The global decline of insects is a complex question, given the general lack of data and monitoring of insect populations, and the vast diversity of insects. Im not an expert when it comes to forest ecosystems but for instance, most pest insects are thriving in agricultural lands. Dozens of local declines have been reported but according to some authors, that is not enough to infer the global state of insect populations. Others disagree. A vast debate is going on in the scientific community concerning this matter, and it stems from the general lack of data. In the context of the sixth mass extinction, reported insect declines are the most alarming, but very little is known about the irreversibility of local declines and extinctions, so we cant know for sure that everything is going down. However, it doesnt look good, especially given the possible impacts of climate change. I kind of summarized my phd thesis introduction here, I dont know if it helps you in anyway given that I did not answer your question