Minimising damage to grapevines caused by the native grass grub

16 October, 2007

Whilst the native grass grub is predominantly considered a pest of turf and grasslands it is not particularly fussy and will eat roots and leaves of many plants. A survey in Marlborough indicated that feeding by grass grubs may contribute to vine decline. Grass grub is prevalent in all viticulture regions and there has been growing interest in the monitoring and treatment of this pest.

Dealing with grass grub

When planting grapevines in a block that has a history of high grass grub pressure, some form of control should be considered. First, test to see if you have a grass grub problem. The best time for sampling is February.

Soil sampling

 

• Take 10 spade squares of turf (soil) randomly across the block
• Sift carefully through each sample to determine how many grubs are present
• Based on pasture data, if more than 30 grubs are found in total across the 10 samples, treatment will be necessary
• Process soil samples as you go. If 15 or more grubs are found in the first five samples, dig no further. Treatment will be required.

 

Note: The grass grub number threshold for pasture takes into account the short rotation and low replacement cost of grass. A lower grub number threshold may be appropriate for higher cost, longer rotation plant material like grapes.

Typical damage to the roots

Root damage is caused by grass grub larvae (juvenile stages in the life-cycle). Young developing feeder roots are preferred. Feeding by grass grub larvae will limit both the number of roots and the extent of the root system developed as illustrated in Figure 1.

Figure 1 Grapevine root development in grass grub infested soil (left) compared to soil free of grass grub (right). Photo by Richard Townsend of AgResearch, 2003

Typical damage to the shoots

Damage to leaves is caused by adult grass grub beetles. Adult beetles cut and chew leaf tissue rather than sucking or rasping so sections of leaf will disappear (generally between the veins) when beetles are flying and feeding.

Feeding by grass grub larvae will limit both the number of roots and the extent of the root system developed.

The grass grub life cycle

Adult female beetles lay eggs in the soil to depth of 10-15 cm following their main flight in October and November. Within a few weeks the eggs hatch and the grubs feed on the roots and other organic matter in the soil.

In August they move deeper down into the soil and pupation occurs. By November, male and female beetles emerge, flight and mating occur, egg laying starts the next generation.

From a grape growing perspective key points to note are:

 

• Grass grub larvae do most of their growing and thus are feeding on roots, during February, March and April.
• They do not feed significantly when they are overwintering deep in the soil (June, July) or at all whilst they are preparing to pupate (August through October).
• From February through April young grapevines will also be actively growing. Damage to the roots during this period will often not cause observable effects above ground.
• Damage may go undetected until the vine is placed under stress during bud break, flowering or veraison (fruit softening) in subsequent seasons.

Figure 2 Lifecycle of grass grub during vineyard establishment.

• Wounds to the roots caused by grass grubs can allow subsequent infection by fungi or bacteria resulting in poor performance or death of vines later on.
• Adult beetles are flying mainly in October and November. Damage to leaves will not occur outside this period.

 

How many grass grubs are required to cause damage?

In an experiment using potted plants we found the amount of damage was linearly related to the number of grubs. In other words, two grubs did twice as much damage as one grub. There was no evidence that damage rates differed among rootstocks: all rootstocks were vulnerable to damage from grass grub larvae. Plant growth was not affected by grub number during the first growing season. This is consistent with general field observations. Plant decline or death is typically observed one or more seasons after damage is likely to have occurred.

Treatments to control grass grub

Cultivation in late September and early October can help destroy pupae before they hatch into adults. Rolling or heavy stocking in the late autumn / early winter period, before larvae move further underground can cause larval death.

Chemical control of larvae can also be used. Granular formulations which can be broadcast or direct drilled, seem to give best results. Chemicals should be applied between mid February and late March when larval feeding activity and therefore damage is at its peak. It is difficult to control adult grass grubs as they tend to feed at night and fly in and out of the crop rather than remain on the crop for any length of time. It is better to concentrate on reducing the number of larvae. This will reduce the number of adults developing.

A biological control alternative

BioShield™ is a highly effective biological control agent that can decrease grub populations in pasture. It contains the natural soil bacteria, commonly known to cause amber disease.

Research indicates that this biological treatment can provide a 50% decline in grass grub larval numbers in the year of application compared to untreated pastures. Once applied, the bacteria will cycle through the grass grub populations and survive in the soil for up to five years providing long term grass grub control.

The experiment showed that all grapevine rootstocks were vulnerable to damage from grass grub larvae.

References

 

• CAB International. (2002). Crop Protection Compendium. Costelytra zealandica
• D. Mundy 2003, “Performance of grapevines grown on 101-14 rootstock in Marlborough”. The Australian & New Zealand Grapegrower & Winemaker No.477, October 2003, Pages 70- 75
• Wightman, J.A. (1998). Grass Grub Life Cycle. HortFACT. http://www.hortnet.co.nz/publications/hortfacts/hf401004.html (6/7/04)
• Ferro, D.N. (editor) 1976. New Zealand insect pests. Lincoln University College of Agriculture, Canterbury, New Zealand

 

Sources

New Zealand Pest and Beneficial Insects HortFact sheet and BioShield™ information.

For further information please contact: Dion Mundy, HortResearch Marlborough Wine Research Centre: dmundy@hortresearch.co.nz or 03 577 2373 Mart Verstappen, Corbans Viticulture: mart@corbansviticulture.co.nz or 0274 413 714