Estimation of the population density of the sweetpotato weevils on the Mariana Islands

Abstract

The sweetpotato Ipomoea batatas L. (Convolvulaceae) has been one of the most important foods for Pacific islanders for centuries. However, the yield levels have been declining in the recent past due to the presence of sweetpotato weevils Cylas formicarius (Fabricius) (Coleoptera, Brentidae), Euscepes postfasciatus (Fairmaire) and Daealus tuberosus (Zimmer man) (Coleoptera, Curculionidae). Therefore, urgent management or eradication methods are sought in the Mariana Islands (Guam, Rota, Saipan, and Tinian). However, the management or eradication of these weevil pests requires accurate assessments of the target pest density. Currently, no advice is provided to growers on the best method for sampling sweetpotato for weevil pests, although pheromone-based traps or chemicals are being used. This study defines the results of field counts designed to adjust relative sampling techniques for three sweetpotato weevil pests by inspecting plants visually and at random in the field with an absolute measure of population density. Significant relationships were detected between the relative four sampling sites between the three weevil pests. In the dry and wet season, 90% and 35.5%, respectively, of population density of C. formicarius was noticed in Rota. This density of the population levels of this species is significantly lower in Saipan, Guam and Tinian. No incidence of E. postfasciatus and D. tuberosus was observed on Guam. However, E. postfasciatus is identified as the second most destructive pest in Rota, Tinian and Saipan in both the dry and wet seasons. Likewise, D. tuberosus is the third major pest as the recorded population density ranged from 12.5% to 2.5%. Also, it is evident from the sampling study that the population densities of all three weevils are significantly higher in the dry season than the wet season.