What are those stickers?
PLU codes are 4 or 5 digit numbers which have been used by supermarkets since 1990 to make check-out and inventory control easier, faster, and more accurate. They ensure that the correct price is paid by consumers by removing the need for cashiers to identify the product; e.g., whether or not it is conventionally or organically grown. They are primarily assigned to identify individual bulk fresh produce (and related items such as nuts and herbs) and will appear on a small sticker applied to the individual piece of fresh produce. The PLU number identifies produce items based upon various attributes which can include the commodity, the variety, the growing methodology (e.g., organic) and the size group.
What do they mean?
These codes typically begin with a 3 or 4 and indicate that your fruit was grown conventionally, which includes the presence of pesticides. Don’t panic, though: the pesticide levels are so low they aren’t harmful. Source
Are they GMO?
If the label’s code has five digits and begins with an 8, then the fruit or vegetable has been genetically modified (also known as GMOs, or genetically modified organisms). Twenty-six countries currently ban or have strict restrictions on GMO produce (Poland and Italy signed on to ban Monsanto GMO corn in 2013). Source
For more reading, including the Dirty Dozen and Clean Fifteen, head over to the EWG here.