Each year, pests eat more than one-fifth of the crops grown around the world. Many farmers turn to insecticides to protect their harvest, but some opt for a gentler approach: They perfume their crops with behavior-influencing chemicals called pheromones that can confuse insects and prevent them from finding mates.
But the high price of pheromones—commercial products can cost $400 per hectare—has prevented the widespread adoption of the tactic. Now, a new, cheaper method of manufacturing artificial pheromones could allow more farmers to add this weapon to their arsenals.
“It could revolutionize how pheromones are produced for crop protection,” says Lukasz Stelinski, an entomologist at the University of Florida, Gainesville, who was not involved in the work. “I expect that it’s going to catch on and make pheromone disruption much cheaper and easier to apply in practice.”
Farmers worldwide use more than 400,000 tons of insecticide annually. These pesticides can harm farm workers and cause collateral damage to pollinators and other wildlife. Meanwhile, insects have already evolved 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 to mate. By flooding their fields and orchards with fake pheromones designed to appeal to specific insects, farmers can overwhelm these signals and prevent reproduction. Females then lay sterile eggs, which don’t hatch into hungry caterpillars.
The pheromone mating call is usually a mixture of compounds. Traps are designed to attract a particular species—to monitor for the presence of a pest, for example—so a precise cocktail is usually needed. But to sabotage mating, a broad-spectrum component can work because many related species use the same basic compounds as pheromone components.
Synthesizing this chemical smokescreen is nevertheless a complex, expensive proposition. It can cost anywhere from $1000 to $3500 to produce just 1 kilogram of artificial pheromones. Deploying it can cost between $40 and $400 per hectare, depending on the type of pest.
That’s why pheromones are typically only used to protect crops that require relatively little land to turn a decent profit, such as fruits and nuts. Farmers who grow crops that don’t sell for as much per hectare, such as corn or soybeans, often can’t afford to use pheromones to defend their vast fields. It also requires some experience to deploy pheromones effectively. “You’re talking about razor-thin profit lines for a family farm and then asking them to invest not only in the product, but in the labor it takes to get the product in the field,” says Monique Rivera, an entomologist at Cornell University. “It’s a tough ask.”
In a bid to lower costs, Christer Löfstedt, a chemical ecologist at Lund University, and his collaborators in several countries have for the past decade been modifying plants to produce the chemical building blocks needed for synthesizing pheromones. Their crop of choice is Camelina, a flowering plant related to canola with seeds rich in fatty acids—key ingredients in coaxing plants to produce these raw materials.
Löfstedt and colleagues relied on genetic engineering to outfit Camelina with a gene from the navel orangeworm which causes Camelina seeds to produce a fatty acid called (Z)-11-hexadecenoic acid. In insects, this fatty acid is a precursor to mating pheromones. The researchers began to grow their genetically modified Camelina in experimental plots in Nebraska and Sweden in 2016, selectively cultivating the plants that produced the highest 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 the crop could be an efficient source of the raw materials needed to produce pheromones. Next, the researchers purified the oil and converted it into a liquid cocktail of pheromone molecules designed to appeal to the diamondback moth (Plutella xylostella), a pest that presents a particular problem in the Brassica, a group of plants including cabbage, kale, and broccoli.
In 2017, the team tested this pheromone blend in China. They put pheromone traps on sticks about 10 to 15 meters apart in a plot of the leafy Brassica choy sum. The traps worked just as well as commercial synthetic pheromones, the team reports today in Nature Sustainability. Another test in bean fields in Brazil revealed that a single plantmade pheromone could disrupt the mating patterns of the destructive cotton bollworm (Helicoverpa armigera) just as well as a synthetic pheromone.
ISCA Inc., a pest control company in Riverside, California, that participated in the research, estimates it would cost between $70 and $125 per kilogram to grow the Camelina and make the pheromones, less than half the cost of current synthesizing methods. That would put the costs on par with pesticides. The authors note that a liquified version of these pheromones could be dripped on fields, which would require less labor than manually placing traps.
A lower price might make the pheromones accessible to farmers in the developing world, says entomologist Muni Muniappan at the Virginia Polytechnic Institute and State University, who was not involved in the research. But because these pheromones work best when applied to large areas and most farmers in developing regions work small fields, farmers would likely need to work together to see the benefits, he says. “You need to have farmer education and outreach in order to make that successful.”
Getting regulatory approval to grow the genetically modified Camelina on commercial farms would take several years, the researchers note. But existing experimental permits already enable researchers to grow more than enough engineered Camelina to meet the current worldwide demand for pheromone control of diamondback moths and cotton bollworms, says Agenor Mafra-Neto, CEO of ISCA.
Several hurdles remain to applying the approach to other kinds of pests, such as beetles and leafhoppers. Doing so will likely require finding and adding other genes to Camelina. Still, says Junwei Zhu, a chemical ecologist with the U.S. Department of Agriculture, the new work “is a very good start.”
Source: Science Mag