Bidens is a genus of flowering plants in the aster family, Asteraceae. The common names beggarticks, black jack, burr marigolds, cobbler's pegs, Spanish needles, stickseeds, tickseeds and tickseed sunflowers refer to the fruits of the plants, most of which are bristly and barbed, with two sharp pappi at the end. The generic name refers to the same character; Bidens comes from the Latin bis ("two") and dens ("tooth").
The plants are zoochorous; their seeds will stick to clothing, fur or feathers, and be carried to new habitat. This has enabled them to colonize a wide range, including many oceanic islands. Some of these species occur only in a very restricted range and several are now threatened with extinction, notably in the Hawaiian Islands. Due to the absence of native mammals on these islands, some of the oceanic island taxa have reduced burrs, evolving features that seem to aid in dispersal by the wind instead.
Bidens is distributed throughout the tropical and warm temperate regions of the world. Most species occur in the Americas, Africa, and Polynesia, and there are some in Europe and Asia.Bidens is closely related to the American genus Coreopsis, and the genera are sometimes difficult to tell apart; in addition, neither is monophyletic.
On the Hawaiian Islands, Bidens are called kokoʻolau or koʻokoʻolau.
Nodding beggarticks (B. cernua) and hairy beggarticks (B. pilosa) are useful as honey plants. Several Bidens species are used as food by the caterpillars of certain Lepidoptera, such as the noctuid moth Hypercompe hambletoni and the brush-footed butterfly Vanessa cardui, the Painted Lady. The Bidens mottle virus, a plant pathogen, was first isolated from B. pilosa, and it infects many other Asteraceae and plants of other families.
The taxonomy of Bidens has been described as "chaotic", and it is not clear how many taxa are included in its bounds. There are probably at least 150 to 250 species, and some estimates fall around 230.