I've read that holes in grapefruits can be caused by birds or pack rats. However, my tree does not have the extent to be reachable by pack rats, and the holes do not look like bird holes. The tree itself is about 6 foot high and slightly wider in the middle. It looks like somebody drilled a hole with a drill about 3/8 inch or so in diameter. This is happening to every grapefruit around the time that it turns yellow. Another site claimed it could be insect damage, and added "insect damage of this type requires no control." I certainly disagree as without control every grapefruit is going to be ruined!
How can I deal with this problem?
Update: Here is a picture of what they typically look like. I didn't measure the depth of the whole but they seem to be between half an inch and one inch. In one case there was a large brown bruise-like ring and the interior hole had some kind of flying bugs in it. This particular one had no noticable bugs:
To test the theory of whether it is birds or something else I am leaving both pieces of fruit on the ground to see if they are attacked further. Hopefully this will attract whatever it is so it will leave the tree alone.
Update: This weekend I found a large hole in my bird netting, and a ruined grapefruit on the ground outside the net. I came out later in the day and startled a roadrunner near the tree. Given its long beak, size, and tendency to walk along the ground I'm now thinking that it may be the culprit - although it seems strange that it would like grapefruit instead of lizards. I put up a temporary metal fence around the tree using rabbit wire, so we will see if that fixes the problem for good. However it would be nice to find a better solution, short of attracting coyotes which in turn would be trying to eat the kids. ;'-(