In a perfect world, pests are controlled by farm ecosystems, not the farmers. However, farmers play a pivotal role in setting the scene for the drama to unfold. They can provide habitat for “biocontrol allies,” which are enemies of pests. Farmers can also ensure crops receive a balanced supply of nutrients to help plants resist pest damage. A whole cast of players – from microorganisms to spiders to birds – is involved.
Biological pest control resembles a Shakespearean tragedy more than a simple food chain. In nature and on farms, food webs are much more complex, sometimes even circular. For example, beetles eat mites, but mites eat beetle larvae. Intricate interactions between pests and their enemies have been studied by Canadian researchers since 2009 as part of the Organic Science Cluster (OSC). In all three OSCs, scientists have explored how farmers can create habitat to support beneficial insects including predators of pests, use trap crops to pull pests away from crops, and release biological controls of crop pests.
HABITAT MANIPULATION (IF YOU BUILD IT, THEY WILL COME)
Organic farms tend to have a greater abundance and diversity of biocontrol agents than non-organic farms.1 To keep a strong and stable community of such allies, farmers can provide ‘SNAP’ – shelter, nectar, alternative prey and pollen by planting flowering strips (insectary strips) or leaving wild areas.
Wild areas and flowering strips can provide habitat for pollinators, which can lead to higher crop yields. Also, insectary strips and wild areas fulfill the new biodiversity requirement of the 2020 Canadian Organic Standards to take “measures to promote and protect ecosystem health on the operation.”2
OSC3, Activity 28: Flowers for beneficials
Dr. Jason Gibbs (University of Manitoba) and his team have been exploring the benefits of providing flowering habitats on field margins to support beneficial organisms. He found more than three times the number of ground beetles in flowering strips compared to grassy edges of control fields. Ground beetles consume many pests, including aphids, mites, beetle larvae, slugs and even weed seeds.
It can be challenging, however, to establish flowering strips and maintain the desired species composition. Perennials need to survive the winters and spread. Annuals must self-seed. Aggressive plants, or undesirable plants such as weeds should not be allowed to dominate the plant community.
THE CAST
Biocontrol (biological control) agents – living organisms that are introduced, supported or encouraged by humans with the intent to harm pests of crops. Biocontrol agents are sometimes called “enemies” because they attack pests, but they are really the allies of farmers. Biocontrol agents include:
• predators – eat the pests;
• parasitoids – lay eggs in pests and the larvae eat pests;
• parasites – feed on living pests;
• pathogens – cause disease in pests.
Beneficial organisms – organisms that help crops, including pollinators, symbiotic soil life and biocontrol agents.
Pests – this article focuses on invertebrates that damage crops, but a pest is defined in the Canadian Organic Standard as an “organism causing damage to humans or to resources used by humans, such as certain viruses, bacteria, fungi, weeds, parasites, arthropods and rodents.” This ranges from fungi that inhibit germination, to deer that eat crops, to mice that consume organic products in storage.
“We’ve been reseeding to help keep these going, which might be practical if you’re using purely domesticated plants with relatively cheap seeds,” says Gibbs. Native perennials “hopefully persist and require less input over time. My main takeaway with perennial plantings is to do it very deliberately and take your time. If I was a grower, I’d probably spend a couple of years just preparing the site to make sure it’s weed free.”
OSC1, Activity D1: Flowers for bicontrol agents
Dr. Josée Boisclair (Institut de Recherche et de Développement en agroenvironement) studied the link between flowering strips, pests and their predators. Her team found the most ladybird beetles (ladybugs) in marigolds, nasturtiums, cosmos and yarrow (in that order) compared to coriander, sweet alyssum, alfalfa, petunia, phacelia and white mustard. Ladybird beetles voraciously consume soft-bodied pests, including aphids, mealybugs, whiteflies and thrips.
However, the flowers also attracted pests, including tarnished plant bug in phacelia and flea beetles in sweet alyssum, white mustard and nasturtiums. “The use of flowering strips to encourage natural enemies around their crops will surely increase biodiversity on their farms but the actual impact of this practice on insect pest control is yet not well understood,” Boisclair concluded. “As an example, certain plant species have been shown to favour the presence and activity of natural enemies, such as predators and parasitoids. However, consideration should be taken to make sure that these same plant species do not become a reservoir for insect pests.”
TRAP CROPS
If a certain plant attracts pests, farmers can use this to their advantage. So called “trap crops” divert pest pressure from crops. They also concentrate the pest population. Farmers can then destroy the pests by cutting or tilling the trap crop or applying a botanical pesticide.
OSC3, Activity 12: Trap crops and baby greens
To reduce flea beetle damage in organic baby greens, Dr. Annabelle Firlej (Institut de Recherche et de Développement en agroenvironement) planted trap crops of flowers and a mix of amaranth, rapeseed and mustard. Preliminary results show these flowering plants attract flea beetles more than the other plants and (most importantly) more than the spinach. The more diverse the trap crop, the more pests it attracted.
In weedy or dry conditions, however, the trap crop struggled to get established. Even when the trap crop was robust, it was less effective in weedy conditions because the pests preferred certain weeds over the trap crop. The researchers conclude it is important to have good weed management and irrigation to use trap crops to control pests.
OSC3, Activity 18: Trap crops for strawberries
OSC researchers are exploring various ways to reduce tarnished plant bug (Lygus) damage on strawberries. Lygus is a significant pest in many fruits and vegetables and can cause stunted and deformed strawberries. Lygus damage is a serious barrier to transition to organic strawberry production.
Non-organic producers use broad-spectrum insecticides to control Lygus. However, as Dr. Caroline Provost (Centre de recherche agroalimentaire de Mirabel) explains, these insecticides also kill the predatory mites that naturally control levels of other pests. “Killing predatory mites doubles the number of pests. This is a vicious circle that shows that insecticide is really not a preferred option.”
TIPS FOR PROVIDING HABITAT FOR BIOCONTROL AGENTS:
- Be on the edge. The greatest diversity and abundance of beneficials is found on field borders where cultivated and wild areas meet.
- Small is beautiful. The smaller the fields, the more “fringe benefits” (above) in terms of greater species richness and numbers of insects that attack pests. Farmers can reduce the distance beneficials need to travel from wild areas to crops.
- Plant flowers. Flowers provide parasitoids and predators of pests with food (pollen, nectar and prey). The ideal flowering strip has different types of flowers blooming throughout the growing season.
Dr. François Dumont (Centre de recherche agroalimentaire de Mirabel) “observed that buckwheat, mustard and canola are good summer trap crops, whereas mullein and sunflower are good autumn hosts. We suggest using both summer and autumn trap crops to act on Lygus population for an extended period of time.”
Certain trap crops, such as mullein, are also hosts for predators of Lygus, such as damsel bugs. These trap crops also provide habitat for its predators. Damsel bugs appear to be more effective at controlling Lygus in the trap crops compared to the strawberry plants. The trap crops draw the pests away from the strawberries – once lured into the trap crops, Lygus is attacked by damsel bugs.
RELEASING BIOCONTROL AGENTS
OSC researchers have conducted field releases of biocontrol agents, including in the strawberry-Lygus study.
OSC3 Activity 18: Predation in strawberry fields
To control the tarnished plant bug (Lygus) in strawberry fields, Drs. Provost and Dumont released two predators: damsel bugs and minute pirate bugs. After the releases, the population of Lygus dropped for two weeks.
Both predators eat a variety of species, including other pests. The damsel bug attacks aphids and the minute pirate bug feeds on thrips, spider mites, whiteflies. Their general diet allows the predators to survive even when Lygus isn’t abundant.
Even though these predators are common on organic farms, the researchers conclude that a series of well-timed releases is the most effective way to control Lygus. The damsel bugs attack adult and larval Lygus. Pirate bugs consume eggs and small nymphs. The scientists also used the damsel bug to successfully control Lygus on greenhouse cucumbers.
In organic strawberry fields, the researchers are developing strategies to defeat Lygus. Dumont concludes, “We can envisage building an environment favourable to predators, estimating that these predators will kill 85% of the pest population, and that the remaining 15% will cause damage at a low level that can be controlled.”
MICROBIAL INTRODUCTIONS
OSC2 Activity C30: Viral pesticides
While Dumont compared the attack of a damsel bug on Lygus to a “tiger attacking a sheep,” Dr. Deborah Henderson (Kwantlen Polytechnic University) is investigating a less dramatic form of biological control using microorganisms to control caterpillar pests in brassicas (cabbage, broccoli, kale, etc.).
Currently, many organic growers use Bacillus thuringiensis (Bt), a commercial product containing soil bacteria, to control the diamondback moth, cabbage looper and imported cabbageworm. Unfortunately, many pests have developed resistance to Bt.
Applications of certain viruses and fungi (e.g., Beauveria bassiana) that naturally affect caterpillars can control caterpillar pests, as revealed by Henderson’s research.
OSC3 Activity 21: Wireworms and Metarhizium
Dr. Todd Kabaluk (Agriculture and Agri- Food Canada) studies wireworms (click beetle larvae). These pests feed on the roots and tubers of crops including cereals, carrots, potatoes, lettuce and peas leading to death of seedlings in the spring, and damage to root crops in the fall affecting quality and storability.
Kabaluk found a strain of a soil fungus that kills wireworms. He cultured the strain, Metarhizium brunneum LRC112, and tested it on organic farms.
Amara Farm, a certified organic vegetable and fruit farm on Vancouver Island, has “incredibly high wireworm pressure” that makes it difficult to grow many crops, even head lettuce. According to the farmer, Arzeena Hamir, applications of Metarhizium led to greater survival of lettuce and bok choi and much less damage in Japanese turnips.
“We actually had damage-free potatoes,” Hamir says. Before, wireworm damage was so bad that they couldn’t harvest a decent crop of potatoes.
When asked if they would continue to use Metarhizium, Hamir replies “Absolutely!!! This treatment has given us a lot of hope that we can now include these crops in our farm plan.”
Kabaluk is fine-tuning the application with an “attract and kill technique.” He developed granules of pheromones, chemicals that attract the adult click beetles, and combined these with M. brunneum LRC112. This disrupts mating behaviour and may lead to greater uptake of the lethal fungus.
CONCLUSION
Based on the OSC research findings, the key to successful biological control of pests appears to be a multi-pronged approach. If providing habitat through insectary strips isn’t sufficient, consider introducing organisms that attack pests. If that isn’t enough, consider pheromone disruption or plant-based pesticides.
“The integration of biological control as a preventative approach should be a priority in order to minimize pest problems,” Josée Boisclair concludes. “A better knowledge of the impact of landscape on natural enemies’ abundance and activity on farms is needed to improve the impact of conservation biological control.”
For more information about the research projects mentioned in this article, see www.dal.ca/oacc/OSC.
SOURCE
- Lichtenberg et al., 2017. A global synthesis of the effects of diversified farming systems on arthropod diversity within fields and across agricultural landscapes. https://doi.org/10.1111/gcb.13714
- Organic production systems: General principles and management standards CAN/CGSB-32.310-2020. Sub- clause 5.2.4. https://www.inspection.gc.ca/organic-products/standards/eng/1300368619837/1300368673172
(OSC3) Originally published in Organic Science Canada magazine, Issue #3, Spring 2021
