AgPest » Manuka beetle

Manuka beetle adults

Manuka beetle grub

Left - grass-grub. Right - manuka beetle larvae

Manuka beetle larval damage to pasture

Manuka beetle larvae under damaged pasture

Left - grass-grub. Right - manuka beetle larvae

Manuka beetle is the common name for a group of species of native scarab beetles (their larvae are very similar in appearance to the New Zealand grass grub). Manuka beetle adults are 9mm or less long and are usually coloured a bright metallic green
The beetles are only present for a few weeks in late spring/early summer each year. This period varies by a month or so in different regions and at different altitudes
Manuka beetles fly as adults in warm conditions during the day but are often not noticed against the background of our green spring pastures. Occasionally in very hot calm conditions a massed flight of beetles occurs – very noticeable to anyone passing through the swarm
Most species of manuka beetle are strongly attracted as adults to manuka/kanuka trees or bushes where they can be seen among the young foliage
Adults of most species of manuka beetles will feed on the young foliage of these bushes.

Manuka beetle larvae (grubs) are soil dwelling grey/white grubs up to 10 mm long with tan coloured heads. They are very similar to the ubiquitous New Zealand grass grub larvae but generally much smaller
Typically found lying in a C shape
Larvae will feed on the roots of many sown pasture species and some pasture weeds
Damage can occur in discrete patches in pasture often in areas close to native bush in Taranaki and Hawkes Bay or in the Otago/Southland or Canterbury high country. More extensive damage can occur in hump & hollow and flipped pastures on the West Coast of the South Island
Damage initially appears as yellowed stressed areas of pasture. Later in these damaged areas plants are pulled easily from the ground during stock grazing due to the lack of roots caused by larval feeding
Damage patches may feel soft underfoot.

The immature larval stages of manuka beetle (Pyronota sp.), are generally considered a minor pasture pests but when populations occur in excess of 350/m2 they can cause damage similar to the native New Zealand grass grub
The adult beetles live for a few weeks during late spring and early summer. During this time the beetles mate and the females lay their eggs in pasture soils at depths of up to10 cm. Each female can lay 30 – 40 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 takes almost one year. Root damage usually becomes evident after the grubs have reached the third stage. Damage is usually noticed in autumn and early winter
Several other beetle species have larvae resembling manuka beetle larvae (especially the common New Zealand grass grub) and their feeding can appear identical. The larvae of such beetles are very similar and expert advice may be required to correctly identify them.

Manuka beetle outbreaks most often occur in young pastures 2-3 years after development from areas of native vegetation or bush regrowth. Damage may also occur in recently cultivated pastures, again typically in their second or third autumn or winter
Manuka beetle populations can be naturally attacked by several diseases which commonly only persist in a pasture if there are some grubs present. Research with the native New Zealand grass grub has shown that population outbreaks, and therefore damage to pastures, can occur when the grass grub/disease associations are disrupted and it is highly likely that the same applies with manuka beetle. Cultivation or the use of insecticides have been shown to cause lower levels of disease in subsequent grass grub populations. Pasture renovation by direct drilling appears to favours retention of diseases in the soil reducing likelihood of pasture damage.

Immediately after hatching young manuka beetle larvae feed on plant roots and soil organic matter. As they grow they appear to 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 2nd stage can cause damage it is usually 3rd stage grubs that do so. Most pasture grasses and clovers are suitable food plants. Some grass species such as tall fescue and cocksfoot appear to show greater tolerance to feeding than others
Manuka beetle larvae can occur in aggregations and cause damage in patches of pasture. Within these patches the density of grubs can be very high (2000+ /m2) 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 damage caused increases. In established pasture an average density of 300grubs/m2 and above will cause noticeable pasture damage. Pasture production will be affected and areas of the paddock, especially the tops and sides of dryer ridges, will be opened up for weed invasion with a subsequent loss of pasture quality. In some areas, particularly on the upper West Coast of the South Island, populations can greatly exceed this level
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.

Significant feeding damage by the beetles has only been noticed on manuka trees and bushes. There appear to be no reports of serious feeding on brassica crops or on exotic trees or vegetables.

Awareness and early recognition of damage will allow remediation actions to be taken and avoid severe pasture loss and prolong pasture life. Manuka beetle numbers should be assessed 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 100 mm deep and hand sort soil to find grubs. Take at least 10 spade squares per paddock. In February/March most grubs will be small. 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
- Visual assessment of damage patches. Areas of poor growth in the pasture may become evident from March onwards. Spade samples should be taken to determine the presence of manuka beetle larvae and the area of pasture affected estimated prior to taking a decision on control action
- Densities greater than an average of 12 larvae per spade square (300/m2) may cause damage.

There are no chemicals registered for manuka beetle larvae control
Below are a range of chemicals registered for control of the common New Zealand grass grub larvae in pasture.

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