Rake the bugs from canola

Densities of bronzed field beetle larvae in stubble

RAKING to remove stubble may be one of the best ways to halt the attack of the bronzed field beetle in canol a crops in high- and medium-rainfall Western Australia and South Australia.

Hope for tackling this voracious canol a pest comes at a time when conventional pre-crop spraying is not solving all pest problems. The bronzed field beetle (Adelium brevicorne) is widespread in WA and SA where it commonly causes thinning of the crop through to destruction of large areas of canola. Removing plant residues and places for beetles to shelter is just one of the strategies growers could consider in light of recent research supported by growers and the Federal Government through the GRDC.

"Our experiments at Kendenup, WA, showed very clearly that the bronzed field beetle requires surface plant residues for shelter and for breeding," said Phil Michael of the WA Department of Agriculture.

"We compared stubble plots with those that had been lightly raked in February before the beetles laid their eggs," Mr Michael said. "Counts of soil cores showed that there were almost no larvae in the raked plots but there were large numbers in the stubble plots."

Sampling with pitfall traps and carpet squares showed that adult beetles actually moved freely between stubble and raked plots at night but did not breed in the raked plots, Mr Michael added. Their destructive ways Most damage results from the larvae chewing through the stems of young plants at ground level.

However, large numbers of big beetles can topple older canol a plants, according to Mr Michael. Adult beetles shelter over summer and autumn and females lay eggs in autumn. "In I square metre of stubble, 15 adults can reproduce 1,500 larvae, enough to kill most canola seedlings," he said. The larvae reach 5 mm in length before the crop is seeded and they start changing into adult beetles in August.

With most WA growers being committed to stubble retention and minimal soil disturbance, other control methods are required. Researchers have not been able to successfully control the adult beetles with baits or sprays prior to seeding, and seed dressings have not shown the same promise in the field as they did in plot trials.

In one plot containing lupin and cereal residues, researchers tried a triple spray (preemergence) at night when the larvae became active. "Even with the tri pie spray the level of control was not high except with an uneconomic dose of Regent," Mr Michael said. Another experiment showed the devastating impact bronzed field beetles can have on canol a yields.

Researchers sprayed one treatment three times at very high dosages and compared the results with unsprayed plots. "The sprayed treatment grew more than 40 plants/m' and yielded 2.8 tlha while the unsprayed plots averaged less than 10 plants/m' and the yield was only 0.6tlha," Mr Michael reported. "That amounts to an 80 per cent loss in yield in the unsprayed plot."

On top of the yield losses, these beetles are often harvested along with canola and barley and threaten export markets, particularly for barley. The adults emerge well before canola and barley crops are swathed and are caught up at harvest time because of their habit of seeking shelter beneath the swath lines.

The bronzed field beetle is just one of a number of bugs that are biting into high-rainfall West Australian canol a crops and are the focus of research.

Program 3 Contact: Mr Phil Michael 0898928485