Methyl benzoate and its derivative, acetophenone, as fumigants to control stored product insects

Since methyl bromide has been phased out and recently banned in many countries, and resistance to phosphine has developed and spread worldwide, alternative fumigants are a high priority for stored product pests. The present study evaluated the fumigant activity of methyl benzoate (MB) and acetophenone (AP) against the lesser grain borer, Rhyzopertha dominica, the rice weevil, Sitophilus oryzae, and the red flour beetle, Tribolium castaneum under laboratory conditions. Methyl benzoate and AP exerted a significant toxicity against the different life stages of tested insects. The LC50 values of MB were 10.52, 12.69, and 15.53 μl/L for R. dominica, S. oryzae, and T. castaneum adults, respectively. The larvae of T. castaneum were the most tolerant stage for that species, with an LC50 value of 44.26 μl/L followed by the pupae (LC50 = 18.54 μl/L) and adults. The LC50 values of AP were 18.59, 13.15, and 25.85 μl/L against adults of R. dominica, S. oryzae, and T. castaneum, respectively. Also, the larvae of T. castaneum were the most tolerant, with an LC50 value of 150.11 μl/L followed by pupae at LC50 = 70.07 μl/L and adults. MB and AP showed significant activity against mixed life stage cultures of these beetles at concentrations of 100, 200, and 300 μl/L compared to untreated controls. However, these concentrations of both compounds were ineffective at achieving 100% control of these species. This poor control of mixed cultures was likely due to tolerance of immature stages and also the potential sorption of fumigants in the host material reducing the gas concentrations available in the treatment chambers. An in vitro inhibition assay of acetylcholine esterase (AChE) in T. castaneum larvae showed that MB and AP significantly inhibited the activity of AChE with IC50 values of 55.38 and 32.89 mM, respectively. The results from this research suggest that MB and AP could be promising alternative fumigants for protecting stored products from insect pests.