Human Health and Environmental Relative Risks of WNV Mosquito Control Products

RESULTS: ENVIRONMENTAL RISKS; LARVICIDES

The following discussion of risks, both absolute and relative, only reflect direct effects on specific populations of non-target species. The use of larvicides has the potential to disrupt the food chain, and prior to instituting a larviciding plan in the state, a detailed review of this issue will be undertaken.

Absolute Risks, Larvicides; Aquatic Species

EPA evaluates the environmental toxicity database with respect to non-target species, models the exposure potential and evaluates associated risks to non-target organisms. Following this assessment, EPA requires certain label language to protect non-target species (Appendix III; section 1). The label information for the larvicides is found in Appendix III; Tables 6 to 10, Environmental Hazards section.

With regard to risk from Bt to non-target species, EPA has concluded:

"that toxicity and infectivity risks due to delta-endotoxin effects to nontarget avian, freshwater fish, freshwater aquatic invertebrates, estuarine and marine animals, arthropod predators/ parasites, honey bees, annelids and mammalian wildlife will be minimal to nonexistent at the label use rates of registered B. thuringiensis active ingredients".

The classic environmental risk assessment paradigm (Risk Quotient) cannot be used for Bt. This is due to the way Bt concentrations are characterized in the products. Bt is measured in International units which factor in the potency of each batch of Bt (Appendix VI table 3). In addition, the toxicity study results are reported in terms of Colony forming units (Appendix IV Table 5). The situation is similar for B. sphaericus .

The POE MMF also does not lend itself to traditional risk assessment, for an entirely different reason. Its effectiveness is dependent on its dispersion on the surface of the water causing obstruction of breathing in the mosquito larvae, rather than the concentration in the water (34).

EPA environmental risk assessments for methoprene are not currently available. There is a 1991 EPA RED for methoprene (3) that contains errors according to Wellmark (22).

Temephos lends itself to traditional risk assessment and EPA's most recent evaluation for aquatic risks are found in Table 8.

Relative Risks, Larvicides; Aquatic Species

See the discussion above regarding absolute risks for biological larvicides and the monomolecular film oil. Methoprene and temephos are the only two larvicides which lend themselves to relative risk analysis. The relative risk for temephos and methoprene are summarized in Table 9.

Absolute Risks, Larvicides; Avian Species

See discussion of absolute risk, low potential for exposure and low levels of toxicity of larvicides above. Temephos, being an organophosphate is the only larvicide warranting an avian risk assessment. Because temephos is applied directly to water, terrestrial birds are not expected to be exposed. However, certain species swim and drink from water which could receive temephos. EPA (4) evaluated the dietary and water intake for the mallard duck and concluded that they were below the level of concern. The other pathway in which birds may be exposed to temephos is through the consumption of fish from treated ponds. The EPA absolute risk assessment information is presented in Table 10.

Relative Risks, Larvicides; Avian Species

Relative risk is not a useful tool when considering a single active ingredient.

(a) Emulsifiable concentrates for Temephos are not currently registered in Maine.

(b) EEC = Estimated Environmental Concentration. For larvicides the assumption for receiving waters is a one hectare (2.471 acres); 15 or 30 cm deep intermittent pond.

(a) Measured for methoprene (22b); Highest level from EPA's Risk Assessment (see Table 8.)
(2) Emulsifiable Concentrate not currently registered in Maine

(1) Emulsifiable concentrates for Temephos are not currently registered in Maine.
(b) BFC = Bioconcentration factor

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