Potential of using entomop athogenic nematodes in the management of diamondback moth on kale (brassica oleracea var. Acephala)
Kale, Brassica oleracea var. acephala L., is one of the most important vegetables grown in Kenya. However, it is prone to serious damage by diamondback moth (DBM), Plutella xylostella L. Extensive and regular use of insecticides to control DBM has led local populations of Plutella xylostella to develop resistance to the commonly used chemicals and exclusion of natural enemies. This study was carried out with the aim of determining the occurrence of entomopathogenic nematodes (EPN s) in different agroecosystems, determining the efficacy of EPNs in the management of DBM and determining the effect of varying application rates and intervals on the efficacy of EPNs in the management ofDBM. Soil samples were taken from a coffee field, vegetable garden, pasture and a planted forest. Entomopathogenic nematodes (EPNs) were isolated from the soil using Galleria mellonella as the bait insect. The lethal time fifty (LT50) (time till 50% lethality) of the EPN isolates to DBM larvae was determined by -leaf disc bioassay method. Five isolates of EPNs were used in the experiments as treatments. Glasshouse experiments were conducted to determine the most appropriate application rate and application interval of Steinernema wesieri and Heterorhabditis indica in the management ofDBM larvae. Kale plants were sprayed with infective juveniles (Us) at one of three concentrations (1.25xl09 IJs/ha, 2.5xl09 IJs/ha or 5.0x109 Ils/ha) and application intervals (1 week, 2 weeks or 3 weeks). Field experiments were conducted at Kabete Campus Field Station to test the efficacy of three isolates of EPNs namely: S. wesieri, Steinernema sp.! and H indica. Bacillus thuringiensis subsp. kurstald and tap water treatments were included as controls. This study revealed that the frequency of occurrence of EPN s is different in different agroecosystems. The frequency of occurrence of EPNs was lowest in the soil from vegetable garden, 27%, followed by forest soil, 33%. Soil from pasture ecosystem had a moderate v frequency of occurrence of EPNs, 50%. EPNs were most frequently isolated from coffee field, which is rated as a moderately disturbed ecosystem, 77%. H indica, S. karii, S. wesieri, Steinernema sp., and Heterorhabditis sp. caused 96.0%, 93.3%, 92.0%, 88.0% and 86.7% mortality in DBM larvae, within 72 hours under labolatory conditions, respectively. The mortalities were not significantly different among the EPNs tested. S. wesieri showed the greatest ability to penetrate into DBM larvae,75.0% penetration, while the least level of penetration, 19.2% penetration, was recorded on Heterorhabditis sp. Applications of S. wesieri, H indica, and Steinernema sp. infective juveniles (IJs) reduced DBM damage on kale by 21.0%, 9.0%, and 7.0%, respectively under field conditions. The application of Bacillus thuringiensis subsp. kurstaki caused a 20.0% reduction in DBM damage on kales. Application of EPNs at a rate of 2.5xl09 IJs/ha and 5.0x109 IJs/ha consistently suppressed the population of DBM larvae in the glasshouse. At half recommended application rate, 1.25x109 IJs/ha, the DBM larvae and pupae population was high. The effect of altering the application interval of EPNs on percentage mortality of the DBM larvae was not significant. This study has shown that occurrence of EPNs is different in different agroecosystems. This study has also demonstrated that some EPN isolates have a great potential that may be exploited in the . management ofDBM and foliar application should be done at 2.5x109 Ils/ha (2.5xI05 IJs/m2) at weekly intervals and targeting 3rdnd instar DBM larvae. Farmers need training to increase their understanding of EPN s and the target pest's biology and ecology to facilitate appropriate timing and more effective and efficient application technology of EPNs. EPNs need to be used along with other suitable integrated pest management (IPM) strategies in the management of DBM so as to ensure commercial quality of kale.