Bronzed field beetles

Adult beetles are being found widely in NSW, but it’s more likely their larvae (false wireworms) will cause crop loss

 

Where have they been reported?

Widespread infestations of bronzed field beetle adults have been reported from across the Riverina district of NSW (Big Springs, Mangoplah & Griffith) and at Temora in the Central West Slopes and Plains district of NSW. In one instance, beetles have been observed widely across 200 km of Riverina canola crops. (Thanks for the reports!*). In most paddocks, the adult beetles have not caused substantial damage although pest densities were high enough to cause some chewing damage to canola seedlings. It was reported that many beetles were found to be dying at the base of plants, suggesting that imidacloprid seed dressings were providing some control.

The more concerning aspect is that the larvae of this beetle belong to the broader group of false wireworms, which are likely to become active over the next few weeks, and remain a threat throughout the establishment phase. We have received reports of bronzed field beetle larvae ring-barking triticale and wheat plants in paddocks south of Wagga Wagga. This has resulted in bare patches where early tillering plants have been completely killed. It is not practical to control the larvae without re-sowing.

Bronzed field beetle habits and behaviour

Bronzed field beetle (Adelium brevicorne) larvae are generally found within the top 10 mm of the soil, commonly under plant material, clods of soil, rocks or wood. The larvae may be present in the soil with little or no damage to plant seedlings. This is because they primarily feed on dead organic matter and high numbers are required to cause serious crop damage. They mostly feed on plants at night but can sometimes be seen during the day. Bronzed field beetles are important pests of establishing canola, and attack cereals and pulse crops less frequently.

Over summer and autumn, the adult beetles, which cannot fly, shelter in crop residue, stumps or tufts of grass. The beetles become active and immediately begin egg-laying soon after the first autumn rains. When there is early moisture, as there has been this year, these larvae may reach a length of 5 mm or more before the crop is seeded. In August larvae begin changing to the pupal stage and new adults appear soon after. Bronzed field beetles generally have only one generation per year. Click here for further information on bronzed field beetles.

Our advice

In the likely event that crops have already been sown, the key is to monitor for bronzed field beetles and their damage from sowing through crop establishment. Note that larvae may be difficult to find during the day. If damage is observed, monitor the crop at night when larvae are most active and feeding at the base of plants. Controlling false wireworm larvae is difficult because they are largely subterranean and will not be exposed to insecticides. In severe cases, re-sowing patches or the entire crop may be the only viable alternative.

In pre-sowing situations, assessing beetle numbers using refuges may give an indication of the likely larval population. Approximately 15 adults in one square metre may produce over 1500 larvae, which is enough to damage most canola seedlings. Because of the flightless nature of this pest, infestations are more likely to carry over from one year to the next in the same paddock. Hence, an understanding of which paddocks are harbouring large populations provides a basis for more satisfactory control in subsequent years.

Control options

Bronzed field beetle larvae mostly damage crops sown into paddocks with crop residues and abundant surface plant material. Trials in WA show the adult beetles require surface plant residues for shelter and breeding. The removal of this material in early autumn, where it is considered to be a suitable practice, will reduce or eliminate this pest. This may be achieved through cutting and baling, grazing or burning. Cultivation before seeding will also create a hostile environment for the larvae and can reduce population sizes.

Insecticide seed treatments are registered for false wireworm control in certain broadacre crops, but only in some states.

Description

Bronzed field beetle adults are up to 11 mm long and are shiny black in colour and can have a slight bronzed appearance. The larvae are dark brown, up to 12 mm long and 3 mm wide. They have twelve body segments, the last one having two distinct upturned spines. The larvae are often confused with the grey false wireworm, which is a serious pest in Victoria and parts of southern NSW. The larvae of the grey false wireworm have a flatter appearance and the adults are dull brown in colour and smaller in size. Click here for images of bronzed field beetle larvae.

 

* Sources of field reports of bronzed field beetle

Allan Edis - Agronomist, Landmark (NSW Central West Slopes & Plains)

Barry Haskins - Agronomist, Ag Grow Agronomy & Research (NSW Riverina)

Chris Minehan - Agronomist, Rural Management Strategies (NSW Riverina)

Greg Parker - Agronomist, AGnVET Services (NSW Riverina)