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Case Study

How to Identify Grass Grub Larvae

BioShield® Grass Grub liquid biopesticide.

How to Identify:

BioShield Grass Grub Liquid is only effective on New Zealand grass grub larvae at the 2nd and 3rd instar stage, when larvae are actively feeding (February to April).

There can be many different larvae species living in the soil, and it is essential that the New Zealand grass grub larvae is correctly identified over the February to April period. 

  • Development from egg to adult usually takes 1 year.
  • Root damage results mainly when grubs reach the 3rd instar stage in autumn and early winter. 
  • Under severe environmental conditions, the development of some of the grub larvae may be extended over two years. The two-year life cycle is found more commonly in Otago and Southland than elsewhere and is in response to cool soil conditions. Drought may also cause the grubs to enter a two-year life cycle.
  • Those grubs entering a two-year cycle do not reach the third stage until spring/summer causing damage in summer when sufficient numbers are present




October to November:

If grass grub beetles are flying October or November then apply BioShield Grass Grub Liquid mid Feb to mid April

  • Grass-grub beetles, or brown beetles, are the adult form of the soil dwelling grass-grub.
    They are about 10 mm long, roundish and tan coloured.
  • The beetles are only present for a few weeks in spring/early summer each year.
    This period varies in different regions.
  • They are generally not seen during the day but fly after dusk and are often attracted to lights in large numbers.
  • They have two flight phases, the first is a mating flight that occurs soon after the beetles emerge from the soil, the second consists of feeding flights and may occur for 2-3 weeks.
  • Grass-grub beetles feed on the young foliage of a wide range of plants including grape vines, kiwifruit, blueberries, cereals, brassicas, ornamental and shelter trees and bushes.
  • Damage appears as large irregular areas of leaf eaten from leaf margins.
  • Each female grass grub beetle lays about 30 eggs about 100-150 mm under the soil.  Most of the eggs are laid close to the point of emergence, thereby localising infestations within an area.  The females then fly to new site and lay the remaining few eggs.




Mid February to Mid April:

If NZ grass grub larvae populations are 100m2 to 300m2 then apply BioShield Grass Grub liquid.

  • On hatching the larvae are 5 mm long and feed on roots of a wide range of plants
    especially clover.
  • Larvae are C shaped when relaxed, have a light tan head with prominent jaws
    and a cream white body.
  • They have 6 legs and the soil in their gut is visible.
  • As they grow they pass through three larval stages.
  • From Feb – April look for 2nd and 3rd instar larvae by digging spade squares.
  • Larvae are approx. 1 -1.5cm long during this period and are generally less than 50 mm below the soil surface. 




Comparison of Larve:

Comparison of larvae species found in the soil over the Feb – Apr period

Several other beetle species resemble grass-grub beetles and their feeding can appear identical. The larvae of such beetles are very similar and expert advice may be required to correctly identify them. Below is a guide to help tell the difference between NZ grass grub larvae and other larvae species which can be found in New Zealand soils over the February to April period.

Grass Grub comparison.png
African Black Beetle

  • The African black beetle larvae will be much larger in comparison to the NZ grass grub over the February to April period.
  • It is up to 2.5 cm long versus the NZ grass grub at 1cm.
  • It has prominent breathing holes on the side which are not visible on the NZ grass grub larvae.
  • From April onward it is in a pupae stage while the NZ grass grub is in larvae stage.
  • It does not eat clover so pastures where it exists will be clover dominant. NZ grass grub eats clover first.
  • It is only found in warmer areas; North Taranaki/Waikato, BOP, Northland, Auckland, Gisborne.


Tasmanian Grass Grub 

  • The Tasmanian Grass Grub are at beetle stage over January to March. If beetles are reported over January to March then it is not NZ grass grub.
  • The Tasmanian Grass Grub is in larvae stage March to October but can be identified by its dark brown/black head (NZ grass grub larvae have a tan head).
  • The Tasmanian Grass Grub larvae travel and feed on the surface at night. If you observe eaten leaves on the surface then it is not NZ grass grub as they feed below the soil surface
    on roots.
  • Endemic in Waikato. Not found in Otago and Southland.



Manuka Beetle

  • The Manuka Beetle larvae looks very similar to the NZ grass grub larvae so it can be very hard to distinguish between the two. Expert advice may be required.
  • It is easier to tell the difference by the beetle stage. Make a note of the colour and timing of the beetle adult activity.
  • The Manuka Beetle is green while the NZ grass grub is brown.
  • The Manuka Beetle is active from October to February while the NZ grass grub is only active October and November.
  • Manuka beetles are more likely to exist in cooler, wetter climates like the west coast of the South Island.




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