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Cultivating with Natural Predators Gets Farmers Off the Pesticide Treadmill, According to Study
(Beyond Pesticides, July 12, 2023) A study by University of Delaware entomologist Thabu Mugala and colleagues finds that modifications to their farming methods can reduce slug damage when those changes also encourage natural slug predators, allowing farmers to avoid the endless cycle of pesticide dependency, pest resistance, genetically engineered crops, and synthetic fertilizers. With insects as the target for tens of millions of pounds of agricultural use, growers of the highest-production crops in the U.S., corn and soybeans, continue to find slugs to be a serious problem. Corn and soybean growers who have adopted no-till or conservation tillage and cover crops often think these practices worsen the problem by increasing moisture and decaying plant material in fields, which slugs love. But the cause-and-effect picture is more nuanced and requires strategies that nurture ecological balance.
Slugs are the most damaging non-arthropod pest in no-till corn production in the U.S., and truly effective chemical deterrents do not exist at agricultural scale, as Beyond Pesticides noted here, although biological methods may be on the horizon, such as a parasitic nematode already used in Europe that shows promise. The most voracious natural slug hunters are ground beetles, but harvestmen (daddy longlegs), and wolf spiders also eat them.
The Mugala study, “Ground beetles suppress slugs in corn and soybean under conservation agriculture,†investigates 41 fields in Mid-Atlantic states through two growing seasons. The researchers looked at the interactions among cover cropping, tillage, pre-plant insecticide applications, weather, and natural enemies on slug populations and activity.
Slugs are mollusks and generalists, making use of both living and decaying plants, and cool, wet weather often triggers a slug outbreak. Farmers dealing with slugs know that tillage disrupts the soil microclimates that slugs like, and are tempted to use it, especially because, once started, a slug outbreak is difficult to suppress by chemical means. The available chemicals are expensive, do not work well in damp environments, and kill wildlife, according to Mugala et al., who also observe that while “there is no commercially available biological control agent for slugs in North America, there is an array of native and exotic predatory and parasitic natural enemies of slugs present.†Many of the pesticides used against insects, including neonicotinoid seed treatments, also kill these other beneficial arthropods, as well as other soil invertebrates important to cycling nutrients naturally.
Some slug baits are also problematic. Many contain metaldehyde, which as Beyond Pesticides reported in March, hampers the growth of vegetables and is quite toxic to many animals. Other anti-slug weapons may be difficult to use on field scales, such as bread-dough or beer bait. One Lithuanian study found that invasive Spanish slugs would not eat a lethal dose of either metaldehyde or iron phosphate pellets, and about 17 percent of the pellets were removed nightly from the study area by earthworms.
Less toxic regenerative methods may help slugs, but they also help their predators, and some tweaks to tillage and cover cropping may discourage slugs while encouraging their enemies. Mugala et al. report that the timing of cover crop removal affects slugs’ depredations—doing it too soon before planting gives slugs a leg up, so to speak. While a 2022 study of chemical-intensive corn production found reduced need for slug bait with the use of row cleaners to remove plant debris in seed rows and the application of nitrogen fertilizers at night, this approach ignores the value of natural predators and ecosystem services (see more).
Adding to the uncertainty about the best way to deal with slugs, some of the research data can appear contradictory; a 2013 survey of Shenandoah Valley farmers found that 13 percent of no-till fields planted with corn and soybeans showed slug damage, while only 1 percent was reported for conventionally-farmed fields. But another study found that farmers who always used insecticide at planting reported the most slug damage, independent of their tilling practices. This may be because their arthropod predators suffer sharp declines in fields applied with pesticides and where seeds have been treated with neonicotinoids. Farmers may be blaming regenerative methods for damage that is actually caused by pesticides.
The pesticide industry has long tried to monkey-wrench agricultural independence; Monsanto introduced Roundup-Ready soybeans in 1996 and claimed genetically modified seeds would enable sustainable (and now regenerative) agriculture by eliminating the need for tillage. Unfortunately (but inevitably) the target weeds became resistant to Roundup, and many farmers returned to tillage and even stronger chemicals. The first insect resistance to a pesticide (sulfur-lime) was noted in 1914. With each iteration of this Darwinian process, the industry’s response is to develop a variant of the failed pesticide rather than developing ecologically-based pest management and abandoning chemical-intensive agricultural practices that ignore the ecosystem in which they operate.
Where once the industry touted the Green Revolution and the utter dependence of agriculture on its products to feed the world, now it is trying to convince people that it is on the sustainability bandwagon, all the while continuing to market its non-regenerative products. In Syngenta’s words, “Although the green revolution has been successful in feeding a rapidly growing human population, it has also depleted the Earth’s soil and its biodiversity and contributed to climate change. These extractive practices are not sustainable. We must move quickly to transform agriculture by employing a suite of practices known as regenerative agriculture.†The company manufactures the herbicide atrazine, a notorious endocrine disrupter.
Even as it claims progressive goals, the industry also continues its old-school scaremongering. CropLife America, the agricultural chemical industry’s powerful lobby group, claims that “Without pesticides, farmers would need twice as much land to grow the same amount of food due to reduced yields.†This is not true. Many farmers have reduced or eliminated pesticides without significant loss of yields or profits.
Despite the industry’s use of the right buzzwords and its attempts to clothe itself in the virtues of regenerative practices, pesticide use has not decreased. Just the opposite. In the U.S. about 196 million pounds of pesticides were used in agriculture in 1960; by 1981 it was 632 million pounds; by 2020 it was up to more than a million tons. The U.N. Food and Agricultural Organization put global usage in 2022 at just over four million tons, with the U.S. in the lead and Brazil second.Â
Farmers are already familiar with Integrated Pest Management (IPM), which looked like a step in the right direction when President Richard Nixon directed federal agencies to integrate it into agriculture in 1972. The U.S. Department of Agriculture (USDA) update in 2018 describes IPM as “a science-based, sustainable decision-making process that uses information on pest biology, environmental data, and technology to manage pest damage in a way that minimizes both economic costs and risks to people, property, and the environment.†But it took two decades for the USDA, the Environmental Protection Agency (EPA), and the Food and Drug Administration (FDA) to jointly agree to get IPM in place on 75% of U.S. acres by 2000. IPM has not been universally popular, and it has not weaned agriculture off pesticides. By 2001, some kind of IPM had been practiced on 70% of crop acreage, but pesticide use increased during the same interval, with little decline in the use of the most toxic pesticides.
More recently, organic and regenerative agriculture has been expanding. Between 2012 and 2017, U.S. cover crop usage increased by 50%. Still, cover crops are in use in less than 5 percent of croplands nationwide, reflecting a stubborn resistance to a core practice of regenerative agriculture. There remains among many farmers a fear, encouraged by the pesticide industry, that abandoning pesticides will result in pest apocalypse, yield reduction, and penury.
Agriculture will likely only survive and thrive if pesticide use declines rapidly. Nontarget effects of pesticides ranging from neonicotinoid insecticides to dicamba are wreaking havoc with the balance between plants, animals and humans. It should not take yet another generation to make the transition to sustainable food production, whether you call it integrated pest management or regenerative agriculture. See Beyond Pesticides webpages on Organic Agriculture and Keeping Organic Strong.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source: Mugala T, Brichler K, Clark B, Powell GS, Taylor S, Crossley MS. Ground beetles suppress slugs in corn and soybean under conservation agriculture. Environ Entomol. 2023 May 26:nvad047. https://pubmed.ncbi.nlm.nih.gov/37235638/
