Water fleas, mosquitoes and the elderly

On August 8 - 10, 2005 county mosquito control in Sacramento, California aerially sprayed the pyrethrin pesticides with piperonyl butoxide (PBO) over 85 square miles. On August 20 - 22 they did it again, this time covering 104 square miles. Their objective was to kill adult mosquitoes that carried West Nile Virus after two dozen cases of the disease appeared in area residents. Use of pyrethroids with PBO is common for mosquito adulticiding. There is a general belief the pyrethroids are relatively benign, although there is not much data. The addition of PBO as a knockdown agent has come under suspicion because it works by inhibiting cytochrome P450 function which both insects and we use to detoxify foreign chemicals. It has also never been shown that mosquito adulticiding interrupts the transmission of disease, so the risk benefit trade-off is murky on both sides of the equation (see our earlier post here).

Now University of California - Berkeley researchers report results of increased toxicity of sediment and water samples from creeks taken after the August applications.

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. (ScienceDaily)

The PBO from the mosquito spray doubled the toxicity of the pyrethroids, just as it is meant to do -- in mosquitoes. But doing it in the creek is not one of the expected benefits. It turned out it wasn't pyrethroids from the mosquito spray that were the toxic ingredient in the creek waters but pyrethroids already resident in creek sediments. Pyrethroids are used in many commercial products. It's estimated there are more than 1200 pyrethroid containing products are sold in California and apparently a fair amount winds up in the environment, including creek sediments via urban and agricultural runoff.

As a result the California Department of Pesticide Regulation (DPR) is doing a re-evaluation of pyrethroid products in California. But they won't be looking at its use for mosquito adulticiding:

[Donald Weston, UC Berkeley adjunct professor of integrative biology and lead author of the study] does not foresee a dramatic change in mosquito control strategies as a result of this study's findings, particularly when weighing the risks of mosquito-borne diseases. "Public health risk still trumps everything, and pyrethrin and PBO exposed to sunlight degrade easily into innocuous substances after a day or two," he said.

Well, apparently not, according to Weston's own study. Public health may trump everything else, but before you do the trumping you should be sure the public health intervention is working. In this case there really is not evidence that it is. Moreover the use of pyrethroids is only being examined by California regulators, not elsewhere, as far as we know. Given the anti-regulatory stance of the Bush administration we shouldn't expect the federal government to lift a finger here. They could care less about water fleas, which is the way they will see it.

Bit if PBO is enhancing toxicity of other compounds in the creek environment, it is reasonable to consider what effects it might have on humans. In particular we should be examining drug metabolism in the elderly, where effects on the cytochrome system might shorten or prolong the time action curves of various medicines, many of which already have narrow margins between therapeutic and toxic doses.

This study on tiny organisms in California creeks is a warning flag for large organisms that walk on land.

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There have to be better ways than waiting until the adult part of the cycle. (And does the spray even get where it needs to go, to get the mosquitos?)

Too easy to act like even environmental impacts have no detriment to human health...

By crfullmoon (not verified) on 04 Aug 2006 #permalink

Indeed, the Central European Honeybee is marching toward threatened status, due to habitat destruction, pesticides, and other stresses. An extinct honeybee. That's quite an achievement.

It's all part of the secret war between the Freemasons and the Mormons.

By Ground Zero Homeboy (not verified) on 05 Aug 2006 #permalink

Aerial spraying of pesticides does kill huge numbers of beneficial species in addition to the intended target.

I lived in an area of California which was aggressively sprayed with malathion in order to suppress an infestation of the Mediterranean fruit fly. It worked. We saw no medflies thereafter. Nor any butterflies or moths. Nor any honeybees nor bumblebees. A few ants and houseflies survived.

The two years post-spraying were straight out of Rachel Carson. Sharply reduced insect populations. And, as second- and third-order effects, reduced populations of birds, bats, lizards and other insectivores. Our garden and orchard became ill and drab. Several plants failed.

There is a metaphor in English about using a sledgehammer to kill a fly. I think that it is conceptually inaccurate. Sledgehammers do a lot of damage, but only in a small area, and the damage does not continue past the time of application. What this called to mind, instead, was using a flamethrower, but even that does not convey the extent of the ruin which resulted.

Allow me to commend to your attention the excellent book by Buchmann and Nabhan, The Forgotten Pollinators. It makes quite clear how much permanent harm is being done by the anthropogenic poisoning of crucial insect and animal species on which plants are crucially dependent for their propagation.

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