The hot, humid climate in the Southeast lends itself to nearly year-round insect, weed and disease pressure, and growing is especially tough if you’re an organic farmer.
Chemical inputs are limited in organic production, requiring more emphasis on managing site-specific conditions by incorporating cultural, biological and mechanical practices. What’s more, the body of science-based research and support for organic growers in the state is limited, leaving growers fewer resources to draw from in organic systems.
To address these constraints, Assistant Professor Carmen Blubaugh and her team have implemented a series of research trials at the Durham Horticulture Research Farm in Watkinsville, Georgia, as well as on-farm studies with a network of more than 40 organic growers throughout the state. The team is looking at various components of organic management that could lead to establishing practical control methods for organic farmers in the Southeast.
“Much of our work examines how agricultural biodiversity influences pest and weed control services,” Blubaugh explained. “We want to provide strategies for organic farmers to leverage biodiversity and natural processes to nourish and protect their crops profitably while reducing the need for costly external inputs that simply meet the nutritional needs of crops in the short-term.”
Establishing beneficial insect refuge
Master’s student Allison Stawara, part of Blubaugh’s team, is looking at the benefit of incorporating rows of living mulch between organically managed vegetable crops. Many of the available pesticides in both conventional and organic production have broad spectrum coverage that negatively impacts beneficial insects, so the living mulch will serve as a refuge for them during pesticide applications. Sometimes referred to as cover crops or green manure, living mulch provides the added benefit of building soil's organic matter over time, as growers are mechanically incorporating plant materials into the soil once cash crops are removed.
From an applied research perspective, Stawara hopes to demonstrate that by creating refuge for beneficial insects, organic farmers will capitalize on the ecosystem services they provide by preying upon unwanted insect pests.
Blubaugh is concurrently applying the same practice throughout the state on organically managed farms. She’ll collect data over the growing season to compare results from her on-farm trials with Stawara’s research findings to see whether the living mulch reduces mortality rates of beneficial insects.
“We’re hoping to see the living mulches buffer natural enemy populations from collateral damage from pesticides, and enable them to recolonize crops, naturally suppress pests and reduce the need for future pesticide applications. Pesticides are never 100% in their efficacy, and if we can promote more diverse shelter, alternative prey and floral resources for beneficial and predatory insects, we will hopefully see additional pest control from those insects that have the resources they need to stay in crop fields for a longer period of time,” said Stawara.
Understanding insect defense mechanisms
Meanwhile, Christiana Huss and Jon Golan, also part of Blubaugh's lab, are attempting to fill a gap in the scientific literature on an increasingly damaging insect pest, the yellow margined leaf beetle (YMLB). The invasive beetle from South America has recently become a major insect pest of brassica crops in organic production.
“Surprisingly, not much is known about the yellow margined leaf beetle,” Huss said. “They’re not always listed on Extension bulletins as pests of brassicas, perhaps because they look so much like flea beetles and the damage to host plants is similar as well,” she explained. “It may also be that because they are so well managed in conventional systems with traditional pesticides, they haven’t garnered a lot of attention. But for organic growers, they are a major problem for which there are limited options for managing.”
Huss and Golan are looking to identify the defense mechanisms that yellow margined leaf beetle uses to ward off predatory beneficial insects. Many pests of the brassica family sequester a potent family of compounds found in the plant tissues of all Brassicaceae, called glucosinolates. As these pests consume the plant material throughout their life cycle, the concentration of glucosinolates increase and are used as a defense mechanism against insects that would otherwise eat them, such as spined soldier bugs, lacewings and lady beetles.
Huss and Golan are examining the possibility that yellow margined leaf beetles use a similar mechanism to defend themselves against predators by conducting a series of behavior experiments and analyzing glucosinolate concentrations in insect and plant tissues. To identify potential predators of the pest, they’re currently collecting beneficial predators and analyzing their gut material to learn whether yellow margined leaf beetle DNA is inside.
“Identifying the mechanism that yellow margined leaf beetles use for predator defense will help us better understand why invasive insects can become successful pests. I will also be conducting a field experiment in spring 2022 to test the use of intercrops, trap crops and habitat manipulation for control of the pest,” Huss said of her future work.
Gauging the impact of excessive nitrogen
Another aspect of organic farming is the priority given to fertility management. Master’s student Julia Berliner is studying the ecological consequences of surplus fertility, particularly nitrogen in the form of organic fertilizer.
“Most of the available organic fertilizers typically have nutrient ratios that are less than ideal, so farmers often need to over-apply nitrogen to meet crops’ other nutrient requirements,” said Berliner.
It is well understood that the excessive use of fertilizers is the leading cause of eutrophication — an excessive richness of nutrients in a lake or other body of water — as surplus nitrogen leaches out of the soil, potentially polluting waterways or causing harmful algae blooms, she explained. However, less is understood about how surplus nutrients affect agroecosystems, particularly how they affect the beneficial predators of insect pests.
“There are all kinds of crazy complicated ways that nutrient availability in the soil can cascade through insect food webs, affecting nutrient concentrations in plants and insect herbivores that can influence the effectiveness of biological control by natural enemies,” Berliner explained.
Plants that receive an abundance of nitrogen respond by putting out excessive growth, which results in increased herbivore damage from insect pests. Plants that receive too little nitrogen will invest more in plant defense mechanisms from herbivores. They do this by detecting chemicals contained in the saliva of chewing insects and respond by releasing volatile organic compounds that both help repel certain insect herbivores and attract specialized beneficial predators that prey on the insect pests.
“I hope that my work will not only expand our knowledge of basic ecology, but also aid farmers’ fertility decision-making in order to maximize biological control and yield and minimize time and money spent on fertilizing and weeding,” Berliner said.
You can follow the work in Blubaugh's lab by visiting the Insect Ecology in Sustainable Agroecosystems website, where you’ll also find her publications and a list of farmer collaborators.