Garden is – black beetles
Our gardenia is covered with big black beetles. They have a smell similar to the gardenia but its offensive because it smells so strong. They come once a year. Will they damage the Bush and what are they? In the past I have removed them by hand but this year there are too many of them.
Certified GKH Gardening Expert
It sounds like this is Thrips. Thrips can overwinter in the plant debris so keep the area clean.
Neem Oil can be used to treat your plant.
Here are some links with more infomation.