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A Sugar Pill That May Be Hard to Swallow: Eucalyptus Trees
Used to Clean Up Selenium May Now Be a Danger to Migratory
Birds
Eucalyptus trees were introduced to California from Australia
more than a century ago. Today, eucalyptus trees are almost
as common a feature of the California landscape as they are
of the Australian landscape. However, if you've ever visited
a grove of eucalyptus trees, you may have noticed that they
seem unusually quiet. That is because eucalyptus leaves release
certain chemicals that make them an unattractive food source
for most of California's native insects. Therefore, groves
of mature eucalyptus trees have traditionally been devoid
of insects and, thus, insect eaters such as migratory birds.
Feeding Frenzy
Recently (1998), the inevitable happened. An
Australian eucalyptus insect, the red gum lerp psyllid, was
accidentally introduced to California and is rapidly colonizing
California's eucalyptus trees. The lerp psyllid larvae excrete
a scale-like, protective covering composed of crystallized
honeydew; that is, a hard capsule that the larva lives inside.
This hard capsule is called a "lerp." The honeydew that crystallizes
into a lerp is a sugary substance produced from the sap of
the eucalyptus tree. A single eucalyptus leaf can have 30-50
of these "sugar pills" attached to it. These sugar
pills have proven to be an extraordinary energy source for
migratory birds. Now, the once quiet eucalyptus groves are
being described in Audubon newsletters as having been turned
into ". . . a three-ringed circus of birds gorging on lerps!"
The birdwatchers are beside themselves with joy as they watch
large numbers of woodpeckers, crows, jays, titmice, finches,
warblers, tanagers, grosbeaks, orioles, flycatchers, and hummingbirds
(among other species) converge on lerp-infected eucalyptus
groves.
Sounds like a story with a happy ending? Maybe, maybe not.
To understand the possible problem, you need to know a bit
of California's history.
Dangerous Dirt
In the early 1980s, scientists documented that waterfowl in
marshy areas of California's San Joaquin Valley were producing
offspring that were stillborn or deformed. Scientists determined
that selenium (chemical symbol Se) was the cause. Selenium
is an essential trace element that occurs naturally in the
environment. Although small amounts of selenium are important
for both wildlife and people, too much is toxic. Selenium
poisoning in animals results when they eat selenium-tainted
insects, plants or animals. In parts of the western United
States, the soils contain rather high levels of selenium.
There, selenium is most commonly found in rocks and soil that
originated in the ocean (marine sedimentary deposits) such
as those found in the mountains that border California's San
Joaquin Valley (Valley). The Valley is a broad basin perfect
for collecting the sediment carried down from the mountains
by streams and rivers. Over tens of thousands of years, large
amounts of sediments containing trace elements and salt have
been deposited in the Valley.
Poisonous Ponds
The Valley is one of the world's most productive farming areas.
However, the long summers and hot, dry climate that makes
the Valley so attractive for farming also demands the extensive
use of irrigation. Unfortunately, long-term irrigation flushes
out the salt, selenium and other trace elements found in the
Valley's soil into irrigation return flows. Traditionally,
these selenium-tainted agricultural waters have been disposed
of in evaporation ponds. However, evaporation ponds are an
attractive nuisance to birds that stop to feed on plants and
animals found at the ponds. These plants and animals often
contain high concentrations of selenium, which the waterbirds
then accumulate in their tissue, causing the kind of problems
scientists began observing in the Valley in the 1980s.
Pollution Solution?
To avoid the problem of toxic evaporation ponds, the "agroforestry"
method for disposing of selenium-tainted irrigation drainage
water was developed, with eucalyptus trees as its primary
component. Since the late 1980's, more than 40 agroforestry
sites have gone into operation in California's San Joaquin
Valley alone. Hundreds of thousands, if not millions, of eucalyptus
trees have been planted for the purpose of disposing of selenium-tainted
water. The trees can be thought of
as biological pumps; their main function is to use and evaporate
large volumes of contaminated water. This has been viewed
as an environmentally friendlier way to dispose of irrigation
wastewater than using evaporation ponds because it decreases
the exposure of birds and other animals to selenium. In addition,
analyses done for the agroforestry program showed that little
selenium accumulated in the eucalyptus leaves or wood fiber;
however, it was noted that high concentrations of selenium
accumulated in the sap. Now, faster than you can say LERP!!!,
a major pathway to connect the selenium-tainted eucalyptus
sap to migratory birds via the red gum lerp psyllid may have
opened.
Stay Tuned
At present, we do not know if migratory birds are at risk
for selenium poisoning through exposure to lerps or, if they
are at risk, which bird species may be most at risk. The Sacramento
Field Office's Division of Environmental Contaminants has
begun an investigation that aims, in part, to examine the
extent to which eucalyptus trees used for disposing of selenium-tainted
irrigation drainage water are infected with lerps, and to
collect samples of lerps from these trees to determine their
selenium content. Stay tuned for the rest of the story as
it unfolds.
Links:
Palomar College. Wayne's World: A Newsletter of Natural History
Trivia. The
Red Gum Lerp - A Tiny Insect That Attacks Eucalyptus
University of California Statewide Integrated
Pest Management Project. Biological Control of the Red
Gum Lerp Psyllid, a pest of Eucalyptus species in California
- http://www.CNR.Berkeley.EDU/biocon/dahlsten/rglp/index.htm
Updated: May 7, 2001 |
