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Orcas students find scientific use for tent caterpillars
The Orcas Middle School Science Club was looking for a good indicator of toxic accumulation in the recently completed constructed wetland in Eastsound. After trying a number of different kinds of traps for flying and aquatic insects, none of which collected much testable biomass, the solution, well simply dropped on them.
As toxic chemicals from motor vehicles wash off roads and into the recently built detention ponds, some of the contaminants will stick to clay and organic matter and stay on the bottom. Trees and shrubs around the ponds may take up some of the contaminants and could spread them to insects and birds eating leaves, flowers and fruits. Monitoring changes in the amount of pollution leaving the ponds this way is technically challenging, said Kwiáht ecologist Russel Barsh, who advises the science club. Indicator species must be found that are abundant but short-lived, accumulate toxics quickly from wetland plants they eat, and are easy to collect and extract.
Tent caterpillars should be excellent indicators, he says. They eat voraciously and convert many times their own weight in leaves into body fat each day for up to six weeks before they pupate. Even very small amounts of pollution in plants should be detectable in the body fat of tent caterpillars.
Extracting caterpillars is “really gross,” science club members said, but the result is a clear green liquid that reflects the kinds of plants each caterpillar was eating. Individual caterpillars are quick frozen, ground up into a paste with powdered silica and methanol is squeezed through the mixture under high pressure. Toxic compounds dissolved in body fats are extracted into the methanol; the fats stay in the silica.
Science club decided to focus on polycyclic aromatic hydrocarbons because they are found in gasoline, motor oil, road tar and automobile exhaust, and can be highly carcinogenic. Antibodies are used to determine the amount of PAHs in extracts from insects and shellfish—it’s the same technology hospitals use to test blood and urine, and can detect less than a billionth of an ounce of PAHs in a caterpillar.
Tent caterpillars collected from trees around the constructed wetland in early June had PAH concentrations of more than 100 parts per billion. This is several times higher than what science club found in Indian Island clams last fall, suggesting that a lot of street runoff toxics are already being retained by plants in Eastsound—and transferred to insects and to animals that eat insects such as frogs, birds and bats. A 2010 Kwiáht study of the largely agricultural False Bay watershed on San Juan Island found only one-third to one-fifth as much PAHs in aquatic insects.
Science classes at Orcas Middle and High Schools and Orcas Christian School studied recently constructed bioswales this past year, and with the staff of Kwiáht, built and tested scale models of their own design using plants and fungi to capture. The Orcas Island Community Foundation, the San Juan Nature Institute, and People for Puget Sound all contributed to this year’s focus on wetlands and managing runoff.
Kwiáht landscape ecologist Nathan Hodges hopes to attract more local support for more student projects in “green engineering” which he describes as “not just complaining about environmental threats, but building and testing locally meaningful solutions.”