Maize for Silage
Insight 328


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Adult black beetles (Heteronychus arator) are a rich brown glossy black (Figure 1). They are robust beetles 10 - 15 mm long.

Figure 1: Black beetle adult

Eggs are pearly white in colour and around 2 mm long. They are laid one-by-one and are oval swelling to an almost spherical shape before hatching.

Larvae are around 2.5 cm when fully grown. The head is light brown and the body greyish or creamy white except for the hind end that appears black due to the gut contents showing through. Larvae develop through three stages. The second and third stage larvae have light brown head capsules and orange spots along the sides of the thorax and abdomen (Figure 2). Like grass grub, the larvae are found curled up in the soil.

Figure 2: Black beetle larvae

Pupae when first formed are pale yellow but turn reddish brown before adult emergence. They are found in earthen cells and are around 15 mm long. 


Climatic factors limit the distribution of black beetles to the northern regions including Northland and South Auckland, northern parts of the Waikato, the Bay of Plenty and coastal strips of Taranaki, Hawke’s Bay and Poverty Bay. Black beetles prefer free-draining, sandy or peaty soils.


Adult beetles overwinter from June to September in free draining soils. Eggs are laid singly, near the soil surface from October – January with peak numbers in early November. Depending on the soil temperature, eggs can take six weeks or less to hatch.

Larvae are soil dwelling. They develop through three stages at about monthly intervals over the summer. First stage larva feed mainly on soil organic matter and high soil moisture levels can result in high death rates. Second and third stage larvae feed mainly on plant roots (especially grasses).

Fully-grown grubs burrow down about 100 mm into the soil to pupate. New adults emerge in the autumn. Beetles disperse to other sites by crawling or flying. In years of high numbers massed dispersal flights of hundreds of thousands can occur. These usually take place on warm evenings after the autumn rains have started. Adults then remain dormant from June to September although flights can occur on warm winter days. Approximately 70% of the larvae in an infested paddock arise from eggs laid by adult females that overwintered there.


Black beetles do not occur every season but when they do, they are a very damaging pest in seedling maize with numbers as low as 1/m2 causing economic losses. High levels of paspalum or kikuyu in pasture ensure higher survival rates of the adults over the winter which later results in greater larval populations and pasture damage.

Black beetle adults move around the soil surface until they find a maize plant. They test bite the stem and burrow into the soil where they slowly feed on the stem near its base. Black beetle damage is characterised by rough, rasped plant tissue damage in the bottom 3 cm of the stem. Visible black damage may not peak until the maize is in the 3rd or 4th leaf stage. Damage to the plant growing tip results in plant death. Each beetle kills one plant on average. Stunted plants and plants with multiple tillers can also occur as a result of black beetle damage.


  1. When to control

    The climate has a huge impact on black beetle numbers. Mild winters and warm dry summers favour an increase in black beetle numbers and a population explosion can occur after successive dry summers.

    Even in years of low beetle numbers, most pastures contain sufficient beetle numbers (1 - 2/m2) in the spring to cause economic loss in maize. Adult numbers are highest (up to 40/m2) in pastures containing paspalum or kikuyu. A control measure is recommended when maize is sown from any pasture.

    For maize crop paddocks previously planted in maize crops, the risk of damage will be low if black beetle numbers are low (usually indicated by few, if any, beetle flying the preceding autumn). Before cultivation begins, check grass weeds for tiller damage (tillers reddish or yellowish with rough feeding damage at the base). Sort through the top 100 mm of the soil under grass weeds for adult beetles. Control measures will be necessary if there are 1 - 2 black beetles per square metre.

    Black beetles are able to overwinter successfully in stubble or grass weeds. Winter cereal or annual ryegrass crops greatly enhance beetle survival rates and increase the risk of damage in subsequent maize crops. If there are a large number of black beetles flying in the autumn, all areas being planted in maize are likely to need some form of black beetle control.

    The best control option is to plant Poncho® insecticide treated seed Poncho® also controls Argentine stem weevil and greasy cut worm. It has a withholding period of 42 days.

    Pioneer Premium Seed Treatment offers a number of industry-leading insecticide options to protect your crop from insect damage. To carry the Pioneer Premium Seed Treatment stamp of approval. Every bag of Pioneer® brand maize seed must meet stringent quality control standards which ensure the precise dose of seed treatment is applied to each and every seed. For your protection every bag of Pioneer® brand seed is mechanically stitched closed with green and white bi-colour tamper proof string. This “locks- in” the Pioneer warranty and Seed Replant Risk Cover and guarantees there are a minimum of 80,000 kernels in each and every bag.

  2. Pre plant control options

    Increasing the fallow period will not reduce the risk or severity of black beetle damage. Delaying planting to mid-November will reduce the risk of black beetle damage however this is usually not practical or economic. The best control is achieved through the use of Poncho® insecticide treated maize seed.

  3. Post emergence control options

    Currently there are no chemicals registered for the control of black beetles in emerged maize crops. Some of the contact insecticides registered for greasy cutworm can give black beetle control. These should be banded along the maize rows and they require adequate rainfall (5 - 10 mm) to wash them into the soil to contact the beetles at the base of the maize seedlings. Contact your merchant representative, contractor or Pioneer Representative for crop specific advice.


Black Beetle Biology, Damage and Control (1981) Aglink FPP 45, MAF
New Zealand Pests and Beneficial Insects (1984) Scott, R.R (Ed) Lincoln University
The advice of Paul Addison (AgResearch Ruakura) in the preparation of this bulletin is gratefully acknowledged.

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Revised: June 2015
Expires: June 2018