Lethal and sublethal effects of different ecotypes of Melia azedarach against Plutella xylostella (Lepidoptera: Plutellidae)

Abstract

Shade-dried, powdered drupes of Melia azedarach L. were selected from three states in India, each representing a different geographical region, viz. Rajasthan (arid), Punjab (open plains), and Himachal Pradesh (hilly). Drupes were extracted in chloroform: methanol (9:1) and evaluated against second instar larvae of Plutella xylostella (Linnaeus) using a ‘leaf-disc dip’ bioassay. The ecotype from the arid region of Rajasthan yielded the lowest LC50 (1.47%), followed by that from the open plains of Punjab (1.79%) and the hilly areas of Himachal Pradesh, which yielded the maximum LC50 (2.29%). The survivors demonstrated various types of sublethal effects such as prolongation of larval period (7.0–7.8 days compared to 6.3–6.5 days in the control), reduction in pupal weight (36.0–38.4mg/10 pupae in comparison to 54.4–56.0 mg in the control), deformation of pupae (up to 69.7%) and inhibition of adult emergence (up to 92%). A highly pronounced curtailment of fecundity (up to 10.4 against 73.2 in the control), significant reduction in oviposition period of the adults (up to 3.5 days compared to 7.4 days in the control) and longevity (5.5–7.8 days against 8.9–10.3 days in the control), were also recorded. The extracts produced from trees in the arid region of Rajasthan caused both lethal and sublethal effects following treatment of the second instar larvae of P. xylostella. Only 1.0% normal adults survived at the end of the study compared to 3.5 and 5.5% adult survival in the extracts from the open plains and hilly area ecotypes, respectively. An understanding of the bases of these variations in the toxicity of the crude extracts from the M. azedarach ecotypes growing in different climate conditions may open up the possibility of propagation and genetic improvement of the desired traits in the future.