PROPOSAL TO UPDATE NON-TARGET PLANT TOXICITY TESTING UNDER NAFTA
Michael Davy, Richard Petrie, and Jerry Smrchek, EPA Ted Kuchnicki
and Derek Francois, PMRA
Joint Presentation By: Health Canada - Pest Management Regulatory
Agency
and USEPA - Office of Prevention, Pesticides, and Toxic Substances
2.5.5 Floating Vascular Species
Of the floating aquatic plant species, duckweed (i.e., Lemna
sp.)
is perhaps the most commonly used in toxicity testing. Actually,
“duckweed” can refer to both Lemna sp. and Spirodela
sp. (greater duckweed), although, it is usually associated with Lemna
sp. (Newmaster et al.
1997). Duckweeds are floating non-rooted aquatic plants with a
reduced
root system and lack stems and true leaves. It has been speculated that
the
roots serve as anchors to keep the fronds right side up and to form the
tangled
masses which are of some importance in dispersal and protection from
water
movement. Mats of duckweed are habitat for small invertebrates, for
example,
the ephydrid fly (Lemnaphila scotlandae) and the rhyncophorous beetle (Tansyphyrus
lemnae). Other invertebrates such as hydras, flatworms and snails
are
common just beneath the duckweed mat (Hillman 1961). Waterfowl and
marsh
birds such as coots, black ducks, mallards, teals, wood ducks,
buffleheads
and rails eat duckweed in large quantities. Duckweed also provides food
and
shelter for fish (Newmaster et al. 1997).
Most toxicological studies with aquatic vascular plants expose the
test organism to the toxicant through the exposure medium. Testing
procedures can
follow the ASTM (1998b), Environment Canada (1999), APHA (1998b) or the
EPA
(1996b) test method guidelines. The methodology recommended for use in
pesticide
registration is a static test with Lemna sp., similar to the
flask
method for algae. Briefly, the test is initiated by the introduction of
Lemna
fronds into test vessels. Colonies are inspected for changes in frond
number
and appearance at initial stages of exposure and at the end of
exposure,
the total number of living and/or dead fronds are enumerated.
Testing with other floating vascular plants is not well
documented. Some
of the obvious choices for other floating species are pond lily (Nuphar
sp.), water lily (Nymphaea sp.) and greater duckweed (Spirodela
sp.). There are however, no known test protocols for these
species. There are currently no required data for the exposure of
aquatic vascular plants to pesticide spray
drift. Lemna species can easily be used for the examination
of pesticide exposure through water, for the study of pesticide drift
and research into the effects of surface
films at the air-water interface (Swanson 1989, Taraldesen and
Norberg-King
1990). Lockhart et al. (1989) [ summary
] have shown that the sensitivity of Lemna to glyphosate
increased several-fold with a foliar exposure compared to the
conventional exposure through the growth medium. In this method,
glyphosate was first sprayed onto
Petri dishes containing Lemna fronds in growth medium. The
sprayer
consisted of a spray nozzle which moved along a rigid track. The
sprayer
was calibrated using dyes to allow for selection of the desired
application
rate. After spraying, fronds were allowed to stand for 6-24 hours
before
being removed t o flasks containing clean culture medium.
Subsequently,
fronds were counted several times over a 2-week period.
The difficulty with recommending this type of test is that there
is only
one published report where exposure was examined through aerial deposit
(Lockhart
et al. 1989). In the case of herbicides, it may be useful to consider
the
mechanism of action before requiring this study. For example, if a
herbicide
is intended as a contact toxicant, then it would be useful to explore a
foliar
test. By contrast, it may not be necessary to examine systemic
herbicides
by this type of exposure provided that the mechanism of action is well
characterized.
2.5.6 Submersed Vascular Species
...
Duckweed is the only aquatic vascular plant that is currently required
for
pesticide and chemical testing. Development of submersed vascular plant
tests
is recommended as no single species can be representative of the
majority
of vascular plant species even if Lemna has a global
distribution. Lemna is somewhat unique in its floating,
unrooted growth habit
and exposure can be both aerial and aquatic. Furthermore, Lemna
is
a monocotyledon species. As there may be differences in herbicide
sensitivity
between monocot and dicot species, it may be beneficial to include
dicot
species in the testing scheme.
The references cited above can be found in the PDF file,
http://www.epa.gov/scipoly/sap/2001/june/sap14.pdf
Selected references:
- Day, K.E. and V. Hodge. 1996. The toxicity of the herbicide
metolachlor, some transformation products and a commercial safener to
an alga (Selenastrum capricornutum), a cyanophyte (Anabaena cylindrica)
and a macrophyte (Lemnagibba). Water Quality Research
Journal Canada. 31(1): 197 - 214.
- Environment Canada. 1999. Biological Test Method: Test for
Measuring
the Inhibition of Growth Using the Freshwater Macrophyte Lemna minor.
Method Development and Application Section. Environmental Technology
Centre. Environment Canada. Ottawa, ON. Report EPS
- 1/RM/37. March 1999. 106 pp.
- Fairchild, J.F., D.S. Ruessler, P.S. Heverland and A.R.
Carlson. 1997. "Comparative Sensitivity of Selenastrum capricornutum
and Lemna minor to Sixteen Herbicides." Arch. Environ.
Contam. Toxicol., 32: 353-357.
- Hillman, W.S. 1961. The Lemnaceae or Duckweeds: A review of
the description and experimental literature. Bot. Rev., 27:
221-287.
- Taraldsen, J.E. and T.J. Norberg-King. 1990. New method for
determining effluent toxicity using duckweed (Lemna minor). Environmental
Toxicology
and Chemistry. 9: 761 - 767.
US EPA. 1996b. Ecological Effects Test Guidelines. OPPTS 850.4400
Aquatic Plant Toxicity Test Using Lemna spp., Tiers I and II.
EPA 712-C-96-156.
