Large hog barns that use liquid slurry manure systems apply large amounts to fields close to the barns. This increases the worm population by over 5 fold. It also forces the worms to the surface. Cluster fly infestations are much more likely where liquid manure slurry is being applied to fields close by. Also the farm fields being fertilized have no pollen so the flies move to adjacent acres that have flowers.
Ive been trading for 2 years, last year I did one cluster fly job and have had a couple this year. Ive been using maxi foggers from 1env. Seems to work nicely.