I've lived all over this country (USA), while I was growing up and then after becoming an adult. When I was much younger, while in Southern California (2nd grade), I first spotted a Monarch chrysalis, about twenty feet up, in the crotch of a large ******** tree (Eucalyptus). I plucked it, along with the piece of bark it was attached to, and placed it in a glass fish bowl, where I observed it emerge. I was hooked. I asked my teacher about them, and she referred me to the library, where I learned much more. I soon was able to locate tiger caterpillars on milkweed plants that were growing on the edges of our school playground.
I've observed Monarch's and milkweed in many parts of the USA. Here in Southern Arizona (the desert southwest), there are only two species of milkweed that are native, Asclepias subulata and Asclepias albicans. There is also an invasive species of milkweed present, a vining one. These native milkweed species are rather restricted in their ranges, though many have been planted by the city of Tucson and Pima county in the landscaping associated with public roads in our area. I'm sure this is an effort to help increase habitat for Monarch and Queen butterflies. It is curious that the two native milkweed species only rarely have leaves, the plants are more usually just a roughly vase shaped collection of upright blue-green stems (2-3 feet high), with clusters of yellow-white flowers, then more typical milkweed seedpods and airborne seeds. When they do have leaves, the leaves are tiny and don't last long.
The only weed spraying I've noticed, is when they spot spray weeds that have invaded this landscaping (they use dye in the herbicide, so it's easy to notice when it's recently used).
In my flower garden, I have sweet clover, alfalfa, sainfoin, holy basil, moringa trees, butterfly bush (Buddleia davidii) and many different native and non-native milkweed species.
Usually there are many different kinds of butterflies, flies, bees (including honey bees), grasshoppers, leaf hoppers, praying mantids, walking sticks, hover flies, dragonflies, and many other insects, frequenting my small flower garden (most are planted in large drip irrigated pots). I have never used any pesticides in my garden. I even remove aphids and other pests, by hand, or with a small spray bottle of water.
I plan to ask the local extension office, to determine what they are using on the alfalfa. I suspect the alfalfa, since there has always been some alfalfa planted in this area, but only in the past 2-3 years has the acreage planted in it, skyrocketed. And, around here some of it is in bloom, every month of the year. So, in the winter months, it would be a major draw for insects that feed on nectar/pollen.
Here's to continued hope that this is simply a natural and cyclical issue, and nothing more. In the next few days, when I'm in town, I plan to stop at some of the local plant nurseries to see what insects are visiting their flowering plant, if any. I've done this often, in the past, and was always rewarded by the presence of many nectar/pollen foraging insects.