How did Colonel Lloyd keep the slave boys from taking his fruit?

From the text:

The colonel had to resort to all kinds of stratagems to keep his slaves out of the garden. The last and most successful one was that of tarring his fence all around; after which, if a slave was caught with any tar upon his person, it was deemed sufficient proof that he had either been into the garden, or had tried to get in. In either case, he was severely whipped by the chief gardener.