When many living creatures, including plants, are dying in large numbers, English natives often say, they are "dropping like flies."
Few high school graduates in Western countries are unaware of fruit fly experiments. Fruit flies have a short life span; that is why science studies them extensively. A group of fruit flies that has reached the end of that life span will result in many small, dead flies lying on their backs. This is the image created by the expression, dropping like flies.
Example: "Many elderly people have been dropping like flies during the heat wave in Russia. Please, check up on your loved ones and make sure they are safe and sound." (Safe and sound is another idiom that means "safe and in sound (good) health.")