plant louse

There are many plant louses on the roses.→There are many plant lice on the roses.

“plant louse”的复数形式是“plant lice”，不是“plant louses”。英语中“louse”的复数不规则变化为“lice”，因此“plant louse”的复数也应遵循此规则。

I saw plant louse on the leaf.→I saw a plant louse on the leaf.

“plant louse”是可数名词，单数形式前需加冠词“a”或“the”。漏掉冠词是常见错误，因为中文中可数名词可以不带量词。