Theres not enough of them in the game :sadvaultboy: . I liked how in F3 when you would go to a place noone has been in ages, it would be full of roaches. F:NV has roaches AND mantis, I think they could have placed them at alot more places. Like the police station near primm, it had alot of giant mantis in it.
Killing them is not a problem, especially the mantis, you can just walk over them and kill some of them, i think the nymphs. Its about the environment.