"What is the reason for limiting the number of applications per year?"

The EPA limits the amount of pesticide residue a plant or its product may contain for general health reasons.

"How seriously do I need to adhere to that?"

A pesticide label is considered a Federally mandated legal document. The instructions are to ALWAYS follow label directions. Period!

Just an FYI but more is not always better. In many cases, the heavily repeated use of various pesticides will cause the insects they are intended to target to build up resistance, making them virtually ineffectve.

As gardengal said...works same for ppl. And as the type of 'medicine' goes: don't give penicillin minute you hear your child/any person cough, or it has headache. Many ppl have to use different medicine now because they build up resistance by too frequent and not necessary use...Find out what the problem is and treat accordingly (not saying that you shouldn't use it on thrips!)

Hi GardenGal and Rina, thanks very much for your comments. Yeah, that makes perfect sense. I thought I might use spinosad as a preventative, but it sounds like that mightn't be a very good idea :)

Other than some systemics, which have their own set of challenges and tend to be pretty heavy duty (and virtually never recommended for any edibles), insecticides do not work preventatively. In most cases, they either have to be in direct contact with the insect to be effective or treated plant material needs to be eaten by the insect for them to be effective. No flashing neon signs telling the insects to stay away!!