In southern England, adult Longitarsus nigerrimus spend most of their lives in moss and tussocks in boggy areas.
If these tussocks are pushed under water, the beetles readily float up to the surface.
Although most flea-beetles flounder if they land on water, Longitarsus nigerrimus will happily sit upright on the water’s surface.
The beetles use their front two pairs of legs to swim breast-stroke style. They kick backwards with their hind legs to propel themselves forwards. However, they cannot swim underwater.
In mid to late July, as water levels fall, adult beetles sit on the moss surface or crawl up plant stems.