1 ONION SEEDLING PROTECTION AGAINST THRIPS TABACI LINDEMAN AND ITS COMPARATIVE ECONOMIC BENEFIT OVER PESTICIDE SPRAY M.M. Waiganjo 1 , S. Sithanantham 2 , J.M. Mueke 3 and L.M. Gitonga 4 1 KARI-Thika, P.O Box, 220, Thika 2 International Centre of Insect Physiology and Ecology (ICIPE). P.O Box 30772, Nairobi 3 Department of Zoology, Kenyatta University. P.O Box 43844, Nairobi 4 Jomo Kenyatta University of Agriculture and Technology, P.O Box 62000, Nairobi Abstract The onion thrips, Thrips tabaci Lindeman is a major constraint in onion production worldwide. Two field trials were conducted to assess the potential of onion seedling management options against onion thrips and their economic benefits in comparison with routine pesticide application. The design of the trials was randomised complete block design of 8 treatments replicated 4 times. The treatments were onion seed coating with Thiamethoxam 350g/l a.i (Cruiser®), seedling drenching with Neem oil Azadirachtin 0.15%w/w (Achook®), seedling drenching with neem powder Azadirachtin, 0.03% w/w (Neemros®), seedling drenching with Imidacloprid 200g/l a.i (Confidor®), overnight seedling dipping in Confidor® and soil placement with Carbofuran 5% a.i (Furadan®) during transplanting. These single dose treatments were compared with Profenofos 400g/l a.i plus Cypermethrin 40g/l a.i (Polytrin®) fortnightly foliar spray and unprotected control. All the preventive treatments reduced thrips infestation significantly (P=0.05) during the first and second month after transplanting in both trials, except Neemros® seedling drenching which was not effective during the second month in the second trial. Confidor® seedling drenching and Furadan® soil placement had the longest residual effect and significantly increased onion bulb yield and quality. There was significant negative relationship between onion yield and thrips infestation, which was best described by a third order polynomial regression model. Confidor® seedling drenching gave the highest economic benefits among the treatments and resulted in comparable yield increase to routine fortnightly spray with Polytrin®. The results suggest that onion thrips management may be optimised through systemic pesticide seedling drenching application. Introduction The bulb onion Allium cepa L. is an important source of income for smallholder farmers and businessmen in Kenya. Despite there being favourable conditions for onion production, the demand exceeds the supply thus necessitating importation. Onion yields in Kenya range between 5 and 20 t ha -1 , compared to over 30 tonnes ha - 1 in countries such as Spain, Japan and China (FAO, 1999). The low yields are attributed to low yielding cultivars, poor agronomic techniques, diseases and insect pests such as onion thrips, Thrips tabaci Lindeman. Yield losses ranging from 18 to 60% have been reported in Kenya due to thrips damage (Waiganjo, 2004). Onion protection against thrips infestation is therefore essential for economic production of the crop. Control of onion thrips in Kenya and other sub-Saharan countries is mostly by pesticide sprays. Although pesticides provide the most potent means of suppressing pests, their use should be minimised due to their numerous hazards. To protect the resource- poor farmers from pesticide contamination and high production costs, there is need to develop low-cost technologies that will reduce pesticide exposure and the high cost of onion production. The objective of this study therefore was to determine the potential of single dose preventive pesticides in controlling onion thrips and to assess their economic benefits with the aim of identifying promising options for incorporation in integrated pest management (IPM). Materials and methods Two field trials were conducted at the Kenya Agricultural Research Institute (KARI) Mwea farm in Kirinyaga District. The onion seed variety Red creole was sown on 15 th February and 26 th May, 2001 for the first and second trial respectively. The trial plots of 3x3m each were arranged in randomised complete block design of 8 treatments, replicated 4 times. The 8 treatments tested were seed coating with Thiamethoxam 350g/l a.i (Cruiser®) during planting, seedling drenching with Imidacloprid, 200g/l a.i. (Confidor®), overnight seedling dipping in Confidor® solution, seedling drenching with neem extracts consisting of Azadirachtin, 0.15%w/w (Achook®) and Azadirachtin, 0.03% w/w (Neemros®), soil treatment with Carbofuran 5% a.i (Furadan®) during transplanting and an unprotected control. The preventive treatments were compared with Profenofos 400g/l a.i plus Cypermethrin 40g/l a.i mixture (Polytrin®) sprayed fortnightly as the standard. Recommended rates on the manufacturer’s label were used for application of all the pesticides. Absolute thrips density estimation was carried out by whole plant cutting and bagging (Freuler and Fischer, 1984), starting from 14 days after transplanting and at weekly intervals thereafter, until the onion crop matured. Thrips extraction from the cut onion plants was done in the laboratory, using 70% alcohol and counting under a dissecting microscope. The onion crop was harvested when more than 50 % of the leaf bundles had lodged. Bulb yield