Relative insecticidal value: An index for identifying neem trees with high insecticidal yield

摘要

Insecticidal property of 38 neem trees, sampled from six locations in Karnataka, was evaluated through laboratory bioassays of the neem seed kernel extract (NSKE) against the second instar larvae of cabbage diamondback moth, Plutella xylostella (L.). The assays revealed a four-fold difference between trees for LC50 values generated at 96 h after treatment. This large variation in the insecticidal property of trees, which can be attributed to both qualitative and quantitative differences in neem seed chemicals, has important implications, especially for the identification of trees with high insecticidal yield for propagation. An index, viz. Relative Insecticidal Value (RIV) was developed using the LC50 values and the 100 seed weight (test weight). This index can be used to identify elite trees of neem from the point of view of insecticidal yield. Further, in the absence of any information on the quality of seeds obtained from different trees, the farmers can be advised to mix seeds from as many trees as possible for preparing NSKE and thus avoid the possibility of using seeds from trees with poor insecticidal activity. [References: 15]