Nectar is used to turn Pikmin from leaf or bud stages and into the flower stage. There are numerous places where nectar is found; Eggs, the bugs in the Wistful Wild, Iridescent Flint Beetles and relatives, Nectar Weed patches, piles of rocky Rubble, Mitites, Honeywisps, and yellow Unmarked Spectralids can all drop globs of nectar. In Pikmin 1, the Pikmin can only drink it if the blob is at "full size".
There is glitch that occurs if a drop of nectar, such as one from a petrified enemy, falls on top of a pile of sticks used to build a bridge. If the sticks are removed from under the drop by building the bridge, the drop will remain suspended in the air. Pikmin can be made to drink the nectar by throwing them at it, and they will fall back to the ground once the nectar is all used up.
