INFESTATIONS OF diamondback moths devastated canola crops in southern NSW and northern Victoria in 2002, according to entomologist Joanne Holloway from the Wagga Wagga Agricultural Institute.
Advice commonly provided by agronomists is that the infestation is associated with dry conditions and that, given a more normal season in 2003, the moths are unlikely to be a serious pest. SARDI entomologist Greg Baker who's been researching the moth, particularly in vegetable crops, agrees that the season is likely to have a big bearing on the potential for another outbreak, but says that it is too simplistic to write them off as a seasonal phenomenon.
"We don't yet fully understand the relationship between the season and the population of diamondback moths, " he says. "Conditions in the cereal country of SA this year were almost as bad as those prevailing in the eastern states, yet we saw smaller populations of the moth than expected.
"Recent history in WA saw a major outbreak follow a February rain that germinated volunteer canola and other brassica weed plants. Diamondback moths bred on this early green bridge, migrated to newly sown canola crops and devastated them. "
Mr Baker says that the interplay of moisture and temperature is critical in the build-up of the moth population. "High rainfall and low temperatures, the 'typical' mid-winter situation, slow down the lifecycle and result in high mortality rates. That's what's needed to reduce population numbers. "
Natural pest control
Diamondback moths are vulnerable to a naturally occurring fungus Zoopthora radicans. CSIRO entomologist Richard Vickers has been studying the impact of the fungus on the moths. He says that under ideal conditions it can have a significant impact on populations of the moth.
"Unfortunately that's often at the end of the season. You need warm humid temperatures. Under those conditions the fungus can get going and knock out the moths. That usually happens too late in the season to prevent damage, but it can reduce the level of carry-over into the next season. "
Unfortunately, conditions didn't favour the fungus in southern NSW at the end of the 2002 season.
Patchwork chemical resistance
An alarming trend last year was the number of moths resistant to currently registered chemicals. Dr Holloway estimates that as many as 20 per cent of each population was resistant to the synthetic pyrethroids. Mr Baker agrees, saying that the relatively short lifecycle of the pest gives it the opportunity to build up resistance quickly. However, the resistance tends to be a mosaic with some populations showing no sign of resistance.
Will the moths be a menace in 2003? There's been a big build-up in population numbers in NSW, but Mr Baker says the question will be how many of those moths survive the summer. "They need host plants to survive, and that means green brassica, anything in the cabbage family. Given the current state of the drought, that suggests that irrigated vegetable crops will be the main source of any infestation next season. "
Contact: Dr Joanne Holloway 02 6938 1999 Mr Greg Baker 08 8303 9400 Dr Richard Vickers 07 3214 2824