BRENDON CANT brendon at iinet.net.au
Wed Jul 30 04:55:07 CEST 2008

GRDC Media Release – 30.07.08


Canola growers finding Diamondback moth (DBM) grubs in a crop by early
August may have a problem, according to entomologist, Kevin Walden, of the
Department of Agriculture and Food WA (DAFWA) 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 80 per cent.

“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

“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

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.

“In high risk situations a second spray should be applied within seven days
of the first,” he said.

Mature DBM larvae are 12 millimeters long, pale yellowish-green and tapered
at both ends.

Moths are the same length, with a white, diamond patterned stripe of uneven
width down the centre of the back when the wings are folded over the body.

More information is available from DAFWA’s PestFax newsletter, which advises
on outbreaks and recommended treatment and can be viewed at

PestFax, supported by the National Invertebrate Pest Initiative, is a GRDC
project offering national pest management information and promoting
integrated pest management principles.


Authorised by GRDC and issued on its behalf by Brendon Cant & Associates,
Tel 08 9384 1122
MEDIA CONTACT: Kevin Walden, Tel 08 9956 8539

GRDC REF: DiamondBackMoth.doc/DAW0041/Rainbow300708
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