Monarch butteflies need milkweed to feed on and to lay their eggs. In recent years, herbicides have destroyed milkweed, and with it the population of monarch butterflies. “No milkweed, no monarchs” is a popular saying.
   
“Where there is no milkweed there are no monarchs” was cited in print in 1993. “No milkweed = no Monarchs” was cited in 1998.
 
   
Wikipedia: Monarch butterfly
The monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae. It may be the most familiar North American butterfly. Its wings feature an easily recognizable orange and black pattern, with a wingspan of 8.9–10.2 cm (3½–4 in)[3] The viceroy butterfly appears similar in color and pattern, but is markedly smaller and has an extra black stripe across the hind wing
   
Google Books 
Canadian Geographic
1993  
Pg. 43:
Where there is no milkweed there are no monarchs.
 
Google Groups: alt.wedding
NYT article on butterfly releases
J&G Wood
10/18/98
(...)
The milkweed is the plant/weed that Monarch butterflies have to have to complete their lifecycle. No milkweed = no Monarchs.
 
17 September 2005, New Castle (PA) News, “Milkweed,” pg. C3, col. 3:
The point is, no milkweed, no monarchs.
       
Turkey Hill Brook Farm
October 31, 2009
Where are the Monarchs?
Where oh where have the Monarch butterflies gone? I heard this question many times this past summer. Mostly, it seems the weather was to blame, at least in New England. Butterflies need sun and warmth in order to fly, and they need to fly to locate Milkweed plants to lay their eggs. Their wings are like little solar chargers, soaking up the sun to fuel their flight. This year’s cold and rainy weather in New England provided few opportunities for female Monarchs to fly to areas containing Milkweed plants (Asclepias species), which is the only plant that Monarch butterfly caterpillars can use as a food source.
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Remember, no Milkweed, no Monarchs. Let’s help these unique winged wonders survive for future generations to enjoy by growing Milkweeds anywhere we can!
 
Twitter
Alan Watson
‏@DietHeartNews
NY Times 7-11-11: No Milkweed & No Monarchs in 100 million acres of corn and soybean fields sprayed with Roundup.
6:20 PM - 17 Sep 2011
   
Google Books
Poison Spring:
The Secret History of Pollution and the EPA
By E.G. Vallianatos with McKay Jenkin
New York, NY: Bloomsbury Press
2014
Pg. 211:
Herbicides kill milkweeds, where monarch butterflies both feed and lay their eggs. No milkweed, no monarchs. It’s as simple as that.
 
Twitter
NatureCity
‏@Nature_City
No milkweed, no monarchs: Crop expansion + herbicides contributing to Monarch decline http://www.theglobeandmail.com/technology/science/monarch-butterflies-threatened-by-gm-crops-in-us-study-says/article18994894/ …
8:52 PM - 5 Jun 2014
 
YouTube
No Milkweed, No Monarchs
Nebraska Pheasants Forever
Published on Mar 16, 2015
In this habitat tip, we discuss the importance of milkweed to the monarch butterfly population, as well as six different milkweed species that are commercially available to put in your next habitat project..
   
News & Record (Greensboro, NC)
From Our Gardens: Attracting butterflies is easy
Posted: Sunday, May 17, 2015 2:53 pm
By Dennis Burnette Special to News & Record
(...)
Most butterfly species must have a particular type of plant for their caterpillars to eat. The most famous example of this is that monarch butterfly caterpillars must have milkweed to eat. You might have heard the old saying, “No milkweed, no monarchs,” and it’s true. The caterpillars can’t live on anything else.