Risk Background

Didymosphenia geminata belongs to the Bacillariophyta (diatoms), which are characterised by possessing silica cell walls, which are intricately patterned. Didymosphenia populations grow by vegetative cell division (asexual) with occasional sexual reproduction to exchange genetic material. However, little is known about which factors trigger asexual versus sexual cell division and whether there is a dormant or resting stage which could promote the spread (NZ Biosecurity 2006). Didymo is made up of cells that cannot be seen with the naked eye until a large colony is formed. A single live cell is all that’s required for Didymo to establish and spread (NZ Biosecurity 2006).

The overall risk of entry, establishment, spread and associated consequences of importing ‘Didymo’ via used water craft and fishing equipment is considered to be unacceptable and risk management measures are therefore required.

The department's Pest Risk Analysis on this pest was released in July 2008, and the control measures outlined in this BICON case have been adjusted to incorporate that Agency’s recommendations to reduce the risk of entry, establishment, spread and associated consequences of this diatom to an acceptably low level.