The release of tiny insects to combat an invasive weed is paying off, according to a recent study by the U.S. Department of Agriculture.
Scientists from the USDA’s Agricultural Research Service released arundo gall wasps (Tetramesa romana) and arundo scale insects (Rhizaspidiotus donacis) several years ago as part of a biocontrol program to kill a weed called “giant reed” (Arundo donax) along the Rio Grande in Texas. The weed, also known as “carrizo cane” and “Spanish reed,” clogs streams and irrigation channels, weakens river banks, stifles native vegetation, affects flood control, reduces wildlife habitat, and impedes law enforcement activities along the international border.
Recent research conducted by entomologist John Goolsby demonstrates that these insects have helped control giant reed over more than 550 river miles. Measurements taken in 2014 documented a 22-percent decrease in plant biomass along the Rio Grande since the insects’ release in 2009. Measurements in 2016 show a further decrease of 28 percent and significant recovery of native riparian vegetation.
Giant reed grows between three and seven inches a day and reaches heights of 30 feet along the Rio Grande. The weed increases the population of cattle fever ticks by creating an ideal habitat for them, which makes it difficult for USDA inspectors to detect tick-infested cattle and deer. As the riverbank transitions back to native vegetation, the plant community supports greater abundance and diversity of tick-feeding ants and beetles that act as biological control agents.
To accelerate weed removal, scientists have combined “topping” — mechanically cutting cane — with insect releases. Topping suppresses growth for more than a year and makes plants more susceptible to insect attacks. Combining topping and insect releases gives a high, long-term suppression of cane and allows native trees to grow and start shading giant reed.
“We’ve thinned the cane out significantly,” Goolsby said. “The biggest decline in plants correlates with the greatest number of our biocontrol agents—the wasp and scale.”