Summary of Concerns in Preliminary Ecological Risk Assessments
for Ingredients of the Mosquito Adulticide Spray
for West Nile Virus

The U.S. EPA is halfway through the process to reevaluate the registration of pyrethrins and piperonyl butoxide (PBO) for use in pesticides.  The preliminary risk assessments indicate new areas of concern.  

OPP-2005-0042-0003 Piperonyl Butoxide Overview, U.S. EPA, May 4, 2005

Piperonyl butoxide was evaluated for its ecological and environmental effects independently from any other active ingredient, but because PBO is never used alone, its adverse effects should be considered in light of the other active ingredients it is formulated with, most typically insecticides like pyrethroids or natural pyrethrins. PBO is generally less toxic to the species of concern, such as aquatic invertebrates, than pyrethrins and pyrethroids. Depending on the different fate characteristics of PBO and other active ingredients in formulated product, they may separate in the environment, affecting the potential for adverse effects to species at risk.  

Available data indicate that risk quotients (RQs) discussed in this section reflect PBO alone. The toxicity from PBO plus another active ingredient like pyrethrins or pyrethroids, can show greater adverse effects to invertebrates than if exposure was to the pyrethrin/pyrethroid alone.  [This statement indicates that the reverse is also true, that pyrethrins were evaluated without the synergistic effects of PBO.]

Environmental Fate and Transport

• PBO degrades rapidly (8.4-hour half- life) in the environment by photolysis in water, and is metabolized by soil microorganisms.
• Other tested routes of degradation, such as hydrolysis, aerobic and anaerobic aqueous metabolism, are very slow or have questionable rates due to experimental difficulties (soil photodegradation).
• The estimated atmospheric half- life of PBO is 3.4 hours, based on the estimated reaction rate with hydroxyl radicals.
• PBO is moderately mobile in soil-water systems.
• Little volatilization from soil or water is expected, however PBO may enter the atmosphere as an aerosol as a result of spraying.
• There is no concern for risk to predators via consumption of contaminated fish.
• The major degradates PBO-alcohol, PBO-aldehyde, and PBO-acid are expected to be more soluble in water than the parent and therefore more mobile in soil-water systems.

Endangered Species

• Federally- listed (endangered and threatened) freshwater aquatic organisms may be at acute risk from exposure to contaminated water bodies receiving run-off and spray drift from PBO treated sites.
• Listed birds and mammals are also at chronic risk of reproductive effects if present in treated areas.

To be meaningful, a listed species assessment for PBO would have to consider simultaneous exposure to the insecticides it is used to synergize, including pyrethrins and pyrethroids, as well as the co-synergist MGK-264.

Environmental Fate and Ecological Effects Data Needs
…. In the avian reproduction toxicity studies on piperonyl butoxide, there was toxicologically significant evidence for PBO to be a potential candidate to test for endocrine disruptor effects once the endocrine disruptor program is formalized.


 OPP-2005-0043-0003 Overview of the Pyrethrins Risk Assessment, U.S. EPA, April 27, 2005

Environmental Fate and Transport

• ….Pyrethrins are moderately persistent under aerobic metabolism (t_= 10.5 days), but more persistent under anaerobic conditions (t_= 86.1 days).
• Pyrethrins adsorb strongly to soil surfaces and are generally considered immobile in soils (Koc range 12,400 to 37,840); therefore, the potential to leach into groundwater is considered low.
• They could reach surface water via spray drift or runoff events accompanied by erosion where they quickly adsorb to suspended solids in the water column, and partition into the sediment.
• Sediments are likely to be an environmental sink for any non-degraded pyrethrins since they appear to persist under anaerobic conditions.

Aquatic Animal Risk
Agricultural

• The level of concern for acute risk to freshwater and estuarine/marine fish and invertebrates were exceeded for the agricultural uses of pyrethrins.
• The level of concern for chronic risk to estuarine/marine invertebrates was exceeded for the agricultural uses of pyrethrins.
• The level of concern for acute and chronic risk to estuarine/marine sediment dwelling organisms in water bodies near agricultural use sites was exceeded for the agricultural uses of pyrethrins.

Mosquito Abatement

• The mosquito adulticides are applied as mists (very small droplet sizes) that do not deposit rapidly and may drift substantially. The deposition and drift of pyrethrin to adjacent bodies of water was estimated using the AGDISP (AGricultural DISPersal) computer model version 8.07 followed by PRZM/ EXAMS which follows degradation and partitioning between the water column and sediment, and estimates the environmental concentrations.
• The levels of concern for acute restricted use and listed species were exceeded for estuarine/marine invertebrates associated with the mosquito abatement use (RQ range = 0.33 to 0.39).

Terrestrial Animal Risk
Small Mammals

• The acute level of concern was exceeded (RQ = 0.15) for listed small mammals less than 15 grams, that forage primarily on grass.
• The chronic levels of concern for both endangered and nonendangered small mammals would be exceeded (RQ = 1.10) if exposure actually occurs.

Non-target beneficial insects

• Honey bee studies suggest that pyrethrins are generally toxic to nontarget beneficial insects.

Endangered Species

• RQs exceed the Agency’s levels of concern for endangered and threatened species of freshwater and estuarine/ marine fish and invertebrates.
• There are acute risk concerns for endangered small mammals in terrestrial environments.

More information can be found at http://docket.epa.gov/edkpub/index.jsp  , see Docket No. OPP-2005-042 for PBO and Docket No. Opp-2005-0043 for pyrethrins.