Q. Have weeds been observed to develop resistance against paraquat as there has been reported for glyphosate, for example?

A. Yes. Like many other herbicides, some resistance to paraquat has been identified.
However the rapid development of crops resistant to glyphosate (another common herbicide) is stimulating the demand for paraquat as we see more evidence of glyphosate-tolerant or -resistant weeds. Paraquat is an increasingly important part of Integrated Weed Management

Description

A decision support system for crop protection which focuses on long-term prevention or suppression of pest problems with minimum impact on human health, the environment, and non-target organisms. IPM takes into consideration all available pest control techniques and tactics (cultural, mechanical, biological, chemical). IPM emphasizes the growth of healthy crops for better productivity with the least possible disruption to agroecosystems.

Authoritative On-line References and Resources

http://www.ipmcenters.org "The USDA's National Site for the Regional IPM Centers' Information System provides information about US commodities, pests and pest management practices, people and issues."
(IWM) programs associated with glyphosate-tolerant crops and in other situations where glyphosate has been intensively used for long periods of time.
Surveys exist to help farmers and advisors understand the extent of herbicide resistance and plan coping strategies. Examples of these surveys can be found here: http://www.weedscience.org/
http://www.resistancefighter.com/