Grass-grub, brown beetle

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. 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.

Beetles

• Feeding by the beetles occurs during the later phase of beetle flights and is restricted to spring and early summer. Similar beetles, such as Odontria species, also fly at other times of the year and cause similar damage but are usually less common or restricted in their distribution. Grass-grub beetles feed on the young foliage of a very 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 and reduces the amount of photosynthetic leaf area available to the plant.

Information prepared by AgResearch