By Silvia Strobl, Master Gardener

Flowers planted in a vegetable garden aren’t just beautiful, nor just an edible addition to your salads, and not even just a pollinator attractant for your zucchini crop. Did you know that specific flowers that attract beneficial insects by providing pollen and nectar resources, known as insectary plants, are one part of the botanical triad preventative pest management approach used by organic farmers that you can apply in your own garden? The other two parts of the triad are the desired crop (vegetable or fruit) and a trap crop that is more attractive to insect pests than the desired crop, hence luring pest populations away. The trap crop once infested with pests is sacrificed.

Field trials have demonstrated that planting both insectary plants and trap crops is more effective at reducing pest populations than using either method alone (i). Researchers are developing guidelines for the most effective triad plant combinations for a localities’ pest and beneficial insects. Additionally, they are testing optimal planting arrangements, for example, planting insectary plants along field edges and inter-planting trap crops with the desired crop.

In Quebec, researchers evaluated 10 different flowering plants for how well they attracted both (i) beneficial natural insects and (ii) typical insect pests. Weekly surveys found that beneficial ladybeetle (Coccinellidae family) captures were twice as high in Marigold (Targetes patula), Nasturium (Tropaeolum majus), Cosmos (Cosmos bipinnatus) and Common Yarrow (Achillea millefollium) as compared to the other six plants in the study. Meanwhile, captures of Tarnished Plant Bugs (Lygus lineolaris) pests were highest in Bee (or Lady) Phacelia (Phacelia tanacetifolia) while flea beetle pests (Chrysomelidae: Alticinae) were highest in Sweet Alyssum, White Charlock (Sinapis alba), and Marigold (ii).

Trap crops can be in the same family as the desired crop, but have to be more attractive to the pest. For example, in one study 10% percent of the Broccoli crop area was interplanted with the Brassica, Pac Choi to effectively lure away flea beetles (iii). Similarly, Blue Hubbard Squash, susceptible to Squash Vine Borer (Melittia cucurbitae), Squash Bug (Anasa tristis), and spotted and striped cucumber beetles (Diabrotica undecimpunctata, Acalymma vittatum), is used by commercial farmers as a trap crop when growing other squash, cucumbers and melons (iv).

Home gardeners can apply the botanical triad by interplanting trap crops and insectary plants in their vegetable gardens.

Trap Crops

Planting a diversity of trap crops is more effective than planting only one trap crop species. The trap crop should cover at least 10% of the desired crop’s planting area to be effective, and must be destroyed to kill the pest before it moves to the desired crop (iii). A few trap crops that have been found to reduce insect pests on common vegetable crops follow.

Insectary Plants

Flowering annuals like Marigold, Nasturtium, Cosmos, Bee Phacelia and Sweet Alyssum can be started indoors before transplanting the seedlings into vegetable gardens after the frost-free date. These will flower within a month and be able to feed beneficial insects, when pest populations peak and right into late fall. Planting a succession of native flowering plants near the vegetable garden can also provide habitat for beneficial insects as well as support pollinators.

These annuals can be purchased in garden centres, but are often only available as varieties with double flowers that either produce less nectar (because energy is put into petal production) or block pollinator access to nectar and pollen (vii). Choose single or heirloom varieties to maximize pollen and nectar for beneficial insects. It is also more economical to start your own plants, because you can be certain they haven’t been treated with synthetic pesticides. After all you want to attract beneficial insects that will in turn prey on or parasitize insect pests on your vegetables!

As you plan your 2025 vegetable garden, consider whether the botanical triad could help prevent or minimize any pest problems you may have experienced in the past.

References

i. Shesthra, B., D.L. Finke, and J.C. Piñero. 2019. The ‘Botanical Triad’: The Presence of Insectary Plants Enhances Natural Enemy Abundance on Trap Crop Plants in an Organic Cabbage Agro-Ecosystem. Insects: June10(6):181 https://www.mdpi.com/2075-4450/10/6/181

ii. Boisclair, J. E. LeFrancois, M. LeBlanc, M. Grenier, M. Lefebvre, & G. Richard. 2014. Beneficial and pest insects associated with ten flowering plant species grown in Québec, Canada https://orgprints.org/id/eprint/24006/1/24006-OWC2014-Boisclair-Canada-Flowering%20strips_MM.pdf

iii. Parker, J. & W. Snyder. 2014. Diversity by Design: Using Trap Crops to Control the Crucifer Flea Beetle. https://eorganic.info/sites/eorganic.info/files/u461/trap%20cropping%20flea%20beetle%20webinar_Final.pdf

iv. University of Missouri. 2017. Trap cropping: A simple, effective, and affordable Integrated Pest Management strategy to control squash bugs and squash vine borers. https://ipm.missouri.edu/MEG/2017/3/Trap_cropping/

v. Parker, J.E., W.E. Snyder, G. C. Hamilton and C. Rodriguez‐Saona. 2013. Companion Planting and Insect Pest Control. Chapter 1 in Weed and Pest Control – Conventional and New Challenges, Pages 1-30. http://dx.doi.org/10.5772/55044

vi. Hoidal, N. 2021. Companion planting in home gardens. University of Minnesota Extension. https://extension.umn.edu/planting-and-growing-guides/companion-planting-home-gardens

vii. Silva, E. 2021. Less showy flower petals mean more pollinators. Oregon State University Extension Service. https://extension.oregonstate.edu/gardening/pollinators/less-showy-flower-petals-mean-more-pollinators

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