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Yes, they can be very numerous. Reminds me of the time I ran a Malaise trap next to a G.viridis tree. When I set the trap there were 4-5 flies on the tree - when I collected the trap 3 days later there were about 10 flies in the trap and 4-5 on the tree. This pattern went on for 2 weeks - I always caught plenty but there were always 4-5 more on the tree.

Needless to say, I wasn't trapping for G.viridis - they were just unfortunate to be in my trapping area. The population wasn't affected in subsequent years - i just think flies are less territorial and far more mobile in the countryside than most people think.