If you want spikey defense, pick up some Agave. Worst spikey plant ever and if they chop it, it gets even worse with severe contact dermatitis. Realistically, a high deer fence is your safest bet. A dog can alert you to their presence but if you don't have a kennel in that corner odds are they will never know someone is there. 16 acres is a lot of area for a dog to patrol. Its hard to shoot someone if you don't know they are there, and even with the laws in your favor, good luck on shooting someone for stealing your fruit and getting away with it. The libtards would be all over you with how vicious you were in brutally shooting a poor man simply trying to get a piece of fruit from a tree to feed his poor family, they will ignore the facts of the case and you will lose instantly in the court of public opinion.
Criminals stealing fruit will be much less likely to enter your property if it is secured with a high fence, perhaps with concertina on the top. That and they would have a hard time justifying how they were injured on your fence in court. I cant think of any real other defense that would work in your current position. Its expensive but only a hardened bad guy is going to break through it. Perhaps some security cameras on that corner of the yard.