Discover 3

22 DISCOVERMAGAZINE.COM

LEFT: COURTESY OF THE TAMARISK COALITION. RIGHT: DAN BEAN/COLORADO DEPARTMENT OF AGRICULTURE

with multiple shoots resprouting
from the roots left behind. Tamarisk
survives, and even thrives, in fire.
Manual removal with chain saws
and local herbicide application is
prohibitively labor-intensive for
landscape-scale control, and aerial
herbicide sprays are expensive and kill
desired vegetation.
A biological control agent — four
species of Diorhabda beetles in this case
— offers another tool.
The U.S. Department of Agriculture
began searching for a tamarisk biocon-
trol in the mid-1980s. By 2001, they’d
launched the tamarisk beetle program,
releasing the insects at 10 different sites
with the caveat that no releases would
be permitted within 200 miles of a
known flycatcher nest.
But by 2008, after an unsanctioned
beetle release in southern Utah,
Diorhabda carinulata reached the Virgin
River: flycatcher habitat. Ecologists
worried the beetles would destroy the
tamarisks that this unique bird had
come to rely on for nesting. And in
2010, the USDA ended the program.
“Since 2010, it’s been impossible to
transport them across state lines,” says
Dan Bean, an insect physiologist who’s
worked on the beetle release program
since it began and is now director of
Colorado’s Palisade Insectary. “But

like all biocontrols, the insect has abso-

lutely no respect for state boundaries.”

RUSH HOUR

The federal government has stopped

supporting the beetle program, but

the beetles continue to adapt to their

North American home. They’ve

evolved to have life cycles better suited

to lower latitudes, and they’re still

expanding into new watersheds. In

some places, like central New Mexico,

different Diorhabda species overlapped

for the first time this summer.

As Jamison peers into his insect net,

he’s not worried about getting an exact

count. “I don’t care how many beetles

are on this tree versus that tree,” he

says. “I just want to know if there are

none, a few, or a ton of them, and the

impact they’re having.” He wants to

know if there are enough here to eat all

the leaves on the trees, and there are.

The beetles work in big, migrating

groups and produce multiple genera-

tions in a season. “Right about five

o’clock in the afternoon — we call it

the rush hour — the beetles climb to

the top of the trees and release a sex

pheromone that travels on the wind.

You get all these beetles following the

smell, and they congregate and have

a big party and have a ton of sex and

lay eggs all over the tree, and then go

do it again somewhere else,”

Jamison says. A month later,

those eggs hatch into larvae,

which munch on the leaves.

When they pupate into adults,

they fly off to find fresh,

green tamarisk.

Jamison finishes his sweep

of the roadside site, then

heads back to his van with

a few beetles he’ll keep as

samples for genetic analysis.

Sitting on the cooler that

serves as his workbench, food

refrigerator and beetle storage,

he fills a glass vial with ethanol

to preserve the insects. “Right

now, there are two species on

the Rio Grande that look identical,” he

says. One, Diorhabda elongate, which

hails from central Asia, has been mov-

ing south from Colorado. The other,

Diorhabda sublineata, from Tunisia,

has been working its way north. Just

south of here, near Bosque del Apache

National Wildlife Refuge, he expected

to find the two species overlapping for

the first time in the New World.

Tamarisk and the tamarisk beetle are

now permanent features of our western

waterways, says Ben Bloodworth,

who coordinates beetle-monitoring

programs across the western U.S. and

Mexico for the nonprofit Tamarisk

Coalition. There was never an expecta-

tion that the tree would be eradicated,

he says. Rather, the goal is balance —

revegetating the riversides with native

riparian willows and cottonwoods

or, if current hydrology and a chang-

ing climate just won’t support those

flood-adapted plants, perhaps relying

on native grasses and shrubs.

“The name of the game now is

restoration,” says Jamison. “Let’s get

these ecosystems to where the balance

is tipped toward the natives doing

really well and non-natives not taking

over.” D

Katherine Mast is a science writer in Santa Fe,

New Mexico.

Tamarisk beetles feast

on a young tree (above).

The bugs kill these plants

(left) by feeding on the

same ones year after

year, leaving many dead

tamarisk trees along the

Colorado River and other

western waterways.

Notes

From

Earth