ND4SPD

Let alone get a decent shot off on it. Unless you are a serious marksman, you will not hit a small quick moving target with an air rifle. Oh, and have you even IDENTIFIED what you are looking for? You said they burrowed under your fence. Try putting the fence... below ground a foot or two, or even more intelligently building a wooden box underneath the bed so they can't burrow underneath.

SBTT

Beeman markets Theoben Rifles (UK Mfg). Outstanding craftsmanship and accuracy.
You might consider contacting them directly to save a few $.
Recommended calibre: .22
Otherwise, for a more economical (and faster purchase) choice see my reply above to your original post.

SBTT

An Economical Recommendation:
Daisy Model 131 or Gamo Model 440
Break-Barrel / Single-Shot w/
- Minimum 9X Scope and Rings
- .177 Pointed Field Pellets
Whatever your choice, minimum Muzzle Velocity (Feet Per Second) should not be under 600FPS.
Good luck with your Pest Control and sorry about your crops.

Disclaimer:
Although not a firearm, this recommendation assumes the recipient to have (at minimum) Basic Firearm Safety Training. Always remenber, the first and most important safety feature of any gun is the shooter.