Environmental ▼News Bugs boost phytoremediation
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searchers examined how selenium changed in a food chain that consisted of wasp parasitoid (Cotesia marginiventris), whose larvae feed on and eventually kill their host, the beet armyworm (Spodoptera exigua), which feeds on alfalfa. DANEL VICKERMAN
team of international researchers showed for the first time that insects can play a role in cleaning up contaminated soils. Experts call this surprising finding, which is described in this issue (pp 3581–3586), a turning point for the discipline of phytoremediation. In the initial decade of phytoremediation work, researchers focused solely on studying how plants could transform or accumulate pollutants such as pesticides or metal-containing compounds and deferred questions regarding how the altered or sequestered contaminants might affect the safety of other organisms in the food chain, explains Steve McCutcheon, a pioneer in phytoremediation and senior environmental engineer at the U.S. EPA National Exposure Research Laboratory in Athens, Ga. “[This] groundbreaking work shows that not only are plants reducing the toxicity of contaminants, but that the metabolism of insects and higher trophic level [organisms] may be taking a larger part than we expected.” The ES&T research focused on laboratory experiments that included insects associated with alfalfa (Medicago sativa), a forage crop that takes up selenium from contaminated soil but is not considered a hyperaccumulator. Because these insects don’t consume much of the metalloid, their presence in the system may not substantially enhance phytoremediation, but they certainly don’t detract from selenium phytoremediation either, says Danel Vickerman, lead author and currently a postdoctoral fellow in entomology at the University of California at Riverside. While Vickerman was working in entomologist John Trumble’s lab at Riverside, she collaborated with researchers at the Stanford Synchotron Radiation Laboratory. The re-
In a bug-eats-bug world, the environment improves. A wasp larva (white, left) emerges from its host, the beet armyworm (light green, right), which eats seleniumcontaminated alfalfa. New research in this issue of ES&T shows that each level of this food chain transforms selenium to a less toxic form.
Vickerman conducted behavioral studies to confirm that the beet armyworm and wasp larvae would consume selenium and toxicity and life history studies to determine selenium tolerance and effects on insect size and longevity. However, the big surprise came when her colleagues determined the selenium species and discovered that each of the three trophic levels transformed selenium to a lesstoxic form. The plant had partially transformed selenate to organoselenium. The beet armyworm contained only organoselenium, which it absorbed directly from the alfalfa or transformed itself. The wasp cocoons also contained organoselenium, whereas the adult wasps contained organoselenium and trimethylselenonium-like species. The only adverse effect on the insects appeared to be lower weight of the pupae and longer development time for the wasps. However, smaller wasps that take longer to
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develop could be more vulnerable to predation and other hazards. The researchers believe that these insects transform excess inorganic selenium to organic forms through methylation and also release some through volatilization. Vickerman speculates the mechanisms may be due to their complex pupation process, but narrowing down a specific one is difficult. Because selenium is a necessary micronutrient for humans and other invertebrates—not toxic at low levels like lead or mercury— volatilizing selenium could help remediate soils and transport it to places that are deficient, says Bettina Francis, an environmental toxicologist at University of Illinois at Urbana–Champaign. “This [work] is absolutely scratching the surface,” says Murray Isman, an agroecologist at the University of British Columbia in Canada. Because higher organisms play a role, there may be one more part of the ecosystem to manipulate to speed up the process of remediation, he says. For example, if plants can sequester heavy metals and other environmental contaminants out of soil but not convert them into a form that ultimately dissipates them, then perhaps an insect can finish the job. And there may be other insects already doing this work more efficiently than beet armyworms and wasp larvae, he adds. Isman praised the thorough work, which he characterizes as “more a proof of concept than anything with commercial application” because beet armyworms are major agricultural pests. Whether or not insects can really aid on a large scale, McCutcheon says, “From here on out, we will definitely be looking at these systems as a community and not just as a monoculture of plants.” —RACHEL PETKEWICH © 2004 American Chemical Society
