necrotic spots, not burn.. helps deduce what's wrong. (i think it is, anwway)
Are you sure it is a pest? Get your scope out to look under leaves... find seomthign crawling around? plus, the stuff you foliar spray can cause blemishes and issues too.. so consider when things happened or when they worsened in that context too.
DE around pots, neem oil in vege only (in my opinion) sticky tape, all the regular things you can do with bugs... Also, try to give it water only for a few irrigations and se ehow it reacts - *pH'd water, if you find no evidence of pests, otherwise. If it improves with the water, you can rule out pests or microbes under the surface of your substrate.