Review of soldier flies in sugarcane and identification of some key research gaps

Soldier flies (Inopus spp.) (Diptera: Stratiomyidae) feed on the root system of sugarcane and in some cane-growing regions of Australia, especially Central and Southern Queensland, are considered an economic pest complex. Past studies and some current research gaps for sugarcane soldier flies are presented. Low numbers of soldier fly larvae can cause significant damage to sugarcane due to their ability to cause poor ratooning that ultimately leads to poor growth and reduced yield. Soldier flies have a complex life cycle of up to 2 years, unpredictable outbreaks, relatively uncharacterised varietal choice, and patchy distribution. These factors make research activities and management options challenging. Most studies on the biology, ecology and control approaches for soldier fly have focused on a single species I. rubriceps, even though a further species I. flavus is also known to attack sugarcane. These studies are reviewed here. However, there are at least five major genetic groups of soldier fly in Queensland cane fields and at least a further four uncharacterised species. Their geographical distribution and relative economic significance in terms of crop damage across regions have not been determined. This diversity of soldier flies highlights the need to further improve knowledge on the biology, genetic diversity, and ecology of these insect pests, and insufficient baseline information on the soldier fly complex is a potential barrier for development of targeted control methods. Currently, only cultural practices for control of soldier flies are recommended, and reports of infested localities are increasing. Hence, new approaches to research are required. One new approach is the development of an in vitro rearing method using an artificial diet for sugarcane soldier fly larvae to compare the nutritional requirements of species, which would inform varietal selection. Development of a rearing methods is also a prerequisite to establish rapid screening of pesticides, including chemical and biorational options, against healthy soldier fly larvae in the laboratory prior to field trials and varietal screening. A new research approach, focusing on the development of an artificial diet and characterising species distribution, is outlined.