The art and science of adding plants to reduce pests
Companion planting is the practice of growing a diverse set of plants together for synergistic benefits.
Perhaps the most well-known example is the practice of growing the “three sisters” —corn, squash and beans. The relationships between these three plants are easy to visualize and appreciate. We can see that the towering corn plants provide a natural trellis for the beans to climb. We know that beans and their bacterial partners supply the nitrogen that the corn and squash require, and we can see the lush vegetation of the squash plants outcompete weeds for the benefit of all.
To understand how companion planting can be used to manage pests, we must first understand how insects interact with plants in the environment.
Insects rely on different cues to locate their host plants. Airborne chemical cues, called “volatiles,” play a significant role, especially at a distance. They are readily produced by plants and serve different ecological functions. They can aid in plant-to-plant communication, attract pollinators, be used to deter herbivores and attract natural enemies. And yes, insect pests can use volatiles to locate their hosts!
Visual cues also factor in. Signals like the shape of a plant’s leaves, the color of its flowers or its size help a pest find the perfect place to feed or lay its eggs. With this knowledge, we can use different planting arrangements to protect our cash crops from pests.
Anti-pest Companion Crop Systems
Masking is a way to “hide” the cash crop by growing companion plants that physically or chemically obscure it. As a result, the pest has a difficult time orienting to the crop. This is also where the “inappropriate landings theory” comes into play. Research shows that some pests require several consecutive landings on the host plant before they will lay their eggs. If, in a diverse planting of crops, the insect makes an “inappropriate” landing on a non-host plant, they will not achieve that optimal number of consecutive landings and will fly off to look for another plant for egg laying.
Fragrant herbs like sage, dill, hyssop and German chamomile have been shown to effectively mask cole crops from cabbage butterflies; planting basil alongside tomatoes reduces egg laying by hornworm adult moths; and marigolds interplanted with either onions or cole crops have been found to reduce egg laying by onion root maggot and cabbage root flies.
Trap cropping is a practice in which a more attractive crop or mixture of crops is used to draw a pest away from your cash crop. With the pest concentrated on your trap crop, you can now focus management on a more targeted area. That management may be a targeted pesticide application, timely crop destruction, hand removal, vacuuming, etc. The design of a trap crop system requires knowledge of the pests’ biology and behavior. Is the pest highly mobile? That is, how likely are they to travel a distance to reach the trap crop?
Another important factor includes the timing of the trap crop planting. This should occur a few weeks ahead of the cash crop planting so that pests are immediately lured to the trap crop. This may impact whether you choose to plant your trap crop on the perimeter, at the ends of beds or in strips throughout the field.
A well-known trap crop in vegetable systems is Blue Hubbard squash. When planted along the perimeter of squash fields it effectively lures squash vine borer and squash bugs away from the cash crop. Other examples include the use of hot cherry peppers to pull pepper maggots away from bell pepper plantings, various mustard varieties to protect other cole crops from flea beetles, and strips of alfalfa to protect strawberries from tarnished plant bug damage.
Push-pull systems take trap cropping one step further by not only incorporating plants that are more attractive to “pull” a pest away from the crop, but also integrating plants that “push” or deter the pest from the cash crop.
The push-pull system was developed by researchers in Kenya to help small-acreage farmers manage stemborers and a weedy pest called Striga in their maize and sorghum plantings. Desmodium, the “push” plant, not only emits volatiles that repel stemborer moths but also controls the Striga through a combination of competition for resources and allelopathy. Napier grass, the “pull crop,” is a more attractive plant for stemborer moths to lay their eggs on. As a bonus, when the larvae hatch and begin feeding, the grass produces a sticky sap that causes the larvae to get stuck and perish. It’s a win all-around!
In vegetable systems, recent research shows success in using marigolds as a “push” and mustard greens as a “pull” to manage various pests of kale.
Attracting Natural Enemies
The enemy of my enemy is my friend! We can attract insect predators and parasitoids by incorporating beneficial habitat into our farmscape. Beneficial habitats offer natural enemies protection from predators or harsh environmental conditions, sites for reproduction and development, shelter for overwintering, and alternative food sources like pollen and nectar. Including insectary strips in your fields, intercropping with cover crops, incorporating both annual and perennial species into the landscape or planting hedgerows are ways you can improve natural, biological control on your farm.
To have the most success with companion planting for pest management, it is important to consider the biology and behavior of the pest you are trying to manage, how companion plants will affect your overall crop rotation and whether your companion plants will compete with your cash crops for resources like light, water and nutrients.
Caitlin Tucker is an Organic Consultant with Rodale Institute based out of Buffalo, New York. Please visit rodaleinstitute.org/consulting for more information.
