By Ed Ricciuti
Budget-crunched rural communities often take advantage of the old saw about one man’s trash being another’s treasure—by charging a fee to handle big cities’ municipal waste. Taking cash for trash, however, could have hidden costs, because harmful organisms can ride along with waste and gain a foothold in the communities that accept it.
Searching for unwelcome hitchhikers in mountains of waste can be like looking for the proverbial “needle in a haystack,” according to a team of scientists whose recent research may make the task much easier. In a study published in May in the Journal of Economic Entomology, researchers at the United States Department of Agriculture’s Agricultural Research Service (USDA-ARS) and the University of British Columbia describe how they employed a method for detecting DNA, first used in the 1980s to study microbial diversity in the environment, to reveal otherwise covert invasive agricultural insect pests hidden in green yard waste.
DNA can be shed by organisms into the environment through droppings, bodily fluids, skin cells, and tissue fragments, all of which can be collected and sequenced. The sample sequence is then compared with DNA sequences in reference banks in hopes a match is found, identifying the organism.
Called “environmental DNA” (eDNA), at one stroke it provides a snapshot of several different organisms present in an environmental sample and even those that had been there before but no longer were when the sample was collected. The technique is much more efficient than traditional methods, such as surveying by eye, taking photographs, trapping, and picking up droppings. It has been used on investigations as varied as surveying for invasive pythons in Florida to solving crimes.
Given that the annual cost of damage caused by unintentional introductions of species averages in the billions of dollars, say the researchers, “With so much at stake, it is easy to see the value in being able to identify pests even if the whole organism cannot be found in a shipment.”
In their two-year study, the scientists detected the presence of three crop-damaging insects in green waste that was shipped from Vancouver, British Columbia, Canada, to an agricultural region of central Washington state in the U.S. Two of the three, the apple maggot (Rhagoletis pomonella) and the lined click beetle (Agriotes lineatus), known as the wire worm as a larva, are not found in the area studied. The apple maggot damages fruit trees and the beetle attacks potatoes. The third, the codling moth (Cydia pomonella), is already a major pest of apples and pears in the region, a problem that could be intensified by an addition of more moths via waste shipments.
The researchers specifically looked for the apple maggot and codling moth, while the lined click beetle was an unexpected bonus. “This species was not an original target of the study but, using this method, was found nevertheless,” says Lisa G. Neven, Ph.D., a research entomologist at USDA-ARS who led the study.
The title of Neven and colleagues’ report illustrates the hit-or-miss quality of trying to detect often-cryptic organisms in environmental samples: “Using eDNA to Play Whack-a Mole with Invasive Species in Green Yard Waste.”Whack-a-Mole is a carnival game in which contestants try to hit targets, often representing moles, which randomly pop out of holes.
Similarly, there is a random quality to the type of insects that might be found in green yard waste, which can include grass clippings, dead leaves, tree and shrub cuttings, weeds, garden waste, and windfall fruits and nuts. Making it more difficult is that waste may host potential pests across various life stages, with different identification issues. All told, the transfer of green yard waste poses what the researchers call “a significant ecological and economic threat to recipient communities.”
Neven and colleagues predict that the use of eDNA could have far-reaching implications for the USDA’s Plant Protection and Quarantine (PPQ) operations, designed to intercept introduction of harmful pests and diseases. For instance, before they reach grocery store shelves, all imported fresh fruits and vegetables must pass inspection; in 2021, PPQ oversaw the inspection of 3.7 billion pounds of fresh agricultural products from 21 countries, so any tool to improve efficiency is welcome.
“There are programs looking for hitchhiking invertebrates and plants on boats in Washington, which require mandatory inspections and washing,” Neven says. “Quick DNA testing for these species in the effluent water could be useful in detecting very small numbers of these invasive species, those not detectible to the naked eye, or in a large volume of water from washing a boat hull. An eDNA approach could be used in the boat inspection areas where new potentially invasive species could be identified before they become a problem in waterways.”
“Using eDNA to play whack-a-mole with invasive species in green yard waste”
Journal of Economic Entomology
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.
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