Apple,
Actually the LD50 is based on the concentrate formulation of the pesticide purchased, not on the final spray mixture. You call pull it right off the Material Safety Data Sheet (msds) which has recently been renamed simply Safety Data Sheet (sds). The MSDS/SDS has toxicological information (LD50 values-generally for rats) not for the diluted spray mixture (since in most instances the dilution rate is at the discretion of the applicator). Nor is the toxicological information for the active ingredient itself in it pure form, rather the toxicological information is for the formulation in the bottle itself.
The reason for this is that workers who ship/handle the product could be exposed to the concentrate in the bottle (through spills, etc.) In that case, accurate toxicological information would be helpful to physicians/poison control centers to determine if the victim had received a lethal dose. In some cases the MSDS/SDS must accompany the product being shipped for this reason.
If you look at the MSDS/SDS of something like Captan 50 and Captan 80, you will notice the Captan 80 has a lower LD50 value. Of course one would expect a lower LD50 value for the Captan 80 because it has a higher percentage of active ingredient making the formulation more lethal.
This ties back into our previous discussion in that, as you point out, Mustang only has 10% active ingredient and is still roughly as lethal as Imidan with 70% a.i. However, as I mentioned, the use rate of Mustang is 20 times less than Imidan, giving the initial fruit residue a much higher margin of safety (for mammals) vs. Imidan.
Because of all this, in your tick example with Sevin and pyrethroids, it is true that pyrethroids are many times more effective than older compounds if you consider simply the amount of active ingredient it takes to destroy the pest. In that case, pyrethroids are the clear winner because so very little is required to control the pest. However, I agree with you it could be very misleading to people because efficacy trials are based on using the respective pesticide at labeled rates, not based on how little active ingredient is required to do the job. So if someone interpreted a pyrethoid to be 64 times more effective than the alternative because it took 64 times less active ingredient, that person would be mistaken. To determine efficacy one would need to compare both pesticides at labeled rates. That was a good point to bring up. Glad you pointed it out.
Lastly, I want to mention again, I personally would not be afraid to spray Imidan, or eat produce which had been sprayed with it. In my last couple of posts Iām merely trying to offer some rationale as to why EPA is not that hip on organophosphates, which probably has a lot to do with their lower margin of safety. However something with a lower margin of safety does not automatically mean itās dangerous. It simply means it has a lower margin of safety than something else, but still may be entirely safe. Flying in an airplane has a higher margin of safety than automobile travel, but Iām not afraid to do either one.