I know it is an old topic, but very interesting.
I work as a researcher for Lepidoptera (butterflies) in a national park. The decrease in butterflies and bees are due to several factors. Intensive agriculture, insecticides and most likely climate change, among other things.
Butterflies and bees consume a large amount of energy when they fly, which is why they need to suck every little time to move. If there are no wildflowers to feed, they just die.
It would be very important that every few meters there is a "free" space for intensive crops and dedicated only to wild flowers. So they can stop, feed and go on their way.
I cannot explain the whole process in words, as my level of English is very basic, but there is a very good report from the BBC, which explains it clearly.
The large grassy parks are very nice, but are usually cleared of weeds. These weeds and their flowers are what feed both bees and butterflies, which are great pollinators..
As you can see, there are many factors that contribute to their disappearance from our environment. If each of us leaves a small space in our orchards or gardens, so that "weeds" come out and that is also free, we would contribute to create that corridor, so that they feed, pollinate our gardens and continue on their way.