Some studies on Nosema infecting DBM in Malaysia

Field and laboratory studies on Nosema bombycis infecting diamondback moth (DBM), Plutella xylostella (L.), were conducted. Percent N. bombycis infection and mean infection intensity were significantly different (P<0.05) among DBM larvae or pupae collected from either highland (CH) or lowland (SG) areas. Percent infection was significantly higher (P<0.05) for both larvae and pupae (71.3 and 66.8%) in the CH than in the SG (10.0 and 2.4%). It was also noted that the dead DBM larvae and pupae collected from the field had an abundance of Nosema spores especially DBM from the highlands. Percent mortality was significantly (P<0.05) higher in smaller instars (I and II) than larger instars (III and IV) even at lower spore concentration (4,260 spore/μl) and 24 h after treatment. There was also evidence that Nosema had developed resistance to the antibiotic (Fumidil-B) commonly used in making artificial diet for DBM. The recommended Fumidil-B seems to be ineffective in stopping disease development even at 400 ppm (220 ppm – current recommendation). Other antibiotics showed no better effect in controlling the disease infection than Fumidil-B. Temperature treatment was also unable to check the disease development even at 50°C. There was evidence that Diadegma semiclausum was involved in horizontal transmission of Nosema spores among DBM larvae. Nosema was observed to have a negative effect on the diurnal behaviour of DBM and D. semiclausum. Because the time spent by severely infected parasitoids was less than that of less or uninfected parasitoids, the prevalence of disease in the field might have a negative impact on the role of the parasitoid as an effective biological control agent of DBM.