• Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • Moths fly in large numbers during spring and early summer. They do not feed.
  • Each moth can lay up to 3000 eggs which are scattered over the pasture surface
  • Most eggs and very young caterpillars do not survive
  • Caterpillars initially live in silk tunnels on soil surface and construct burrows, up to 30 cm deep, as they grow larger from which they emerge at night to feed
  • On average the caterpillars each moult 7-8 times
  • Caterpillars and the damage they cause are most noticable from April until September
  • Caterpillars appear dirty white with grey back and a deep brown head. When handled they will regurgitate a dark brown substance.

  • Porina are a complex of caterpillars occuring throughout New Zealand but damaging populations are uncommon in the northern half of the North Island. The moths fly in spring, summer, and early autumn depending on location and species present. The moths don’t feed and live only a few days. During these flights female moths may lay up to 3000 eggs scattered over the surface of pasture or grassy areas. The eggs hatch in 10 to 21 days and the young caterpillars construct silk lined casings on the soil surface. As the caterpillars grow they construct permanent burrows in the soil which eventually reach a depth of about 250 mm. They emerge from their burrows at night to feed, severing grass and clover leaves at the base of plants and dragging these back to their burrows where they are eaten. The caterpillars are reluctant to venture far from their burrows and pasture damage initially appears as 5 to 6 cm in diameter bare areas around burrows. As food is depleted they move further from their burrows and if sufficient caterpillars are present large areas of pasture are damaged or destroyed. Development from egg to adult takes 12 months. For those species studied, eight to nine instars (stages between moults) appears to be usual.
  • There are seven porina species (Wiseana cervinata, W.copularis, W. fuliginea, W. jocosa, W. mimica, W. umbraculata and W. signata), 3 of which comprise 2 sub species each, and they are difficult to distinguish from each other. Although traditionally porina have been viewed as a species complex that can be treated as one species, this is no longer considered a valid approach. For pest management purposes it is important for a farmer to know whether spring , summer or autumn flying species predominate on their property.
  • The distributions of the species overlap and as many as four species have been recorded co-existing in pasture. Although, with experience, moths can be visually identified to species the caterpillars cannot and DNA technology must be used. Based on adult flights W. cervinata and W. copularis are traditionally thought to be the major pest species and molecular identification of caterpillars suggests W. copularis (which has early and late flying sub species) to be the most widespread and probably the more significant of the two. It is likely that most species could cause local or occasional damage but W. signata and W. umbraculata appear uncommon in improved pasture. The most significant difference between species from a pest point of view is the timing of moth flights and therefore the timing of caterpillar development. W. cervinata moths emerge and lay eggs in spring and are seldom seen after December. W. copularis moths may also be present in spring but their main emergence in the South Island is in January and in the central and lower North Island in late February early March. In Westland autumn flights of W. copularis also occur and there is some evidence that, currently minor, autumn flights may also occur in the southern South Island. Several months can separate the main flights in any one area. This means that the development of caterpillars of early flying moths may be three months ahead of caterpillers from late flying moths. While this will have minimal effect on the effectiveness of organophosphate insecticides it is an important consideration when applying diflubenzuron which is most effective against early larval stages.

Natural population regulation

  • Porina occur in most pastures most of the time. Whether or not they attain pest status is largely governed by weather over the time eggs hatch and while young larvae are living on the surface, and natural occurrence of diseases. Porina are prone to several diseases which can only persist if porina are present. Porina outbreaks, and therefore damage to pastures, usually occur when the porina/disease association is disrupted. Disruption occurs when porina numbers are too low to enable disease organisms to persist. Commonly this is a result of cultivation but may also be due to drought, occasionally insecticide use and in high rainfall areas, such as Westland, very high soil moisture. Multiple dry or wet years may exacerbate later damage. In the years following disruption, porina populations increase and cause pasture damage, before disease organisms build up enough to reduce porina numbers to below damaging levels. The outbreak most often occurs 2-3 years after the disruption so damage to young pastures, established using cultivation, typically occurs in their second and third winter although 1 and 4 year old pastures may also be hit. Pasture renovation by direct drilling favours retention of porina diseases in the soil reducing likelihood of damage. When the porina outbreaks are the result of drought, pastures of any age can be affected.

  • Porina are grazers. At low densities they are direct competitors with stock for food and reduce the amount of foliage available. At higher densities plants are destroyed allowing inferior plant species to establish reducing long term pasture quality and production.
  • Depending on pasture productivity:
    • 25 – 50 porina/m2 (1-2 per 8 inch spade square) equate to grazing 1 ewe/ha. Plants are seldom destroyed at this level but production losses may occur
    • 50 – 75 porina/m2 (2-3 per spade) will destroy some plants, open up pasture and allow weed species to establish. Pasture production will be reduced
    • 100 or more porina/m2 (4 or more per spade) will result in major pasture damage. Pasture plants will be destroyed, bare areas in pasture will develop and weed species may establish. The amount of pasture produced and the quality of that pasture will be reduced.

Pastures at risk

  • Awareness and early recognition of damage will avoid severe pasture loss and prolong pasture life. The earlier control measures are implemented the greater the economic return. Often the presence of caterpillars from early moth flights is not noticed until May onwards, and from autumn flights as late as July, by which time, in both cases, considerable damage has occurred
  • Porina are present in most pastures – but not always at damaging levels
  • Porina numbers are largely governed by summer weather and association with diseases
  • Outbreaks and damage occur when the porina/disease association is disrupted
  • This can result from cultivation, hot dry summers occasionally insecticide use and very high soil moisture
  • Damage often occurs 1-4 years after cultivation but most often in 2nd or 3rd winter
  • In porina prone areas, 2-3 year old pastures are at particular risk of attack
  • After drought, or very wet years, pastures of any age can be affected and damage may be widespread. Damage often shows 1-3 years after drought
  • Long or dense pasture in summer favours egg and young caterpillar survival, this makes young pastures used for hay production vulnerable
  • May be multiple main flights of porina
  • Spring flying moths – caterpillars cause damage from autumn on
  • Summer flying moths – caterpillars cause damage in winter, spring and early summer. Feeding can often be masked by spring growth
  • Autumn flying moths – caterpillars cause damage in winter, spring and early summer. Feeding can often be masked by spring growth

Assessing populations

In porina prone areas monitor young pastures for porina and assess porina numbers as early as possible. Timing of assessments will depend on whether spring, summer orautumn flying species are predominant. Some options are:

If spring flying species are mainly present:

Summer/ Early Autumn

  • Caterpillars are small (less than 25 mm), live on the soil surface or in small burrows and are very difficult to find
  • Take turves 200 x 200 mm (standard spade spits) x 30 mm deep at least 10 per pasture, invert under a heat source over a container of water overnight, porina will drop into the water where they can be counted and may be very small.

Early / Mid Autumn

  • Dig and hand sort soil, ten spade spits per paddock
  • Use insecticide to indicate pasture loss – Apply insecticide to a strip (2 x 20 m) of pasture where it can be easily seen. If strip appears greener than surrounding pasture after 3-4 weeks a pest is causing damage. Dig and sort soil to determine if that pest is porina.

Autumn onwards

  • Dig and hand sort soil, up to ten spade spits per paddock (if high numbers are present less will do).

Where January species occur:

Early-Mid Autumn

  • A mixture of sizes from early and late flying moths may be present
  • Sample as for small caterpillars above.

Mid Autumn

  • Dig and hand sort soil, large larvae will be causing damage
  • Small larvae found in late autumn and early winter may cause damage into the spring and early summer but this can be hidden by increased pasture growth at this time
  • Densities greater than 2 per spade square (50/m2) will cause damage (see Impact).

Where February/March flying species occur:

(lower and central North Island, Westland)

Late Autumn

  • A mixture of sizes from early and late flying moths may be present
  • Sample as for small caterpillars above.

Early Winter

  • Dig and hand sort soil, large larvae will be causing damage
  • Small larvae found in late autumn and early winter may cause damage into the spring and early summer but this can be hidden by increased pasture growth at this time
  • Densities greater than 2 per spade square (50/m2) will cause damage (see Impact).

Chemical Control

Treat pastures as soon as sampling indicates a damaging porina population is present or damage is noticed Registered chemicals for porina control are:

Active ingredient Action When to apply
Diazinon Contact and ingestion Any time caterpillars are present
Chlorpyrifos Contact, fumigant and ingestion Any time caterpillars are present
Diflubenzuron Must be ingested, no contact activity. Disrupts caterpillar moulting. Must be applied when caterpillars are small, 8 – 10 weeks after moth flights

Consult your farm consultant, industry representative or the New Zealand Agrichemical Manual for more information about chemical control.

Biological control

There is currently no biological control option for porina.


  • Survival of porina eggs and young larvae are reduced by dry conditions
  • Dry hot summers can keep numbers low – Heavy stocking and close grazing in summer can reduce survival
  • Porina are favoured by mild wet summers and long or dense pasture cover – young pastures shut up for hay often affected
  • Cultivation predisposes pastures to damage 2-3 years later
  • Direct drilling retains porina diseases in soil reducing likelihood of damage
  • AR37 ryegrasses, cocksfoot and tall fescue are not palatable to porina – feeding may focus on clovers growing with these grasses.


  • Grass Grub and Porina in Otago and Southland. A guide to management and control. BIP Barratt, van Toor RF, Ferguson CM and Stewart KM. 1990.