Beetle Evolution


The tamarisk leaf beetle is an unlikely citizen of Utah. And until recently it wasn’t found there. The tamarisk-munching bugs that now inhabit Utah, Colorado, and other parts of the West have their roots in Eurasia and hail from northwestern China and Kazakhstan. Scientists brought their ancestors to the U.S. in 2001 and set them loose to cull tamarisk, the invasive, pink-flowered weed that has subjugated the banks of the Colorado, Rio Grande and Pecos rivers, along with other waterways in the Southwest.

The Eurasian beetle has a particular taste for tamarisk leaves – lab tests indicate that it eats only this plant – which makes it an ideal candidate for biocontrol, the use of a natural enemy to keep weeds in check. Working in packs, the beetles defoliate tamarisk trees, turning them brown. Beetle larvae can strip a “15-foot-high tamarisk in four days” but the tough shrubs can withstand about five or six attacks over the years before they die.

While slow to work, the beetles are still effective hired guns. But it took them extra time to perform optimally in the Southwest. The foreign invertebrate is sensitive to the number of daylight hours and felt more at home in northern parts of the U.S. where longer mid-spring and summer days matched those in its homeland. In the south, shorter days forced the bug into early hibernation, which meant populations spent less time procreating and gnawing on tamarisk. It also led to early deaths because dormant bugs burned up their carbohydrate reserves before winter came to an end.

Over time, however, beetle populations in the Southwest evolved, delaying their hibernation so they could emerge from tamarisk leaf litter, where they camp out during winter, at spring time, ready to refuel on fresh tamarisk leaves. Now the beetles enter hibernation two weeks later than they used to, according to a study published in the July edition of journal Evolutionary Applications. For example, between 2003 and 2008 beetles around eastern Colorado’s Arkansas River switched from becoming dormant in late July to keeping active until mid-August.

The adjustment allows beetle populations to survive into the springtime, gives them more time to reproduce, and makes them more effective at controlling tamarisk, explains study co-author Tom Dudley, principal investigator at the University of California, Santa Barbara Riparian Invasive Research Laboratory, in a press release. It has also supported their spread across the Southwest. When ferried by the wind they can move up to 31 miles from where they were born and their range now extends from Utah through Arizona up to Lake Mead, reports the Scientific American.

The beetle’s evolutionary shift speaks to the natural world’s fluidity, but the little creature has also stirred up some controversy. While tamarisk is vilified as a water-sucking alien (although it was later found to use only as much water as native plants like cottonwoods and willows) it also provides nesting ground for the endangered southwestern willow flycatcher in areas where the bird’s native willow habitat has been denuded. Partly due to concerns about flycatcher habitat, in 2010 the U.S. Department of Agriculture suspended the beetle-based biocontrol program, putting a stop to the release of beetles in states where populations aren’t yet established.

Which just goes to show, no matter how adaptable you are, the government can still shoot you down.

Brendon Bosworth is a High Country News intern.

Image courtesy Robert D. Richard, USDA APHIS.