If I were in your situation, here is what I would do.
- Hire a security guard. Pay him minimum wage, pay him as much as your normal employees, whatever. That's not the main issue here. What you need is someone on the premises who will watch over at least one or two of the fields. Have him do regular rounds. If he sees someone on the fields or acting suspiciously (i.e. driving around the outside of the camp, or something like that) then he needs to call the cops.
- Either buy your own security system, or hire someone. If you're buying, make sure that the cameras do not rely on storing hard copies and record to some sort of digital medium that can be retrieved even if the camera is destroyed. This is essential. Buy some motion detectors and set them up at key entry points to the field. Contact your local police station and ask them what security systems that they would recommend and how to set them up so that they either alert you (the owner) or the police themselves, so that response can be rapid.
- Contact your insurance agency and let them know what's going on. Ask them about your policy and if it covers vandalism. If it doesn't, find a company that does cover it. You don't want to be out of commission for any longer than you have to, and insurance monies is your best last resort: if all else fails, then you need to be able to rebuild quickly.
- Get the community involved, not just the police. Make sure the police and the community know what is going on. Get them active. Offer discounts to the police and try and get them invested in your field's cause. Offer discounts to the community and get them invested in it, too. The more support you have behind you, the better.