"PBO shuts down the enzyme that animals use to detoxify pyrethrin, so it gives pyrethrin more punch," said Weston. "Without PBO, you'd need a greater concentration of pyrethrin or pyrethroid to be effective as a pesticide."
To assess the toxicity of the water that was sampled in Sacramento after the aerial mosquito spray, the researchers compared the reproduction and survival rates of a species of water flea, Ceriodaphnia dubia. No detectable levels of pyrethrins were present in the water collected before and 10-34 hours after spraying, but researchers did find PBO. There was no significant increase in the water flea's mortality, although there was a decrease in the flea's reproduction in three of the 11 water samples.
Researchers also looked at the survival rates of a tiny, shrimp-like amphipod called Hyalella azteca, an indicator species for sediment toxicity. More significant effects were seen in the Sacramento creeks, where many of the sediments were toxic to H. azteca. That alone was not surprising, for their earlier studies had shown that pyrethroids from general urban pesticide use were in a high enough concentration to be toxic. The unexpected finding was that PBO left in the water from the mosquito spray made the sediments even more toxic to H. azteca than they already were.
The research team found that the level of PBO from the mosquito spray was high enough to double the toxicity of pyrethroids already present - primarily bifenthrin, one of the more toxic chemicals of the group. PBO, when in a pesticide product, can increase pyrethroid toxicity tenfold or more, but to see even a doubling of toxicity due
Source:University of California - Berkeley