Family AsteraceaeThe plant is native to Central America and adjacent parts of Mexico, but has become an invasive weed in other areas.
Ageratum houstonianum is a cool-season annual plant often grown as bedding in gardens. The plant grows to 0.3–1 m high, with ovate to triangular leaves 2–7 cm long, and blue flowers (sometimes white, pink, or purple). The flower heads are borne in dense corymbs. The ray flowers are threadlike, leading to the common name.
Ageratum has evolved an ingenious method of protecting itself from insects; it produces a precocene compound which interferes with the normal function of the corpus allatum, the organ responsible for secreting juvenile hormone. This chemical triggers the next molting cycle to prematurely develop adult structures, and can render most insects sterile if ingested in large enough quantities.