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trying to understand PHI

last month
last modified: last month

It's kinda quiet here so let me kick things up a bit.

The "pre-harvest interval" (PHI) is a length of time that must have elapsed for harvest, following application of a substance with perhaps some toxicity. The idea seems to be that this length of time minimizes residue on the particular edible. I was looking at the instructions for Sevin, which has been reformulated to use a much less toxic pesticide - zeta-cypermethrin (a pyrethrin, otherwise known as zetamethrin)- than the original carbaryl, which is now banned in many places. For leafy vegetables, cucurbits, fruiting vegetables, and brassicas, the PHI is now ONE DAY for Sevin. Now, this isn't obviously consistent with other instructions which say that, in case of rain, as long as the Sevin has thoroughly dried, as in 24 hours later, the rain won't wash it off. The aerobic foliar half-life of such pyrethrins can be a week or more, though is somewhat shorter in sunlight. So why is the PHI for Sevin only one day for most veggies? In fact, all of this info was pretty much the same for the more dangerous carbaryl, and I believe the PHI was the same for that! I understand that PHIs are established on the basis of residue tests, in that they actually take samples over time and compare with established "tolerances." But all the facts would suggest that the amount of Sevin on your harvested vegetable can be pretty much the same as it was when you applied it yesterday. So where does a PHI of one day come from? One has to assume that the established tolerances are generous, or maybe the assumption is that all the stuff being harvested was sitting in full sunlight. It is true that this new Sevin is not very toxic, but carbaryl certainly was.

Now, if something we're not understanding is making the Sevin largely go away after a day, does that mean that it's insecticidal activity goes away after a day as well?

Curiously, storage doesn't affect PHI. That is, if I harvest a vegetable immediately after pesticide application, but then it sits around for a while before being eaten, the PHI is still considered to be violated. That doesn't make a lot of sense, though I guess if the sun isn't shining on it, the compound can last longer.

Interestingly, the PHIs for common fungicides are a lot longer. Several weeks.

I have a query in to Garden Tech (who markets Sevin) about this, but they have not responded in several weeks. So much for them. I asked our extension folks, and they didn't add much to what I already know. I should add that I actually rarely use Sevin.

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