Environmental fate and properties of pyriproxyfen

Abstract

Pyriproxyfen is a broad-spectrum insect growth regulator (IGR) with insecticidal activity against public health insect pests such as houseflies, mosquitoes and cockroaches. In agriculture and horticulture, pyriproxyfen has registered uses for the control of scale, whitefly, aphids and fire ants. It is used extensively worldwide, particularly in developing countries, although it has no significant uses in California. Pyriproxyfen acts on the endocrine system of insects by mimicking the juvenile hormone, thereby hindering molting and subsequently inhibiting reproduction. IGRs are unique in that they are specific for insects and have very low mammalian toxicity. As such, pyriproxyfen has received U.S. EPA status as a Reduced Risk insecticide and an organophosphate alternative and is the only pesticide approved by the World Health Organization (WHO) for treatment of potable water against mosquito. However, concerns about its environmental persistence and latent toxicity to nontarget organisms have been recently raised and discussed. In this context, a detailed review of the environmental fate and physicochemical properties of pyriproxyfen from the available scientific literature and from data gathered in its development and testing is needed. This paper gathers, combines, and abridges important environmental fate and property data on pyriproxyfen for academics, environmental scientists and agricultural professionals needing ready access to this information.

Effect of illumination on degradation of pyriproxyfen in water-sediment system

“…III, IV and V were then partitioned to either phase depending on their Koc. The Koc values of III and IV are 120 and 2757 in sediment, 10) which could account for their favorable distribution in water and sediment, respectively. Although the Koc value of V was not available, the EPI-Suite program 24) estimated a smaller value than that of I by an order, which explained the fluctuating distribution of V between two phases in the larger vessels in darkness.…”

“…19) The mineralization from [Pyr-14 C]-I but to a less extent than that from [Ph-14 C]-I indicated possible ring opening during microbial metabolism as similarly reported for nitrogen heterocycles. 20) Since the photolysis of [Pyr-14 C]-I in sterile water at pH 7 produced II as a single identified product and I has an absorption maximum in water at 270 nm with its shoulder extending above 300 nm, 10,11) direct photolysis is considered to be predominant. The highest occupied molecular orbital of I is localized at the aromatic carbons attaching to the ether oxygen atoms 18) and electron excitation to higher unoccupied energy levels are most likely to weaken ether bonds, which may result in the formation of II.…”

“…Generally, the contribution of photolysis was reduced by the existing sediment phase with adsorption or diffusion to the lower layer, and the observation showed that photoinduced degradation might occur on the sediment surface. The rapid partition of I from water to sediment due to its partition coefficient (Koc) to sediment of 4980 kg Ϫ1 of organic carbon 10) showed the possible photodegradation of I on the illuminated sediment surface as well as in the water phase. III, IV and V were then partitioned to either phase depending on their Koc.…”

“…The resulting concentration of I was below its water solubility (0.37 ppm). 10) The illuminated study with a dark control was similarly conducted using [Pyr-14 C]-I (1.3 mg/unit with 14 ml acetonitrile) for 30 days to investigate the effect of illumination, especially to know the behavior of II, which is known to the main photoproduct in water.…”

“…9) I is a hydrophobic molecule (log Pϭ5.37) and resistant to hydrolysis under ambient conditions. 10,11) On exposure to UV light (Ͼ290 nm) in water at pH 7 and 25 o C, I degrades with a halflife of 3.7-6.4 days via cleavage of each of the three ether linkages in the molecule to form 2-(pyridin-2-yloxy)propan-1-ol as the main photoproduct (70% of the applied dose). 11) Taking account of the profile of I and the above considerations, we conducted a water-sediment study of I under illumination as well as in darkness to know the effect of illumination on its dissipation profile together with the possible accumulation of the main photoproduct.…”