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"It was everywhere," he says. "Waist high."
The other day, Johnson, an Arapahoe County weed specialist, returned to the same spot. This time the weed barely tickled his ankles.
"It's encouraging," he says "Exciting. Next year I don't expect to see much leafy spurge at all."
The reason? Three tiny European flea beetles named Nigriscuitis, Czwalinae and Lacertosa. On a peaceful sliver of public park, flanked by hiking and biking trails, they've accomplished what no man, machine or beast has been able to do: obliterate the scourge of Euphorbia esula.
"The success out here has just been tremendous," Johnson says.
Leafy spurge arrived in the United States in the early 1800s, an inadvertent import from western Asia. Like diffuse knapweed, it found a favorable climate, friendly soil and little competition in the West. Also like knapweed, it used numerous survival strategies to ensure its livelihood, including sinking root systems as deep as thirty feet. Today leafy spurge infests 100,000 acres in this state.
"It's a monster," Johnson says. "What you see on the surface is only 10 percent of the plant mass. Everything is underground."
That's why mowing, grazing, hand-pulling and spraying are only effective temporarily. But in North Dakota, where the weed has invaded more than a million acres, authorities have spent $100 million a year to control it, and as a result, the spurge acreage has been cut in half within five years. North Dakota's weapon of choice: flea beetles.
Encouraged by dispatches from the front, Johnson visited that state and returned with 55,000 bugs. Instead of spreading the insects countywide -- the strategy of some weed managers -- he dumped them all at Cherry Creek State Park, where 100 bugs had already been deposited back in 1993. With a base camp in place, the insect population rapidly soared to 1.5 million. The weeds, meanwhile, were thinned by 60 percent.
"The insects just move through here," Johnson says, inspecting a plant with sickly yellow spotted leaves. "They feed on the leaves, the flowers, and finally grind the stems to nubs. Then they lay eggs in the soil. And when the eggs hatch, the larvae feed on the roots for ten months out of the year. That's where they do the most damage. What happens aboveground is just the icing on the cake."
With less leafy spurge around, plants such as needle and thread, Western wheat grass and smooth brome now thrive in Cherry Creek State Park. Arapahoe County also has saved thousands of dollars in spraying costs. And the park has produced enough flea beetles to supply "almost anyone who calls," Johnson says.
But he understands the pressure for instant results. He also knows that flea beetles might not work everywhere. The bugs dislike soil with lots of sand or clay, for instance. They hate vibrations from cars and trucks. And they steer clear of ant piles.
He advises weed managers to be patient. Used in conjunction with grazing or mowing, the beetles certainly can "pay off," he says. After all, the bugs don't eat other plants, they don't require a lot of attention, and they're free -- as opposed to other weed-control methods, such as herbicides, which cost up to $25 per acre. Authorities might never completely replace herbicides, mowing or grazing, but in the fight against leafy spurge, they've found another useful weapon.
"You might not see results this year, but the payoffs that you see the year after and the year after that will be worth it," Johnson vows.
With the flea beetle's success against leafy spurge and promising results reported from Tim Seastedt's experiment with diffuse knapweed in Boulder County, it's been a good summer for bugs, Johnson says.
"It's not just one thing or the other," he adds. "There are other ways of doing things than what people did thirty or forty years ago. Now there are options."