RECENT EVIDENCE FOR RESISTANCE TO SOLDIER FLY AMONG SUGARCANE VARIETIES

LARVAE of the sugarcane soldier fly, Inopus rubriceps (Macquart) (Diptera: Stratiomyidae), are widespread soil-dwelling pests of sugarcane in Queensland and New South Wales. Past emphasis on varietal choice for soldier fly management has been on tolerance, i.e. choosing sugarcane varieties that are able to ratoon despite the presence of larvae. There has been less emphasis on choosing varieties that inhibit soldier fly development, mainly because there has been little or no evidence that this type of resistance exists. However, recent results of monitoring of two field trials near Mackay have caused that conclusion to be revised. Crop yields and soldier fly populations were measured annually with 15 or 18 varieties in each trial. Individual weighing of soldier fly larvae gave better discrimination between varieties than weighing of combined samples as done in previous trials. The number and average weight of soldier fly larvae differed significantly among varieties and the two were significantly correlated in some years; varieties hosting fewer larvae also tended to host smaller larvae. The effect of this weight difference on subsequent larval development was assessed in one trial for two varieties, Q138 and Q135, chosen because larvae were smaller under Q138. A lower proportion of larvae had pupated in winter under Q138 than under Q135, suggesting that the smaller larvae under Q138 may have required two years to complete their development whereas most larvae under Q135 may have developed in one year. Q138 ranked second of 18 varieties in sugar yield over a plant and three ratoon crops in that trial. Therefore, it may be possible to choose varieties that are associated with both poorer establishment of larvae in any year as well as a longer generation time and so a slower rate of increase of soldier fly populations, while also having good sugar yields.