Biology

• The immature larval stages of Costelytra zealandica, generally known as grass-grub, are New Zealand’s most important native pasture pest. The adult grass-grub beetles, sometimes called brown beetles, are shiny tan/brown, about 10 mm long, and fly at dusk in spring and early summer. 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 and occurs very soon after the beetles emerge from the soil, the second consists of feeding flights and may occur for 2-3 weeks
• The beetles live for a few weeks. During this phase the beetles mate and the females lay their eggs 100 to 150 mm under the soil. Each female lays about 30 eggs. On hatching the larvae feed on roots of a wide range of plants. As they grow they pass through three larval stages. Development from egg to adult usually takes 1 year but under severe environmental conditions development of some grubs may be extended over two years. Usually both one and two-year life cycle grubs occur together. 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. Root damage results mainly when grubs reach the third stage. For one-year life cycle grubs this is in autumn and early winter. 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
• 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.

Natural population regulation

• Grass-grub populations are regulated by several diseases which can only persist if grass-grubs are present. Grass-grub outbreaks, and therefore damage to pastures, usually occur when the grass-grub/disease association is disrupted. Disruption happens when grass-grub numbers are dramatically reduced and there is no renewal of disease organisms in the soil. Commonly this is a result of cultivation but may also be due to drought and occasionally insecticide use. The low disease levels in the population allow grass-grub numbers to increase causing pasture damage. The disease organisms build up in response to increasing grass-grub numbers but there is a lag period before they again regulate grass-grub 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 or third winter. Pasture renovation by direct drilling favours retention of grass-grub diseases in the soil reducing likelihood of damage. When the grass-grub outbreaks are the result of drought, pastures of any age can be affected.