Commentary for Kiddushin 76:9

Ulla argued that outside the Land, orlah is a halakhah given to Moses at Sinai, a relatively high status, whereas kilayim is only a simple rabbinic prohibition. This helps us make sense of why the rules governing orlah are stricter than those governing kilayim. Outside of Israel, a Jew may buy orlah and kilayim from a Gentile, but with regard to the former, he cannot see him gather the fruit, whereas with the latter, the only prohibition is for the Jew to gather the kilayim vegetables himself. He can see the Gentile gather them.