Diazinon stability

The effect of dietary protein on diazinon toxicity was evaluated in a study with male albino Wistar rats. The study concluded that a purified protein test diet (with 26% casein and 59% cornstarch) did not significantly alter the LD50 value (415 mg/kg) for diazinon for this species. However, a low protein purified test diet (3.5% casein, 82% cornstarch), lowered the LD50 to 215 mg/kg. In addition, this study found that diazinon samples that were time-of-manufacture stabilized (to prevent spontaneous degradation to more toxic monothiotetraethyl pyrophosphate) were less toxic (LD50 value = 466 mg/kg) than samples stabilized after manufacture (LD50 value = 271 mg/kg) (Boyd and Carsky 1969). A subsequent study... 

A recent review has highlighted the stability of chemical species (in speciation studies) with respect to environmental matrices [1], These results are summarized in Table 4.2. It should be noted that samples must be analysed as quickly as possible after collection. The times given in this table are the maximum times that samples should be stored before analysis. It should also be noted that the guidelines given are general in nature. For example, many organophosphorus pesticides (not shown in Table 4.2) can be preserved by the addition of hydrochloric acid. However, as an exception to this, the (organophosphorus) pesticide diazinon breaks down when acidified. 

Stability of Diazinon and Certain Other Organic Pesticides in Unflooded and Flooded Soils"... 

Analyses of residues in the soils after incubation showed that the persistence of diazinon was considerably shorter in the previously treated soil than in the untreated soil. The half-life value for diazinon in previously treated soil was 1.7 days while in the untreated soil it was 9.9 days. Most of the insecticide added to the previously treated soil was lost within 10 days. Paddy water from the same fields were tested also for diazinon-degrading activity (17). Again water from a rice field treated previously with diazinon inactivated the insecticide more rapidly than did the water from an untreated field. In the water from the previously treated field the insecticide dissipated completely within 3-5 days of incubation after an initial lag of 1-2 days (17, 18). Table II summarizes the results of the study on the stability of diazinon in soil and paddy water. The data indicated clearly that a factor capable of degrading diazinon developed in rice fields of the Institute farm after insecticide applications. The diazinon-degrading factor, found in the diazinon-treated rice fields in the Institute farm, was noticed also in three other locations in the Philippines (19). 

Table II. Stability of Diazinon in Soil and Paddy Water...

Freshwater macroinvertebrates were comparatively sensitive to diazinon. Results of large-scale experimental stream studies showed that dose levels of 0.3 p,g diazinon/L caused a 5-8-fold reduction in emergence of mayflies and caddisflies within 3 weeks after 12 weeks, mayflies, damselflies, caddisflies, and amphipods were absent from benthic samples. Elevated (and catastrophic) drift of stream invertebrates was also documented in diazinon-treated streams, especially for amphipods, leeches, and snails. Short-term tests of 5-h duration with rotifers (Brachionus calyciflorus) show a 50% reduction in feeding rate on alga (Nannochloris oculata) at 14.2 mg/L, with long-term implications to population stability. 

Persistence is related to water solubility, vapour pressure and hydrolytic stability - properties which can vary greatly from one insecticide to another (Table 12.32). Malathion, for example, persists in the soil for 1-2 weeks, whereas diazinon may remain for 3-6 months. Early highly toxic pesticides such as TEPP and parathion are now little used. 

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