Marigold flowers emit limonene, which protects tomatoes by repelling whiteflies.

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Monoculture, the practice of planting only one crop, is commonly used in the agriculture industry. In monocultures, there is no biodiversity. This is an issue because a monoculture crop provides an abundant supply of food to a pest that favors it, and the pest can easily reproduce and travel between plants. In order to suppress pests, agriculturalists often use pesticides that damage the environment. Man-made pesticides are typically chemicals that kill other plants or pests along with beneficial organisms, such as bees and soil microbes. 

Healthy ecosystems usually have a large variety of species. An area that consists of many plant, animal, and microbe species is an area with high biodiversity. Species living in that area can provide important services to each other. Pest control is an example of a service a plant can provide to another plant. In agriculture, companion planting, or planting at least two crops in the same field, can be used as a way to help control pests. 

An example of companion planting is planting  tomatoes with marigolds. In a field where only tomatoes are grown, whiteflies are a common pest that can destroy the crop. Long ago farmers solved this problem by planting marigolds next to tomato plants. They believed that doing so protected the tomato plants from whiteflies. Recently, scientists confirmed that whitefly populations were smaller when tomatoes were planted with marigold flowers. They’ve identified that limonene, a chemical substance found in marigolds, is the substance that repels whiteflies. Limonene belongs to a group of chemical substances called volatile plant compounds (VPCs). VPCs are emitted by plants to send signals to other plants or to attract or repel insects. An insect’s antennae or other sensory organs detect the VPCs, just as a human nose detects many chemical compounds. VPCs can be isolated from a plant. The isolated substances can then be sprayed or emitted to repel pests. However, in the case of tomatoes, planting marigolds directly can provide more benefits than using the isolated limonene VPC. It also increases biodiversity and provides nectar for bees.

In nature, distinct VPCs are emitted by different plants. Together, these plants can repel different insect species and provide a safe method for pest control. This suggests that a careful mix of plantings could repel all sorts of pests. Scientists have much more to learn from natural systems regarding methods of pest control. Those lessons should guide scientists towards effective and safe pest control and could reduce or eliminate the need for man-made pesticides.

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“Our work indicates that companion planting short vine tomatoes with French marigolds throughout the growing season will slow development of whitefly populations. Introducing marigolds to unprotected tomatoes after significant whitefly build- up will be less effective. The use of limonene dispensers placed near to tomato plants also shows promise. It is argued that this work supports the possibility of the development of a mixture of tomato companion plants that infer ‘associational resistance’ against many major invertebrate pests of tomato. Such a mixture, if comprising edible or ornamental plants, would be economically viable, would reduce the need for additional chemical and biological control, and, if used outdoors, would generate plant-diverse agro-ecosystems that are better able to harbour invertebrate wildlife.” (Conboy et al. 2019: 1-2)

“This indicates that repellent volatile organic compounds [aka volatile organic compounds from plant origin, or VPCs]  from marigold are the probable cause of the reduction in whitefly performance on tomato intercropped with marigold (Fig 1).” (Conboy et al. 2019: 5)

Journal article
Companion planting with French marigolds protects tomato plants from glasshouse whiteflies through the emission of airborne limonenePLOS ONEMarch 1, 2019
Niall J. A. Conboy, Thomas McDaniel, Adam Ormerod, David George, Angharad M. R. Gatehouse, Ellie Wharton, Paul Donohoe, Rhiannon Curtis, Colin R. Tosh

“Manipulating the odorscapes of herbivorous pest species has already important practical implications in plant protection as an alternative to pesticides… Modifying the odorscape by introducing other plant species that naturally release different VPCs can also reduce the damage caused by pest insects.” (Conchou et al. 2019: 13-14) 

Journal article
Insect Odorscapes: From Plant Volatiles to Natural Olfactory ScenesFrontiers in PhysiologyAugust 2, 2019
Lucie Conchou, Philippe Lucas, Camille Meslin, Magali Proffit, Michael Staudt and Michel Renou

“Our findings indicate that plants may contribute to the suppression

of herbivore populations, not only through low average quality but also through heterogeneity in nutrient levels… A key implication is that agroecosystems may experience outbreaks of herbivores because herbivore performance is increased by artificially low plant heterogeneity owing to landscape simplification, reduced plant species diversity and crops that are bred to minimize variation. Increasing heterogeneity in plant nutrients in agroecosystems may be a key step towards the sustainable control of insect pests. Plant nutrient heterogeneity could be increased by planting greater numbers of crop varieties21, by increasing genetic diversity within crop varieties, or by breeding varieties with increased constitutive or induced nutrient variance within the parts of the plant that are attacked by insect pests.” (Wetzel et al. 2016: 427)

Journal article
Variability in plant nutrients reduces insect herbivore performanceNatureOctober 12, 2016
William C. Wetzel, Heather M. Kharouba, Moria Robinson, Marcel Holyoak & Richard Karban

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