Potential cane and sugar losses from top-shoot borer, Scirpophaga excerptalis (Walker) (Lepidoptera: Crambidae)

Top-shoot borer, Scirpophaga excerptalis (Walker) (Lepidoptera: Crambidae), is one of the moth borers considered as pests of sugarcane in Papua New Guinea. Larvae bore from the top of the cane through the spindle and into the growing point (meristem), killing the stalk. Cane of 4 weeks old through to mature cane can be damaged. Here, the population dynamics of top-shoot borer at Ramu, Papua New Guinea, and estimated sugarcane juice losses are reported. The life cycle of top-shoot borer is 48–62 days and cane of all ages can be attacked. Populations of top-shoot borer remained low from September to the end of April, but numbers increase from May to July, most likely due to no insecticide spraying. Damaged stalks have a severe impact on cane yields and juice quality and, hence, on sugar yields. In selective sampling, estimated yields for undamaged cane were over 100 t/ha compared to 40 t/ha with damage levels higher than 80%. The estimated sugar yields were higher than 12 t/ha with no damage but declined to <3 t/ha when damage levels were more than 80%. In small-plot trials, cane yield was 59 t/ha when the damage levels at harvest were 44%. Cane yield was less than 28 t/ha when the damage level was 76%. Sugar yields were 7.0 t/ha when the damage level was 44% and was 3.5 t/ha when the damage level was 76%. The reduction in sugar yield was about 50%. An average of 0.8 t sugar/ha loss occurred, with estimated total value of US$5.43 million per year. Key words Top-shoot borer, Scirpophaga excerptalis, cane and sugar losses