Bacteria break down a common insecticide, but slowly create a toxic byproduct
Basapuram G, Duttagupta S, Dutta A
Soil Health
If you spray malathion on aphids or squash bugs, this explains what happens to it after it washes off your plants and into nearby soil, ponds, or storm drains.
Malathion is a bug spray many gardeners use, and once it rinses off leaves into water or soil, certain bacteria start eating it apart. Most of the time this breakdown makes the chemical safer within days, especially in warm, mildly alkaline, nutrient-rich water. But one of the breakdown routes briefly turns malathion into malaoxon, a byproduct that's actually more toxic and longer-lasting than the original spray, so it's not automatically over once the smell fades.
Key Findings
Bacteria genera including Pseudomonas, Bacillus, Acinetobacter, Rhodococcus, and Micrococcus produce carboxylesterases and hydrolases that break malathion into malathion monocarboxylic acid (MMC) and malathion dicarboxylic acid (MDC), which are much less toxic and can eventually mineralize to CO2 and phosphate.
An oxidative desulfuration pathway converts malathion into malaoxon, a metabolite substantially more toxic and persistent than the parent compound.
Degradation is fastest at 25-35°C and neutral-to-slightly-alkaline pH, with oxic, nutrient-rich (eutrophic) conditions speeding breakdown and shrinking malathion's environmental half-life from weeks to days, while oxygen-poor, nutrient-poor systems slow it down.
chevron_right Technical Summary
Bacteria in water and sediment can break down malathion, a common insecticide, into far less toxic byproducts within days rather than weeks, though one breakdown pathway briefly produces a more dangerous chemical called malaoxon.
Abstract Preview
Original paper
Malathion biodegradation and biotransformation in water and sediment systems: bacterial pathways, environmental drivers, ecological impacts, and bioremediation strategies.
Malathion is one of the most widely used organophosphate insecticides, and its environmental fate is strongly influenced by microbial transformation in the water column and sediments. This review s...
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