METHODS USED TO MEASURE THE SUSCEPTlBilLlTY OF SUGARCANE VARIETIES TO ATTACK BY ELDANA

A quick method is needed to determine the susceptibility of sugarcane to attack by eldana borer. Plants of three varieties and of differing ages of cane were tested for attractiveness of the plant to an ovipositing moth and for the performance of the larva when feeding. The preference of moths for certain varieties was studied by releasing moths into a large cage containing potted plants of the three varieties, and the number of eggs laid on each plant was counted. To measure larval performance, about 100 eggs were placed on each plant. After a period during which 500 day-degrees above 15°C had accumulated (the minimum duration of one larval stage in sugarcane), the survivors were counted and weighed. The methods used and the problems encountered are described and results from initial trials are presented. Introduction The borer, Eldana saccharina Walker reappeared as a pest of sugarcane in 1970 (Carnegie3) and has since spread to most areas of the South African sugar industry (Paxton7). Numerous research programmes have been initiated to study and control the pest (Carnegie4. 9. Research on the susceptibility of varieties to attack by'eldana has received little attention apart from the assessment of the presence of eldana in variety trials. Some information (Atkinson'; C a ~ n e g i e ~ ~ . ~ ) has been published and is being used as a guide when varieties are recommended. However, the wide variation in eldana numbers in the trials has made it necessary to assess several trials before the results can be used. Infestation by eldana is affected by the age of cane, variety, soil type, soil moisture content and chemical ripeners (Carnegie4.5). Methods that can be repeated and that are not affected by other factors are required to enable the reaction of eldana borers to different varieties to be determined. The two aspects of cane susceptibility to eldana. infestation which were investigated were differences due to the variety and to the age of the cane. Both the attractiveness of the plant to an ovipositing moth and larval performance, measured by the number and biomass of larvae, were assessed. Methods Cane plants Two budded setts of cane were planted in a mixture of sand, soil and compost in 25fdrums (surface area 0,07 m2) on 4 December 198 1. The drums were kept in a glasshouse for the first month until the plants were established. Thereafter they were moved onto a terrace where they were watered regularly and fertilized with 30 g 5.1.5(47) fertilizer on 5 February, 9 March, 27 April and 5 May. Three varieties were planted: NCo 376, the standard commercial variety, N11, which appeared to be more susceptible to eldana attack than NCo 376, and N12 which, in some trials, appeared to be less susceptible than NCo 376. No attempt was made to prevent natural infestation by eldana and other borers, and all drums received the same treatment. Moth preference At the start of each experiment, drums containing the three cane varieties were placed under a frame covered with 40°/o plastic shade cloth through which no moths could enter or escape. The drums were spaced 30 cm apart and the floor was lined with black plastic sheets. Nedy emerged moths were released into the cane each afternoon for the duration of the four experiments. The experiment lasted four days in summer, and nine days in winter, which is the duration of the egg stage. Details of the experiments are contained in Table 1. TABLE l Moth preference trial; details of Experiments 1 to 4 On the predetermined date, the cane in each drum was searched thoroughly for eldana eggs. To avoid any bias, each worker searched three drums. In Experiments 1, 2 and 3 experienced staff searched for and counted the eggs. In Experiment 4, some of the staff were inexperienced, and the eggs were placed in vials and counted later with the aid of a dissecting microscope. The dry leaves and trash from each drum in Experiments 2 and 3 were weighed and in Experiment 4, the dry leaves, which were attached to the stalks, were weighed separately from the loose trash. The number of eggs found in the dry leaves and loose trash were recorded separately. A sucrose analysis was carried out on cane stalks from Experiments 2, 3 and 4. Age of cane (months) Date experiment started No. of replications No. of drums per replication Duration number of nights No. of moths released per night: Males Females Total number of moths released Larval performance The details of three experiments conducted in the plant breeding glasshouse to measure larval performance are contained in Table 2. In Experment 5 the drums of one variety were placed together in a replication and in Experiments 6 and 7 the drums were placed in a randomized latin square design. Plants were placed in the glasshouse two weeks before they were artificially infested so that any larvae from natural infestations could emerge as moths. In addition, light traps were used in the glasshouse so that the emerging moths would not re-infest the plants. Experiment No. Known numbers of eldana eggs which had been laid on tissue paper in an artificial culture and which were about to hatch, were placed between the leaf sheaths and stalks in each drum on three or four consecutive days until there were approxiI