Abstract: During the period 2006–2009 in the experimental field of IFC–Pleven were conducted studies with 8 varieties of alfalfa included in the Official variety catalog of Bulgaria (2005): Europe (France), Prista 2 (Obraztsov Chiflik, Ruse), Prista 3 (Obraztsov Chiflik, Ruse), Prista 4 (Obraztsov Chiflik, Ruse), Obnova 10 (IFC, Pleven), Pleven 6 (IFC, Pleven), Dara (IFC, Pleven) and Multifoliate (Obraztsov Chiflik, Ruse). For determination of the damage from the large alfalfa weevil Otiorrhynchus ligustici on the root system of plants was developed methodology depending on the root area and the damage area. The area of the roots was calculated by the formula S = (π * d * L) / 2, where π is 3.14; d–diameter of the root collar, L–length of the root. The area of the furrows оn the root system was product of length and width of the furrows. With the lowest percentage of furrows and tolerant to O. ligustici were Multifoliate, Prista 2 and Europe–respectively 10.38, 11.41 and 11.68%. Highly sensitive, with high percentage of damage were distinguished Dara, Prista 4 and Prista 3 (respectively 14.28, 14.05 and 13.58%). As a result of the caused damage the content of sugars and saponins increased as depending on the sensitivity of varieties the increase was different. In the tolerant varieties to the pest the increase in sugar content was less pronounced, but in the sensitive ones–stronger. In regard to the saponins the trend was reversed–the increase in the sensitive varieties was less and in the tolerant varieties–stronger which could be explained with the defensive reaction of plants to compensate the losses from damages of the pest.
Key words: alfalfa, varieties, method of damage degree, biochemical composition, Otiorrhynchus ligustici