Many insects are in decline due to the destruction of their habitat, such as frogs disappearing due to wetlands being drained, but there is also research that shows a decline where the habitat is unchanged. An example would be some Monarch butterflies who live at higher altitude where their habitat was intact and yet their population still declined as the temperature increased.
At typical scenario would be that prolonged exposure to temperatures outside the creature’s critical thermal maximum would increase deaths and push their numbers below replacement rate leading to a steady decline.
Humans are warm blooded mammals and can survive in a wide range of temperatures by adjusting diet or habitat. Insects are cold blooded and, largely do not have the luxury of travel or the ability to change diet.
There are already many studies showing a serious decline in insect numbers but most research is short term and does not have the luxury of going back 50 years or more and insects are not a glamourous subject like lions or polar bears so funding for research is scarce. It is also not helped that we spend most of our time trying to kill insects in agriculture and in the home which makes the concept of protecting insects a difficult thing to grasp.
However, insects are vital to out survival as they pollinate our crops and without them our farming of crops for food would not be possible. Living in a horticultural region we are already familiar with the practice of introducing bee hives to a kiwi fruit orchards to pollinate the fruit flowers but if we had to do it to fields of wheat the immense scale would make it impossible.
Of the 550 gigatons of biomass on Earth most are plants and bacteria, animals make up about 2 gigatons, with insects comprising half of that and fish taking up another 0.7 gigatons. Everything else, including mammals, birds, nematodes and mollusks are roughly 0.3 gigatons, with humans weighing in at 0.06 gigatons.
While humans are statistically a tiny part of the planet’s biomass, there are 8 billion of us and we are rabid devourers of the planets resources and have a profound effect on the environment by draining wetlands, keeping vast herds of animals for food, shifting millions of tonnes of rock and soil for mining, agriculture, roads and buildings. Most importantly we have already raised the temperature of the planet 1.1 C and with an additional 1.4 C or more to come.
The UN, COP27 meeting in Egypt is supposed to solve this problem but it looks very doubtful.