I was not at the Nasci talk which Ed attended and took notes at. I did, however, review the Nasci slides of the Fort Collins, 2003, Mosquito Control effort. I also have talked to Ed about the Nasci presentation.
I suppose the slide that is supposed to be most supportive of the pro-spraying argument is the one which states that in treated areas Cx. tarsalis decreased from 57% to 83% while in untreated area Cx tarsalis decrease was 0% to 43%. The figures give for Cx. pipiens: treated areas saw a 73% to 89% decrease, while the untreated areas saw a 50% decrease to a 0% decrease to a 50% increase.
The important thing that Ed and I saw was that the mosquito population was decreasing before the spraying began. The fact that the mosquito population continued to decrease is not surprising. It appeared to decrease without spraying, so the question is how much of the decrease is due to spraying. One cannot really conclude anything from this one example, since (take the case of Cx tarsalis) if you compare the low estimate of the sprayed group--a 57% decrease, with the high estimate of the unsprayed group--43%, it is quite possible that the difference is attributable to random fluctuations (like weather) that have nothing to do with spraying. Similarly for Cx pipiens the low-high comparison is 73% decrease (for the sprayed group) versus 50% decrease (for the unsprayed group). The fact that the measurements for the unsprayed group varied from plus 50% to minus 50% indicates unstable data!! I would be willing to say that spraying kills some mosquitoes, but the decrease in this case that can definitely be attributed to spraying is not at all clear (since the population was decreasing on its own anyway) --and rigorously speaking, it is even less clear any decrease in mosquito populations due to spraying was actually significant from a health perspective.
If I were in charge of the situation I would rather go with a strategy (larvaciding) that makes more mathematical sense and has at least some data that could be interpreted as very supportive of larvaciding. Ed has data from two traps, one near a pond that was treated with larvacide and one near a pond that was not treated. The ponds are within two football field lengths from each other. The trap near the treated pond had zero mosquitoes, the one near the untreated pond had in excess of 600 mosquitoes.
Attacking mosquitoes while they are vulnerable and more localized as larvae makes sense, can be done with less toxic risk to the environment and is very effective if you are very careful to accumulate data of ``hot spots'' over time and rigorously pursue the breeding sites. Treating mosquitoes with spray is like trying to shoot incoming missles with outgoing missles, you are bound to miss a significant number. It is analogous to trying to machinegun mosquitoes in flight as opposed to attacking their life support while they are relatively fixed in the water. The former is mathematically difficult to impossible and dangerous. The latter, while requiring more thinking, is less dangerous and very effective (as shown by data).
The Nasci presentation does not in any way show that the number of cases of West Nile were reduced by spraying. (While there definitely was a risk to the environment posed by the pesticide.) The City of Boulder did not spray and had better results (as measured by number of West Nile cases) than any of the communities around it that did spray. I would have to agree with Dr. Pimentel, that spraying has a large component of politics mixed in with it. If you spray you are visibly ``doing something.'' Unfortunately, the only thing we know we are doing for sure when we spray is that we are adding poison to our environment.
Martin Walter, PhD
Professor of Mathematics
University of Colorado, Boulder