Honey bees of the cardamom ecosystem and the selective toxicity of diafenthiuron to four different bee species in the laboratory

Bees are important pollinators necessary for fruit set in cardamom (Elettaria cardamomum). Apis cerana indica, Apis dorsata, Trigona iridipennis and Amegilla spp. were observed in cardamom plantations of south India. Since cardamom is attacked by an array of insect pests, synthetic insecticides were used in crop management. Diafenthiuron is one such insecticide found to be very effective against the cardamom borer, Conogethes punctiferalis and thrips, Sciothrips cardamomi. Most insecticides used in crop protection are reported to be hazardous to bees, so diafenthiuron was also tested for its toxicity to bees. Diafenthiuron was found to be slightly harmful to A. dorsata, A. cerana indica, A. florea and moderately harmful to T iridipennis. In particular, it was slightly to moderately toxic to bees on contact with treated surfaces in the laboratory. T iridipennis and A. dorsata are found abundantly in the cardamom ecosystem and unfortunately highly susceptible to the insecticide. It was found that diafenthiuron is highly toxic when applied to the thorax of bees but it is less toxic on ingestion, i.e, contact toxicity is higher than oral toxicity. The labellum, the attractive part of the cardamom flower, where the bees land is not heavily exposed to pesticide sprays in the field because of its position and other reasons, so bees have a reduced chance of exposure to the pesticide in the field. Under critical situations, however, spraying of the chemical during peak bee activity should be avoided.