Littering with ポイ捨て
ポイ捨て is short for ポイと捨てる. It refers to littering small pieces of trash in public places such as streets and parks.
There is a connotation of throwing the trash without thinking about it much, so the term is often used with small pieces of trash, such as drink cans, plastic bottles, candy wrappers, and shopping bags.
Further reading
ポイ捨て|俗語 (edewakaru.com)
Examples
ふだつきのキョーコちゃん » Volume 2 » Page 47