Eretmocerus hayati was first released in late Oct 2004 and over the next 2.5 years it has since spread has far south as the Sydney Basin, west into the northern NSW cotton production areas and in Qld from the NSW border to the Burdekin and as far west as St George, Roma and Emerald; coverage throughout this area is now complete. Levels of parasitism of 4th instars across cotton production areas prior to the releases (1995-1999) averaged 0.04±0.01. In April and May 2007 this had increased to 0.23±0.02 an approximately 6 fold increase. Prior to the releases parasitism of the silverleaf whitefly in 585 samples did not exceed 60% of 4th instars whereas in April/May 2007, 68 of the 478 samples exceeded 60% parasitised 4th instars. Eretmocerus hayati is now the most widespread and abundant parasitoid of silverleaf whitefly in Australia.

Assessing impact is not been straightforward, however, a recent survey by Growcom showed that in coastal vegetable production areas growers observed that silverleaf whitefly numbers were considerably lower and many had either not had to intervene against the whitefly or had greatly reduced the need to apply insecticides. However, the drought and the subsequent lack of crops and weeds have made it more difficult to assess probable impact. The reason is that our work in the vegetable production system in Bundaberg has shown that silverleaf whitefly is better able to colonise crops at a distance to the nearest source. This data is still preliminary, but suggests a crop needs to be <2 km from the nearest source in order to achieve optimal colonisation by the parasitoid. The lack of suitable crops and weed refuges is likely to have considerably changed the landscape from what would normally be expected, this is likely to inturn significantly affect the way both the parasitoid and the whitefly interact with the landscape, but the Bundaberg study suggests that parasitoid will be more affected by habitat fragmentation than the pest.

In, the release of E. hayati has been a remarkable success. To be able to demonstrate such wide coverage in a short period of time and a ready capacity to establish suggests that E. hayati holds considerable promise has a control agent. The next step is to combine on farm management decisions with our knowledge of landscape features such as farm layout and cropping composition in order to understand the circumstances which increase or decrease the capacity for the parasitoid to effectively control silverleaf whitefly numbers. Through this we will be able to develop a grower management guide to how best to encourage the parasitoid to colonise crops and in so doing help achieve a sustainable reduction in the use of pesticides to manage silverleaf whitefly and an overall reduction in the impact of the pest on producers’ bottom line.