Silverleaf whitefly is a growing pest threat to a wide range of agricultural crops in eastern Australia; it’s here to stay but it is manageable with integrated cultural, biological and chemical control tactics.

And a surprising amount of help is coming from native parasitic wasps – the Eretmocerus and Encarsia species in particular – and other predatory insects that slashed whitefly numbers in Central Queensland in the 2002-2003 season.

That was the message to private and public sector advisers at the recent Grains Research Update in Goondiwindi from Richard Sequeira, QDPI senior entomologist from Emerald, centre of a coordinated whitefly control program.

Silverleaf whitefly is a major production constraint in many parts of the world, including the United States, Israel and other parts of the Middle East.

It has forced the virtual elimination of legumes, particularly soybeans, from many parts of Arizona and Texas, where it can cost cotton farmers as much as $500 million a year in direct and indirect losses.

The pest is thought to have been entered Australia in nursery material from the United States in the early 1990s. It spread through horticultural areas along the Queensland and northern New South Wales coasts before the first serious outbreak in a broadacre crop occurred in cotton at Emerald in the 2001-02 season.

Its damage potential is greatly enhanced by its ability to survive on more than 500 crop and weed hosts and its propensity to rapid resistance to insecticides. As a consequence the Grains, Cotton and Horticulture Research and Development Corporations to collaborate on a control program headed by CSIRO whitefly specialist Paul De Barro and Dr Sequeira.

The surprising news from Dr Sequeira at Goondiwindi was that an expected explosion in whitefly numbers in the Emerald Irrigation Area in 2002-2003 did not occur, largely due to unexpected control from biological control agents, particularly the native, parasitic wasps.

"Dr De Barro’s sampling of whitefly numbers in the spring of 2002 showed numbers 33 times higher than in August 2001, the beginning
of Australia’s first major outbreak of the pest in broadacre crops," Dr Sequeira said.

" Monitoring of silverleaf whitefly in a range of crops by QDPI entomologists revealed 70 to 90 per cent mortality of immature stages of the whitefly by mid January 2003 as a result of parasitic wasp activity.

"One of the several factors that contributed to the 2001-2002 outbreak in Emerald may have been the result of widespread use of broadspectrum organophosphate insecticides to control mirids in cotton early in the season.

" The avoidance of OP and other broad spectrum insecticides in cotton for most of the 2002-03 growing season is thought to have contributed to a massive buildup of parasitic wasps, which prevented another explosive outbreak of whitefly."

Dr Sequeira said the cross-industry approach to management of whitefly needed to complement the role of predators and parasitoids, with a range of actions including

crop and variety selection and time of sowing.

Farmers should not plant when other crops in the area were finishing and never adjacent to curcurbits. There could be multi-fold differences in tolerance and susceptibility to whitefly between varieties of the same crop;

Using the wrong chemistry would flare silverleaf whitefly, not control it. While the pest was resistant to pyrethroids, organophosphates and carbomates there were newer chemicals offering some control, although they were expensive, and proper plant coverage was essential.

"We still have a lot of research to do, including the development of control protocols for the cotton, grains and horticulture industries," Dr Sequeira said. "We still need a lot of sampling to develop thresholds for each crop and we need to know how often we should sample and what part of the various plants we need to sample.