Saturday, August 22, 2009

All the News that is Fit to Print -- About Mosquitoes


I went for a walk in the woods yesterday evening, and I said, I'll do a post on mosquitos. There is probably some interesting fact about mosquitos -- maybe a good blog topic?

Well no. Generally this topic is too non-appetizing. Giant pictures of biting mosquitos are too gross. (That is why this post has a cartoon mosquito illustration.)

I also find mosquito anatomy gross, and doesn't everyone know about the mosquito life cycle and that only females bite.

Maybe it is interesting the antibody IgG binds to antigens in female mosquito saliva causing inflamation, but mostly it is disgusting.

One interesting tidbit is that mosquitos eat nector, fruit and foliage almost entirely. They spend most of their lives as little green multi-legged larva -- so they are not totally diabolical. A female mosquito may only bite for blood three times in her lifetime.





The other interesting tidbit is that mommy mosquitoes find prey by smelling octenol, which is in sweat and respirated air. It has been disproven that mosquitos sense carbon dioxide in respirated air. I wish I understood why there is octenol in the body, but I don't know. (Maybe someone could leave a comment on that.) Octenol is sold as a decoy for mosquitos too. It might also make a funny practical joke -- a little mean, but still funny.





DEET,
diethyl meta-toluamide, (CAS# 134-62-3) is the universal mosquitto repellant. It was used as an insecticide before, but became popular for mosquitos during World War II. It numbs the noses of mosquitos so they can't smell the octenol.

Predictably and ironically, the MSDS for DEET from the Fisher Scientific website, says to avoid skin contact. It also has the LD50's, which is the dose that kills half the lab animals. (I know it is going to shock my wife that animals died for mosquito repellent.) These LD50's are pretty high -- that is pretty safe, but for the uninitiated it seems shocking. An LD50 of 3180ml means that 3.2 milliliters per kilogram of body weight killed half the rabbits tested. If I lost weight, and weighed 75 kg, then 233 ml of DEET would have a 50% chance of killing me.

LD50/LC50:
Draize test, rabbit, eye: 100 mg; <-- Lots to read about Draize testing here.
Draize test, rabbit, eye: 10 mg Moderate;
Draize test, rabbit, skin: 500 mg Moderate;
Inhalation, rat: LC50 = 5950 mg/m3;
Oral, mouse: LD50 = 1170 mg/kg;
Oral, rabbit: LD50 = 1584 mg/kg;
Oral, rat: LD50 = 1892 mg/kg;
Skin, rabbit: LD50 = 3180 uL/kg;
Skin, rat: LD50 = 5 gm/kg;


My former employer, SC Johnson, makes OFF! mosquito spray, and it contains 10-30% DEET. I could use 1.1 liters of OFF! to give the 233 ml dose with its 50% toxicity. That is 37 ounces or about 6 six ounce bottles, which I 'd need to apply all-at-once.

The National Pesticide Info Center at Oregon State University puts this in context -- so don't be alarmed. The evidence is overwhelming that DEET can be used safely.

There are lots of other mosquito repellents. The government has 30,000 listed. My view is that DEET works better than all natural alternatives.

At last week's ACS (American Chemical Society) meeting, Bernier of the USDA used a neural net program to screen these 30,000 compounds. The program picked several, which they made and tested. Fourteen turned out to work as well as DEET. Of course, who knows how toxic these are, or more importantly whether anyone can afford to do all the toxicity testing to clear EPA, OHSA and REACH regulations?