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toxicity as a result of their physical or narcotic actions (Lipnick 1993) in non-receptor organism (Figure 3-14). Time24 h48 h72 h96 hLC50 in g/LFigure 35LC50 as a function of time with continuous exposure
the The disadvantages of too great a selectivity towards certain pests are mostly economic —too small a market may be available to support development costs. This is especially relevant today with the increasing complexity and expense of toxicology in support of registration (it costs $40 - $80 million to bring a new pesticide to the market). Too great a selectivity may also involve costs to the user because mixtures of several compounds may have to be used to control all pests present in a certain situation. This not only involves greater use of pesticides but may require more frequent use of spray and application equipment. 3.2TOXICITY TESTING FOR ENVIRONMENTAL TOXICOLOGY Standardized test methods are routinely used and required by a number of regulatory agencies. In Canada, several standardized test methods have been suggested by Environment Canada at the federal level and, at the provincial level, test methods have been developed for use in the Municipal and Industrial Strategy for Abatement (MISA) as regulated by the Ontario Ministry of the Environment. In the U.S., a number of test methods have been developed by the U.S. EPA (CFR 2004) and others such as Environment Canada. The Organization for Economic Cooperation and Development (OECD) has published a number of methods (OECD 1981, 1984) and American Society for Testing and Materials (ASTM) has a long history of methods development, testing and validation (ASTM 1991). Guidance for interpreting tests and for analyzing data is also available (Environment Canada 1999b). 3.2.4Terrestrial systems, plants Plant toxicity tests are legion and largely non-standardized, although an attempt has been made to develop a standardized testing system in Canada (Boutin et al. 1993) and an update of this procedure has recently been presented to the USEPA (Davy et al. 2001) for review but has not yet been adopted. Environment Canada has published test methods for plants (Environment Canada 2005). NarcoticReceptor-mediatedConcentrationResponseFigure 3-146Illustration of differences in sensitivity between organisms with and those without the receptor system Leaf exposureThrough thestomateThrough theroots (soil)Figure 3-151Routes of exposure for plants.
Routes of exposure in terrestrial plants include air, soil and water (Figure 3-15). Aerial