Residual toxicity of Methomyl and γ-Cyhalothrin against Papaya mealybugs on Cassava Leaves

Document Type : Original Article

Authors

1 Cebu Normal University, Osmeña Boulevard, Cebu City, Philippines 6000

2 College of Agriculture and Food Sciences, Department of Pest Management, Visayas State University, Baybay City, Leyte 6521-A PhilRootCrops, Philippine Root Crop Research and Training Center, Visayas State University, Baybay City, Leyte 6521-A

Abstract

 
Paracoccos marginatus Williams and Granara de Willink (P. marginatus or papaya mealybugs [PMBs]) is responsible for 75% of the estimated economic loss in cassava growing worldwide. Insecticidal formulations have the ability to manage P. marginatus by deteriorating its wax coats to death; nevertheless, it has received little attention in the the tropical setting. In the present study, we investigated the residual toxicity of methomyl and γ- cyhalothrin against papaya mealybugs (PMBs) (Paracoccos marginatus) in cassava (Manihut esculenta) leaves and determined what stage in the PMB’s life cycle the pest is killed by the chemical. Using the label recommendation rate, each insecticide was sprayed onto the surface of healthy leaves of cassava plants. Leaf samples were taken after  a period of 2 days from a previously sprayed cassava plant.  Using leaf dipping bioassay, cassava leaf samples were cut into circular disc and placed in petri dish lined with wet filter paper, and 20 crawlers, 20 pre-adults, and 20 adult PMBs were introduced into each dish.. The The preparation was under controlled conditions of temperature, relative humidity and photoperiod (25°C; 80% RH; 12h light: 12h dark) in the laboratory and the number of dead insects was recorded. Each group was replicated in three replicates for each stage of the pest's life and for each insecticide tested. Results showed that crawlers were the most susceptible to both insecticides studied. For the residual toxicity test, the half- life of methomyl is 5.1 days while that of γ-cyhalothrin is 4.7 days. These findings suggest that farmers have to wait 5.1 and 4.7 days, respectively for their next spray application schedule.  Furthermore, as part of food safety measures, when fresh cassava leaves are used as human food and animal feed, the waiting time for the insecticides to dissipate from the leaf surface is the same as previously stated.

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