The tiny Trichogramma wasp continues to impress cotton growers through its ability to curb the destructive activities of the major inspect pest of their crops.

Farmers and researchers have found the Trichogramma wasp can destroy most eggs of the destructive heliothis insect pest, earning it a pre-eminent place in cotton pest biocontrol.

Queensland’s Department of Primary Industries IPM extension officer Austin McLennan said that at farmer meetings held on the Darling Downs there were reports of Trichogramma having destroyed 90 to 100 per cent of heliothis eggs in crops.

Mr McLennan said meetings had been held to discuss issues such as insect pest pressure on crops, beneficial insect numbers and Trichogramma monitoring.

"There continues to be enthusiasm at these meeting for biocontrol generally and specifically for the value of Trichogramma," he said.

More cotton growers and their consultants were using Trichogramma to varying extents in heliothis control or integrated pest control (IPM) programs, Mr McLennan said.

DPI entomologist and Trichogramma specialist Brad Scholz said there were several reasons for the impact the wasp was having on heliothis control:

• the release of both transgenic cotton varieties that needed little or no spraying with insecticides and insecticides that were less damaging to populations of Trichogramma and other beneficial insects;
• the variety of crops grown on the Darling, particularly grain sorghum and maize, that acted as nurseries for Trichogramma and other beneficial insects;
• the re-release in 1995 of an exotic species of Trichogramma, T. pretiosum;
• exponential Trichogramma population increases, with one female potentially able to produce several million female offspring in four generations and about 40 days in summer.

Dr Scholz said although some spectacular results had been achieved with Trichogramma, management of beneficial insects was an issue in commercial cotton crops and involved counting heliothis egg and larvae and using the information to consider natural egg mortality when making spraying decisions.

"We have shown that Trichogramma can have a huge impact on heliothis in Ingard cotton, including a season when there was no significant yield difference between a crop that was sprayed seven times and one that was unsprayed. Trichogramma killed 90-100 per cent of eggs from mid-January onwards.

"It's a matter of having enough confidence in the counts to be able to make a decision of whether to spray and risk losing beneficial insects or doing nothing and letting the beneficials do their job.

"Alternatively, heliothis eggs should be collected to determine the levels of egg parasitism. The DPI can provide advice on this."

Dr Scholz said rather than relying on a natural build-up in Trichogramma numbers some farmers were releasing insects into their crops early in the season to speed up the process.

Trials on the Darling Downs, and at Emerald and Goondiwindi, suggest this could help populations increase early and more rapidly, he said.

The Trichogramma wasp destroys heliothis eggs by stinging them. The eggs are parasitised causing them to turn jet black, which helps farmers and consultants find those that have been destroyed.