Answer:
Most native born Americans are unwilling to pick oranges because the job is pays very little, and is very hard. Most Americans either have available other jobs that are comparitively easier, or that pay at least a bit better, or they simply choose not to do that job due to the fact that hey have lived under a higher living standard than your average immigrant.
Immigrants are a whole different story. Most of them come from countries that are poorer than the United States, and many of them carried out similar jobs in their home countries for even lower pay and under more difficult conditions. Their willingless to work does pose challenges to them though, for example, they are often hired for temporal work, with no benefits, without legal permission, and are made to work for longer hours than it is legally mandated.