Toxicity and dissipation of soil insecticides applied in the management of arecanut white grub, Leucopholis burmeisteri Brenk. (Coleoptera: Scarabaeidae)

White grubs, Leucopholis spp. are subterranean pests of arecanut grown in South India. Grub infestation leads to yellowing, stem tapering, and crown size and yield reduction. Use of chemical insecticide to mage the white grubs gives varying degree of success. Hence an attempt was made to screen newer and safer insecticides. Imidacloprid (LC₅₀ at 120 h = 16.849 ppm on III instar larvae), chlorpyriphos (LC₅₀ = 14.242) and bifenthrin (LC₅₀ = 12.797 ppm) were identified as effective insecticides. Evaluation of these insecticide in the field over two year period indicated the following efficacy in reducing larval population: chlorpyriphos @ 4 kg a.i/ha (83.31%) > bifenthrin @ 4 kg a.i./ha (82.83%) > imidacloprid @ 0.24 kg a.i./ha (75.84%) > bifenthrin @ 2 kg a.i./ ha (74.26%) > chlorpyriphos @ 2 kg a.i./ha (69.15%) > chlorpyriphos @ 1 kg a.i./ha (61.79%) > imidacloprid @ 0.12 kg a.i./ha (56.54%) > bifenthrin @ 1 kg a.i./ha (54.34%) > imidacloprid @ 0.06 kg a.i./ha (41.47%). Bifenthrin in soil persisted for a longer period than chlorpyriphos. On the day of application, 59.46 ppm bifenthrin residue was recovered from soil. On 10th day, it was 7.29 ppm which decreased to 2.59 ppm on 30th day and was beyond detection limit on 65th day. Chlorpyriphos exhibited a rapid degradation in the initial stage; 27.46 ppm residue on the day of application, which further reduced to 0.964 ppm on 10th day, and was below the detection limit on the 30th day. Growth of Trichoderma harzianum was not affected by bifenthrin even up to 40 ppm concentration. However, chlorpyriphos affected the growth of Trichoderma at higher than 5 ppm dose. Similarly, imidacloprid inhibited the colony growth from 2 ppm onwards. Having high lipophylic property and contact toxicity, bifenthrin would be an ideal alternative insecticide to chlorpyriphos for the management of white grubs in palm garden, which is safe and long persisting.