Azadirachtin use efficiency in commercial neem formulations

摘要

Biological efficacy of eight commercial neem formulations (CNFs) were investigated in two separate studies. The assays were conducted against second instar larvae of diamondback moth (DBM), Plutella xylostella, at different time intervals from the date of manufacture of formulations. The formulations represented a wide range of azadirachtin content and different formulation techniques. The studies demonstrated a possible loss of biological activity with increasing duration of storage of the CNFs. In both the studies, the LC50 values of the formulations decreased with the concentration of azadirachtin in CNF and the observed decay functions were significant. However, since the decrease was not a linear function, the relationships were re-examined between the azadirachtin content in the CNFs and the LC50 values expressed in ppm of azadirachtin. In both the studies it was observed that the relationships were strongly positive and followed power functions, suggesting lower use efficiency of azadirachtin with higher azadirachtin content in the CNFs. The observed lower efficiency may be due to two possible non-exclusive reasons. One, the concentration of azadirachtin results in the loss of other possibly synergistically active compounds found in neem, which are lost due to efforts of concentration of one chemical. Secondly, azadirachtin has several effects on the biology of insects. It is possible that the different activities may be more pronounced at specific concentrations and that less toxic antifeedant or repellent effects may be evident at higher concentrations that led to the observed results. Which of these two explanations is more plausible is an interesting question that needs to be pursued. Nevertheless the observed results call for a re-examination of efforts towards increasing the concentration of azadirachtin in CNFs.