, and recent surveys indicate onion thrips (OT; Thrips tabaci Lindeman) is becoming increasingly prevalent. This study investigates primary routes of entry for WFT and OT in the Niagara region of Ontario. Imported plant material (chrysanthemum cuttings) was sampled from 2016 to 2019 to confirm thrips presence and species identity. Entry from outside was evaluated from June to November in 2019 at three commercial greenhouses that produced chrysanthemums year-round with a mix of vegetative, budding, and flowering plants present. Sticky cards were mounted inside and outside from June to November, and plant taps were conducted bi-weekly to evaluate thrips establishment in the crop. Cuttings consistently harboured thrips, averaging 0.04 thrips per cutting. In 2019, all but one adult specimen collected on cuttings were WFT, confirming this is an important route of entry for WFT. Onion thrips were not found on cuttings but was prevalent on cards both outside and inside greenhouses. More OT was caught on outside cards and a significant correlation between inside and outside cards (Efron’s pseudo-R2 = 0.64) indicates outside populations are a likely source of OT. Additionally, eastern flower thrips (EFT; Frankliniella tritici Fitch) was abundant on cards, but uncommon in the crop. Indoor cards were poor indicators of species composition within the crop, significantly underrepresenting WFT by 71% and overrepresenting EFT by 246%, compared to plant taps. These findings support the use of cutting dips for imported cuttings, in addition to physical control measures to reduce infestation from outdoor populations. This study also highlights the importance of plant inspections for gathering accurate data when making pest management decisions as sticky cards alone may not reliably reflect thrips populations.