Effects of three neem-based insecticides on diamondback moth (lepidoptera: plutellidae)

Abstract

Three commercial neem-based insecticides, Agroneem™, Ecozin™, and Neemix™, were evaluated for oviposition deterrence, antifeedant effect to larvae, and toxicity to eggs of the diamondback moth, Plutella xylostella L., in the laboratory. All three neem-based insecticides did not exhibit significant oviposition deterrence on P. xylostella. When cabbage leaves were used as an egg-laying substrate, numbers of eggs oviposited by P. xylostella adults on the cabbage leaves treated with the three neem-based insecticides were not significantly different from those treated with water. However, when aluminum foil sheets coated with cabbage juice residue were treated with the three insecticides and were used as egg-laying substrates, significantly fewer eggs were found on the aluminum foil sheets compared with those treated with water. When eggs were treated with Agroneem, Ecozin and Neemix, 61.6%, 66.2%, and 75.2% of P. xylostella eggs developed to neonates, respectively, although the larval hatching rates in the treatment of Neemix were not significantly different from that in water control (81.2%). All larvae of P. xylostella fed on the leaves treated with the three neem insecticides died on or before day 7 compared with 70–74% larvae surviving to adults in the water control. All three neem insecticides exhibited significant antifeedant effect, and P. xylostella larvae on treated leaves quickly stopped feeding and dropped off treated leaves, resulting in no or minimal damage on the treated cabbage leaves. P. xylostella larvae that fed on neem insecticide treated leaves were significantly smaller (0.012–0.016mg/larva, 13.5–14.8mm in length, and 2.0–2.5mm in diameter) compared with those fed on water-treated leaves (0.058mg/larva, 30.2mm in length, and 4.8mm in diameter).