Culex is a mosquito not a fly . There are several method to collect they are divided into two major types of collection Adult collection and immature collections in Adult there are hand catch method using aspirators for resting mosquitoes but for fields one can use light traps the methods of mosquito collection can be obtained from following document

Reference

World Health Organization. Manual on practical entomology in malaria. Part II. Methods and techniques. Manual on practical entomology in malaria. Part II. Methods and techniques. 1975. p. 6+191 pp.

For controlling them the larval control is required in crop field Bti and gambusia fishes and insecticides of chemical nature should be avoided in case adults creating nuisance then fogging is advisable