Typically they're waiting for ideal environmental conditions.

Insects are ectothermic (cold blooded) so they can only operate when its warm outside, and many species use diapause to hibernate safely through the winter. Insects will go into diapause in their eggs or pupae (cocoons) which are much more resilient than their adult bodies, and build their pupae in safe, warm places like underground, deep in trees, or under decaying leaf matter where it doesn't go below freezing during the winter, emerging from their pupae when it gets warm enough.

Many insects also struggle to manage dehydration as adults, so they'll go into diapause during dry seasons and emerge as adults in the wet seasons.

Like the other comments say, we *think* most bugs die to predation rather than old age, but we (zoologists/ entomologists) really have no idea.

Its incredibly difficult to tell because its basically impossible to track individual bugs in the wild. Most of what we know about the lifespan of animals in the wild comes from mark-recapture experiments, where a large number of animals are captured, marked with something like a leg band or other tag, and then some amount of time later another survey is conducted to see how many tagged animals are re-collected. This works really well for populations of vertebrates and gives us a pretty good estimation of how long animals live, based on the dates on the tags or other long term records. However, as far as I know, its virtually impossible to actually do this with insects (right now, at least). Dobzhansky tried a couple times with wild fruit fly populations but never got anything conclusive, and that's the most well known attempt I know of.

In regards to lifespan, it can be highly variable, and depends on how you define lifespan. Most insects typically live actively for a couple days to a couple years, but some insects can be alive for much longer. Insects can enter a state of hibernation called diapause where they remain in stasis for years at a time, like cicadas which famously hibernate for 17 years. Depending on how you define lifespan, there are insects that are technically alive for decades, but they spend the vast majority of it in diapause in their cocoons and are only active adults for a couple days/weeks/months at most.

Ok actual geneticist/ evolutionary biologist here: The two numbers (30,000 and 3 million) refer to different things.

The human genome is made up of about 3 billion nucleotides, like a string with 3 billion beads on it. Each of those nucleotides can be one of 4 variants: A, T, G or C - like having 4 different colours of beads.

There's inherent variation in the genome, different people will have different variants in different orders, so everyone has a unique string of nucleotides. However, that variation is not evenly distributed. There are some positions that are "polymorphic" (variable) where at a specific position different people have different variants, and there are positions that are "fixed" where virtually every person has the same letter at that position.

When we compare two people or two individuals of the same species we compare polymorphic sites, so if you pick two random people, on average 3 million of those 3 billion nucleotides at polymorphic sites will be different from each other.

When we compare species, we compare fixed sites. Those 30,000 differences between humans and neanderthals means that there are 30,000 sites in the genome where virtually all humans have one nucleotide, and virtually all neanderthals have a different nucleotide (eg, virtually all humans have a C while virtually all neanderthals have an A).