The conservationist quoted in the article said "likely killed", but as I explained earlier, milkweed doesn't grow in Roundup Ready alfalfa, corn, or cotton in California, hence the milkweed hasn't been actually been destroyed.
However, herbicides definately have contributed to the decline in milkweed abundance in California. But, ironically, the conservationists have not objected to the specific applications that have caused the damage. Examples of applications conservationists have not objected to:
1) Greatly increased herbicide use along railway lines to improve railroad safety.
2) Greatly increased herbicide use along roadsides to improve road shoulder safety.
Ditto tremendously increased mechanical mowing along California's roadsides to widen the road
shoulder to improve highway safety including wildfire control. Decades ago, conservationists would be willing to sacrifice their safety to a minor degree for the sake of the environment, but not today
Conservationists also havn't objected to recently increased mechanical control of weeds and shrubs along the margins of organic crops following the deadly E. coli contamination problem of organic spinach last year in California. The purpose of this extra mechanical weed control is to destroy habitat for coyotes, skunks, birds and other wild animals that could contaminate the organic crops with deadly E. coli bacteria. Decades ago, conservationists would be willing to sacrifice their health to a minor degree for the sake of the environment, but not today.
The attitudes of conservationists also have taken their toll on California's milkweed. Example: for 30 years conservationists have opposed building new dams. So farmers are forced to conserve water which means the milkweed growing along irrigation ditches is drying up and dissappearing or it is so water stressed, female monarchs won't lay eggs on it.
Todays conservationists also seem to be "death blind" when it comes to appreciating the amount of milkweed permanenently lost due to the accelerating pace of development in California. Example: they have not voiced any objection to the fact that the average home built in California in the 2000's was twice as big as the average home built in the 1950's and 1960's.