Frequently Asked Questions

Could didymo spread to lakes throughout New Zealand?

Yes. Although didymo prefers a river environment with clear water flowing over stable substrate at depths sufficient for light penetration to drive photosynthesis, blooms have been found in South Island Lakes. Lake Wakatipu, for example, contains visible didymo on some parts of the lake shore.

If PCN is found in pre-harvest survey of site 7, and if the grower has used a sprayer on 4 and 6, what is their PCN status?

Sites 4 and 6 are still eligible for export, but cleaning procedures must be put into immediate effect for any machinery moving from site 7. The reason for this is that PCN cysts are stimulated to hatch by young root growth. Any cysts transferred by clods of soil to sites 4 & 6 would be unlikely to emerge and present a significant risk in the current export season. If 4 &6 are registered for Taiwan, all machinery will have been cleaned between sites.

How long does it take after contamination for didymo to be visible in a river?

A number of environmental factors influence how long it will take for didymo to bloom after introduction to a waterway, or whether it will bloom at all, but the process is not well understood. It seems to vary according to river conditions, in some rivers it is visible fairly quickly and in others it is only visible under a microscope.

Why is dog tail docking and dew claw removal allowed only in puppies that are under four days old?

The National Animal Welfare Advisory Committee (NAWAC) considered, on the available evidence, that the amount of pain that a dog experiences when its tail is docked is reduced if the puppy is of a very young age. At this age, some of the connections in the puppy’s brain which carry signals to let the puppy experience pain are not yet fully developed and scientific evidence suggests that, at this age, the puppy does not experience pain as it would if the tail was removed in an older dog. 

Please note this information is provided by way of general guidance only and does not constitute legal advice. Parties are advised to seek independent legal advice in relation to particular fact situations.

I have heard that certain areas are low risk. If didymo is unlikely to form, why bother cleaning?

Low risk doesn't mean no risk. Low risk areas may not have big blooms but if people don't clean they risk spreading didymo to other waterways. Rivers that are suitable for recreational activities such as fishing and kayaking, also tend to be those that are suitable for establishment of didymo. In its microscopic form it is hard to detect so we need everyone to treat all waterways as if they are affected.

Is it cheaper to register all paddocks as one production site, or as several production sites?

In most cases the costs of taking samples or fork sampling will be the same, as 100 soil cores/4 Ha are required. However a separate laboratory test for PCN is required for each production site. Growers need to balance costs against the consequences of PCN being found on a production site (and it being rejected from the programme).

In the diagram in Question 1 (above), a grower could register paddocks 4,6, and 7 as one production site. If PCN is found in paddock 7 all three paddocks are removed from the programme (and may not be re-entered in subsequent years). If they are registered as 3 production sites, only paddock 7 would be removed.

What is MAF doing about avian influenza?

New Zealand is well prepared to respond to an outbreak of avian influenza. MAF has comprehensive response plans and policies for highly pathogenic avian influenza or any H5 and H7 subtypes of avian influenza as well as any other exotic strains of avian influenza of regulatory concern.

As a member of the World Organisation for Animal Health (OIE), New Zealand is expected to conduct surveillance to demonstrate our freedom from highly pathogenic notifiable avian influenza (HPNAI).

Highly pathogenic notifiable avian influenza and all H5 or H7 subtypes of avian influenza virus are classed under the Biosecurity Act as a notifiable exotic disease in New Zealand. This means that any suspect case must be notified to MAF immediately.

How long has Tuber brumale been here?

We don't know. Oaks and hazels have, since European settlement, been brought in from Europe and the tools to detect the presence of T. brumale on tree roots have only become sophisticated in recent years. It is possible that T. brumale arrived long ago, but we know for certain that it is present in trees planted in the mid 1990s, and potentially other trees as well.

Are these Argentine ants in my region yet?

Argentine ants were first detected as an established population in Auckland in1990. They have since spread quickly and are now present in many North Island cities and in two South Island locations. (offsite link to

Argentine ants are often found closely associated with humans and human activities - e.g., in houses, gardens, plant nurseries and industrial areas. A recent study [WARD, ET AL .PDF] found human-mediated dispersal is primarily responsible for the spread of Argentine ants in New Zealand.

These ants hitch a ride in vehicles (including aircraft) and on transported materials of all kinds. They have the potential to spread throughout much of the North Island and northern South Island, as well as some South Island cities. (offsite link to

What is the lifecycle of didymo and could it affect the spread?

Didymo populations grow mainly by asexual cell division. Sexual reproduction is likely, but not yet observed in didymo. The presence of a dormant or resting stage has never been reported. Little is known about which factors trigger asexual versus sexual cell division. Nor is it known what role the mucilaginous stalks have on the lifecycle of didymo especially compared to the more common diatoms which do not produce such extreme amounts of mucilage. It is likely the mucilage protects individual didymo cells from short term dehydration and therefore could aid in dispersal. Free-floating cells of other species of freshwater diatoms have been found to survive only a few hours when completely out of water, but if kept moist, they can survive longer.