Does Imidan go bad and lose its effectiveness after a while? I'm talking about the dry powder, not after it's mixed with water.

I don't know if it's just me and my memory playing tricks on me, or if the bugs are really bad this year or if my Imidan has gone bad, but I spary every two weeks and I seem to lose more and more fruit each week. (I'm just talking about plums and peaches).

Early in the season I had great timing/luck compared to last year and lost very few fruit to the plum curculio. Last year, my timing was a little off and at least 95% of my plums had damage. This year, I lost very few fruit to the PC early in the season on both plum and peach trees, but now as the season is progressing I see fresh wounds each week and I'm finding fruit oozing pectin.

I've kept up the same spray program this year as I've used in the past (once every 2 weeks), but this year the bug damage is pretty steady, while in the past it was bad at the start, but then seemed to go away.

My working theory is that my Imidan has gone bad and it's not doing the job, but that doesn't make 100% sense since I had my best year ever against PC early in the season.

Why would it work wonders at petal fall, but then lose effectiveness now?

Thanks!

Bart