Giving it our best shot in the war against soldier flies - future research directions

Soldier flies are economically damaging pests of sugarcane, particularly in central and southern Queensland. Despite decades of research on soldier fly control, the search for an effective management approach, except for cultural control, remains elusive. Trials were conducted from 2015 to 2017 to identify potential management solutions for soldier flies by assessing insecticide efficacy and varietal tolerance in field conditions. Five field trials were established to determine whether applying insecticide to plant cane would reduce the build-up in soldier fly larvae in subsequent ratoons. Ten products, comprising seven active ingredients, were field-tested at high application rates. Overall, as in most previous studies, none of the insecticides tested reduced the number of larvae in field-trial conditions. The inefficacy of insecticide treatments could be due to products failing to come into contact with soldier fly larvae or simply lacking effective activity. In addition, three field trials, using up to 14 varieties, were conducted, to assess varietal tolerance. Some varieties tended to host fewer larvae than others, suggesting some resistance, in two trials established in southern Queensland. Any future insecticide and varietal screening trials will need to be conducted in both controlled laboratory and field conditions. However, before such trials can be undertaken, a standardised laboratory rearing method and improved field sampling strategy for soldier flies needs to be developed. Soldier fly outbreaks are also unpredictable and developing methods to forecast them (e.g. using climatic data or identifying preferential soil properties) will also be highly beneficial in informing growers of the potential risk of soldier fly establishment in their paddocks and for selecting field-trial sites. Additionally, recent DNA barcoding and morphological studies have revealed that at least six species of soldier flies are found in sugarcane, not two as previously identified. That finding highlights that the distribution of soldier fly species in Australia and the relative damage to sugarcane varieties needs to be resolved to enable the development of targeted species-specific management approaches.