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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.
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.
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.
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