Grass-grub larvae are soil dwelling grey/white grubs up to 15 mm long with tan coloured heads
Typically found lying in a C shape
Feed on a the roots of many pasture species and other plants
Damage occurs in discrete patches in pasture
These initially appear yellower than surrounding pasture and plants are pulled easiliy from the ground
Patches feel soft underfoot and are easily pugged.
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.
Impacts
Larvae
Immediately after hatching young grass-grub feed on plant roots and soil organic matter. As they grow they focus on living plant roots but can survive on dying or dead roots for extended periods. Larvae feed on roots of a range of plants. Although larvae in the second stage can cause damage it is usually third stage grubs that do so. Most pasture grasses and clovers are suitable food plants although some, such as tall fescue and cocksfoot, show greater tolerance to feeding than others. Cereals and brassica crops are often attacked and fodder beet is also at risk. Larvae also feed on a wide range of plants of horticultural and ornamental significance
Grass-grubs occur in aggregations and cause damage in patches of pasture. Within these patches the density of grubs can be very high and cause localised destruction of plants even though the over all density of grubs in a paddock may be low. When the patches become extensive, either in number or size, the amount of damge caused increases. In established pasture a density of around 100 grubs/m2 measured over the whole pasture is largely undamaging although some damage patches will be evident. At an average density of 200/m2 and above damage will be noticable, production affected and large areas of pasture opened up for weed invasion with a subsequent loss of pasture quality. In some areas, particularly Otago and Southland, populations can greatly exceed this level
In pasture when grass-grub numbers are high the turf may feel soft underfoot, and can be rolled back as a result of lack of roots anchoring the plants to the soil. Patches of heavily infested pasture appear yellow relative to undamaged pasture and plants are easily pulled from the soil by stock. Damaged areas are easily pugged. Grass-grub feeding reduces plant productivity and survival and when most severe pastures may need to be renovated. The density of grubs necessary to cause damage varies between regions and is related to soil temperatures and soil friability.
Control
Measuring damage by larvae
Awareness and early recognition of damage will avoid severe pasture loss and prolong pasture life. Assess grass-grub numbers as early as possible. Options for detecting damaging levels include:
Digging – from mid-February. Dig 200 x 200 mm spade squares to a depth of 50 mm deep and hand sort soil to find grubs. Take at least 10 per paddock. In February/March most grubs will be small (less than 10 mm). By April the grubs will be larger but some small ones may still be present. Grubs are usually found in the root zone of the plants but may be deeper in the soil under very dry conditions
Early warning strip – Autumn. Apply insecticide to a marked strip (2 x 20 m) of pasture in an area 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 the soil to determine if pest is grass-grub.
Densities greater than an average of 8 larvae per spade square (200/m2) will cause damage.
Chemical control
Treat pastures as soon as sampling indicates a damaging grass-grub population is present or damage is noticed. In grass-grub prone areas check for grubs before sowing new pasture or crop. Prills and a seed treatment are available to protect seeds and seedlings
Registered chemicals for grass-grub (larvae) control are:
Insecticide
Application
Time of application
chlorpyrifos
granule
drill with seed
diazinon
granule or spray
when grubs are present
imidacloprid
seed treatment
seed treatment
terbufos
granule
drill with seed
Consult your farm consultant, industry representative or the New Zealand Agrichemical Manual for more information about chemical control.
Biological control
Bioshield is a commercial formulation of a naturally occurring bacterium. This should be introduced to a grass-grub population (approx 150/m2) before damaging levels are reached i.e. the year before damage is expected to occur. It is unlikely to provide a high level of initial kill but can restrict the build-up of damaging populations in subsequent years. It is not effective against populations already causing major damage. Apply while soil is warm and moist and grubs are active (late February to mid March).
Management
In grass-grub prone areas cultivation predisposes pastures to damage 2-3 years later (see Biology). Direct drilling retains grass-grub diseases in soil reducing likelihood of damage
Heavy stocking while soil is moist and grubs are at the soil surface can give low levels of control and re-establish plant root soil contact helping pasture recover from damage
Heavy rolling while soil is wet and grubs are at surface can give significant control
Well fertilised pasture will withstand more feeding pressure than infertile pasture
Extra nitrogen applied in autumn will help especially with moderate populations (200-300/m2)
Tall fescue and cocksfoot tolerate grass-grub feeding more so than ryegrass
Some festulolium grasses (perennial ryegrass x meadow fescue cross) contain an endophyte (Neotyphodium uncinatum (U2)) that deters grass grub feeding.
Further information
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.
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