Soontorn Pipithsangchan, Sanan Subhadhirasakul, Thiwa Butpha, Narubodee Phadoongsombat, and Kan Chantrapromma

pp. 221 - 232

Abstract

n-Hexane and methanolic extracts from the seeds of Azadirachta excelsa Jack. (Tiam) and A. indica
var. siamensis Valeton, Bacillus thuringiensis var. kurstaki HD-1 and abamectin were tested for mortality
effect on the 2^nd and the 3^rd instar larvae of diamondback moth (Plutella xylostella Linn.) by topical application and feeding methods. Among the seed extracts, the methanolic extract from A. excelsa was the most active. Its LC₅₀ at 72 hours after testing on the 2^nd and the 3^rd instar larvae of diamondback moth using topical
application was 16,298.4 mg/l and 28,225.4 mg/l, respectively, and that by feeding method was 3,921.1 mg/l
and 5,136.2 mg/l, respectively. The methanolic extract was more active than the n-hexane extract in each
plant. Among the tested samples, abamectin, a chemical insecticide, showed the highest insecticidal effect.
Its LC₅₀ at 72 hours after testing on the 2^nd and the 3^rd instar larvae of diamondback moth using topical
application was 81.3 mg/l and 227.9 mg/l, respectively, and that by feeding was 44.2 mg/l and 112.8 mg/l,
respectively. The methanolic extract of A. excelsa to reduce the population of the 2^nd instar larvae of diamondback moth on Chinese kale (Brassica alboglabra Bailey) was not significantly different from that of the
methanolic extract of A. indica var. siamensis. The n-hexane extract at the concentration of 5% of A. excelsa
to show antioviposition effect of the diamondback moth on Chinese kale was more active than the n-hexane
extract of A. indica var. siamensis.