Each year, pests eat more than a fifth of the crops grown worldwide. Many farmers resort to insecticides to protect their crops, but some take a more gentle approach: they perfume their crops with behavioral chemicals called pheromones, which can confuse insects and prevent them from finding mates.
But the high cost of pheromones — commercial products can cost $400 per hectare — has hindered widespread adoption of the tactic. Now, a new, cheaper method of producing artificial pheromones may allow more farmers to add this weapon to their arsenals.
“This could revolutionize the way pheromones are produced to protect crops,” said Lukasz Stelinski, an entomologist at the University of Florida, Gainesville, who was not involved in the work. “I expect it will catch on and make pheromone disruption much cheaper and easier to implement in practice.”
Farmers worldwide use more than 400,000 tons of insecticides annually. These pesticides can harm farm workers and cause collateral damage to pollinators and other wildlife. Meanwhile, insects have already developed resistance to many pesticides, forcing farmers to apply even more.
For some growers, pheromones provide an attractive alternative. Female insects naturally emit pheromones that attract males for mating. By flooding their fields and orchards with false pheromones designed to attract certain insects, farmers can override these signals and prevent reproduction. Females then lay sterile eggs that do not hatch into hungry caterpillars.
A mating pheromone is usually a mixture of compounds. Traps are designed to attract a specific species – for example, to monitor for the presence of a pest – so a precise cocktail is usually required. But to sabotage mating, a broad-spectrum component can work because many related species use the same basic compounds as pheromone components.
However, synthesizing this chemical smokescreen is a complex and expensive proposition. Producing just 1 kilogram of artificial pheromones can cost anywhere from $1,000 to $3,500. Deploying it can cost between $40 and $400 per hectare, depending on the type of pest.
Therefore, pheromones are usually only used to protect crops that require relatively little land to make a decent profit, such as fruits and nuts. Farmers who grow crops that don’t sell as much per hectare, such as corn or soybeans, often can’t afford to use pheromones to protect their vast fields. Also, some experience is required to use pheromones effectively. “You’re talking about slim profits for a family farm, and then you’re asking them to invest not just in the product, but in the labor it takes to put the product in the field,” says Monique Rivera, an entomologist at Cornell University. “This is a hard question.”
In an effort to cut costs, Christer Lofstedt, a chemical ecologist at Lund University, and his collaborators have over the past decade modified plants to produce the chemical building blocks needed to synthesize pheromones. Their crop of choice is Camelinaa flowering plant related to canola with seeds rich in fatty acids – key ingredients in forcing plants to produce these raw materials.
Löfstedt and his colleagues rely on genetic engineering to clothe themselves Camelina with a gene from the navel orange worm that causes Camelina seeds to produce a fatty acid called (Z)-11-hexadecenoic acid. In insects, this fatty acid is a precursor to mating pheromones. Researchers began growing their genetically modified Camelina in experimental plots in Nebraska and Sweden in 2016, selectively cultivating the plants that produced the greatest amounts of this critical molecule.
After three generations, 20% of the fatty acid content of the seeds consisted of (Z)-11-hexadecenoic acid—enough to suggest that the crop could be an efficient source of the raw materials needed for pheromone production. The researchers then purified the oil and turned it into a liquid cocktail of pheromone molecules designed to attract the diamondback moth (Plutella xylostella), a pest that is a particular problem in Brassicaa group of plants including cabbage, kale and broccoli.
In 2017, the team tested this mixture of pheromones in China, where the co-author works. They set pheromone traps on sticks about 10 to 15 meters apart in a plot of leafy plants. Brassica choi sum The traps worked just as well as commercial synthetic pheromones, the team reports today in Resilience of nature. Another test in bean fields in Brazil revealed that a single pheromone produced by the plant could disrupt the mating patterns of the destructive cotton bollworm (Helicoverpa armigera), as well as a synthetic pheromone.
ISCA Inc., a Riverside, Calif.-based pest control company involved in the research, estimates that it would cost between $70 and $125 per kilogram to grow Camelina and make the pheromones at less than half the cost of current synthesis methods. This would equalize the cost of pesticides. The authors note that a liquefied version of these pheromones could be sprayed onto the field, which would require less labor than manually placing traps.
A lower cost could make pheromones affordable to farmers in the developing world, says entomologist Muni Muniapan of Virginia Polytechnic Institute and State University, who was not involved in the research. But because these pheromones work best when applied over large areas, and most farmers in developing regions work on small fields, farmers will likely have to work together to see the benefits, he says. “You have to have farmer education and outreach to do this successfully.”
Obtaining regulatory approval to grow genetically modified Camelina on commercial farms will take several years, the researchers note. But existing permits for experiments already allow researchers to develop more than enough engineering Camelina to meet the current global demand for pheromone control of diamondback moths and cotton bollworms, says Agenor Mafra-Neto, ISCA CEO.
Several obstacles remain to applying the approach to other pest species, such as beetles and weevils. This will likely require finding and adding other genes to the Camelina. Still, says Junwei Zhu, a chemical ecologist at the U.S. Department of Agriculture, the new work “is a very good start.”