Resistance of Australian sugarcane clones to moth and weevil borers in Papua New Guinea

Papua New Guinea (PNG) has several moth borers (Lepidoptera) that damage commercial crops of sugarcane in the Ramu Valley. These borers are not present in Australia but would almost certainly cause very serious losses if introduced. To prepare for a possible incursion, Australian cultivars and promising clones were tested for their susceptibility to these borers in field trials at Ramu Agri-Industries, PNG. Three field trials were planted, one in each of 2010 (32 clones), 2011 (33 clones) and 2013 (32 clones), and sampled for borers in the plant and first ratoon crops. There were significant differences among clones for internode damage from Sesamia grisescens Warren (Noctuidae) and Chilo terrenellus Pagenstecher (Crambidae) and meristem damage from Scirpophaga excerptalis (Walker) (Pyralidae), as well as internode damage from the weevil borer Rhabdoscelus obscurus (Boisduval) (Coleoptera: Curculionidae). Generally, the range of damage among clones in each trial was less for C. terrenellus than for the other borers. The proportion of damaged internodes was strongly correlated with the proportion of damaged stalks for all borers and so the proportion of damaged stalks could be used as a surrogate estimate of internal stalk damage for rapid sampling. Levels of damage were correlated, positively or negatively, between species of borers in some trials. Damage from R. obscurus was positively correlated with damage from the two stalk borers, S. grisescens and C. terrenellus, which agrees with its status as a mainly secondary pest; its damage was not correlated with damage from Sc. excerptalis which affects only the stalk tops and the apical meristem. Damage correlations among the moth borers could reflect similarities or differences between resistance profiles of clones against different species, or could be due to direct or indirect interactions among species. Thirteen standard clones were included in all three trials. Among data sets, results for the standard clones were least consistent for C. terrenellus and most consistent for Sc. excerptalis. Some evidence was obtained that antixenosis may play a role in influencing resistance of clones to Sc. excerptalis and, perhaps, S. grisescens, but more research would be needed to confirm this.