Black Beetle in New Zealand Pastures: A Silent Threat to Productivity
Black Beetle
(heteronychus arator) is a significant pest in New Zealand's pastoral farming systems, particularly in the warmer regions of the North Island.
Image source: interest.co.nz
Overview
The Black beetle (Heteronychus arator) is a significant pest in New Zealand's pastoral farming systems, particularly in the warmer regions of the North Island. Originally from South Africa, this large, shiny black beetle (approximately 15mm in length) thrives in free-draining, sandy, ash, or loam soils, making it particularly problematic in Northland, Northern Waikato, the coastal districts of Auckland, Bay of Plenty, Gisborne, and northern Taranaki. Due to their preference for lighter soils over heavy clay soils, black beetles are more prevalent in regions where the soil is easier for both adults and larvae to move through.
Life Cycle and Seasonal Activity
Black beetles have a seasonal life cycle that influences their impact on pasture:
Spring to early summer – Adults lay eggs in the soil.
Summer (December – March) – Larvae hatch and feed on plant roots, causing significant pasture damage.
Late summer – early autumn – Larvae pupate and emerge as adults.
Autumn – Adults chew through the base of newly emerging ryegrass plants.
Winter – Beetle populations are typically lower after colder winters, but mild winters can lead to increased numbers in the following season.
Damage to Pasture
Both black beetle adults and larvae contribute to pasture deterioration:
Adults damage newly sown autumn pastures by chewing at the base of emerging ryegrass plants, leading to patchy establishment and reduced pasture density.
Larvae feed on plant roots from December to March, often causing severe root loss. This results in weakened plants that can be easily pulled from the soil, particularly in dry conditions. Larval damage is most noticeable in late summer and dry years when populations exceed 40/m².
Identification of Black Beetle Infestation
To confirm black beetle as the cause of pasture damage, farmers can conduct a simple field test:
Dig 10 soil cubes (each approximately 20cm wide and 10–15cm deep) from different parts of the affected paddock.
Break up the soil and count the number of black beetle larvae and adults.
Multiply the number found in the 10 cubes by 2.5 to estimate black beetle numbers per square metre.
If beetle numbers exceed 15–20/m² in dry summers or 40/m² in wetter conditions, they are likely responsible for the damage.
Management Strategies
1. Sowing Insecticide-Treated Seed
One of the most effective ways to reduce black beetle damage in newly sown pastures is by using insecticide-treated ryegrass seed. This provides early-stage protection for seedlings and reduces the risk of plant loss in the establishment phase.
2. Utilising Endophyte-Enhanced Pastures
Certain ryegrass varieties contain endophytes—symbiotic fungi that produce alkaloids, which enhance plant resistance to insect pests. Some endophytes, such as AR37, NEA2, and RGT18, have been shown to deter black beetle feeding and improve plant persistence in infested regions.
AR37 – Provides strong resistance against black beetle larvae and adults.
NEA2 – Offers moderate resistance and is suitable for areas with variable beetle pressure.
RGT18 – Provides strong resistance against black beetle larvae and adults while maintaining good livestock performance.
Choosing the right endophyte-enhanced pasture mix is crucial for long-term pasture sustainability in black beetle-prone regions.
3. Optimising Grazing Management
Grazing management can also have an impact on black beetles. Maintaining strong plant growth and avoiding overgrazing reduces plant stress and increases resilience against beetle damage. A well-managed rotational grazing system ensures plants have adequate recovery time, strengthening root systems and improving their ability to withstand larval feeding.
4. Cultivation Methods
Direct Drilling Risks – Direct drilling of new pasture into an existing black beetle-infested paddock increases the risk of seedling loss due to adult feeding.
Full Cultivation – Turning over the soil can help disrupt black beetle populations but should be carefully planned as it may also expose the soil to weed competition and moisture loss.
Future Considerations
With climate change bringing warmer temperatures and drier conditions, black beetle populations are expected to expand further south. Farmers should monitor pastures regularly, adopt preventative measures, and consider integrated pest management strategies to reduce the impact of black beetle on pasture productivity.
By understanding the life cycle of black beetles, implementing effective management techniques, and using resistant pasture varieties, farmers can improve the resilience of their pastures and mitigate the economic losses associated with black beetle damage.