Ageratum houstonianum (flossflower, bluemink, blueweed, pussy foot, Mexican paintbrush) syn. Ageratum mexicanum Hort. 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.
The plant is native to Central America and adjacent parts of Mexico, but has become an invasive weed in other areas. 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 in insects. This chemical triggers the next moulting cycle to prematurely develop adult structures, and can render most insects sterile if ingested in large enough quantities.
Ageratum houstonianum is toxic to grazing animals, causing liver damage. It contains pyrrolizidine alkaloids. It is prone to becoming a rampant environmental weed when grown outside of its natural range. It has become an invasive weed in the eastern USA, Australia, Europe, Africa, China, Japan and New Zealand.
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