Canola growers finding Diamondback moth grubs in a crop by early August may have a problem, according to entomologist, Kevin Walden, of the Department of Agriculture and Food WA in Geraldton.
Mr Walden indicated DBM, or Plutella xylostella, has been found in canola crops in the northern agricultural region, which is a concern since yield loss due to plant stress caused by DBM eating canola leaves and stems can be as high as 80pc.
"If DBM infests canola early in the season and there is enough warmth to complete three or four generations, numbers can quickly build, exceeding threshold levels," he warned.
"If DBM can't be found in early August, the risk of economic damage over the rest of the season is very small.
"The key to DBM control is to constantly monitor populations from now on.
"Growers are strongly advised to sample moth numbers using 10 sweeps, with a sweep net at four or five different locations within the crop.
"If DBM are found, there is potential for numbers to increase and growers must resample and contact their local agronomist for advice."
Mr Walden indicated sampling each crop should take up to one hour and growers should base control measures on DBM numbers.
"Some DBM in a crop can be tolerated, but if average grub numbers exceed 10 in 10 sweeps in early August a major outbreak could develop," Mr Walden said.
"Continue weekly crop inspections if average numbers are less than 10 and twice a week if numbers exceed 10. Even if no DBM are found on first sampling, growers should resample in two weeks as infestations can quickly develop."
If numbers reach 50 by mid to late August, spraying is justified. He indicated grub threshold numbers would increase later in the season.
Mr Walden said a recent Grains Research and Development Corporation (GRDC) supported study offered new information on DBM control.
"We discovered that spraying to control DBM should begin earlier in the season, at the end of August, rather than waiting until late September, which had been accepted practice," he said.
"In high risk situations a second spray should be applied within seven days of the first."