CONTROL THRESHOLD ON SWEET-POTATO, IPOMOE A-BATATAS L., DAMAGED BY THE WEST-INDIAN SWEET-POTATO WEEVIL, EUSCEPES POSTFASCIATUS (FAIRMAIRE) (COLEOPTERA, CURCULIONIDAE)

To obtain the control threshold for the West-Indian Sweet Potato Weevi l, the relationship between the occurrence of sweet-potato stems injur ed by the larvae and the proportion of damaged tubers at harvest was s tudied in both insecticide-treated and untreated plots. The ratio of d amaged tubers at harvest was high in plots where injured stems occurre d early and/or the proportion of injured stems was high. The regressio n equation, Y=20.83 + 0.95X (r=0.639) expresses the relationship betw een the proportion of damaged tubers at harvest (Y) and the percentage of injured stems on day 75 day after planting (X). Based on this modi fied equation, the control threshold level of stems injured during the growing period for a tolerable injury level (13.6% of tubers damaged at harvest) was estimated to be 5%. The insecticide application for th is control threshold was as effective as the conventional application; three applications could be reduced to two.