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

Lerps
Scale-like Protective Coverings Secreted by Lerp Psyllid Larvae.
Used with permission from University of California Statewide IPM Project, J. K. Clark, photographer

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?

Lerp Psyllid Larvae.
Used with permission from University of California Statewide IPM Project, J. K. Clark, photographer

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

Last updated: February 13, 2013