Field and laboratory research were conducted to determine the potential of Mamestra dissimilis Knoch., Lepidoptera: Noctuidae to control mossy sorrel (Rumex confertus Willd.). In the natural sorrel habitat a dynamics of the plant growth and the number of Mamestra dissimilis Knoch larvae occurring on that plant over 50 days were researched. The highest number of M. dissimilis larvae was caught at the end of the first decade of June when the host plant rosette was fully developed. In the laboratory, the weight of consumed food by larvae, and larval body weight were measured at 20^oC. The relations between the weight of the mossy sorrel leaves consumed and L₄ and L₅ larvae body weight gains were significant, (r = 0.97 and r = 0.88, respectively). A very progressive feeding of L₄ larvae was noted, which was defined by first degree equation (r = 0.98). It was calculated that each day over 20 days of feeding L4 larvae consumed about 5.9 mg more leaves than L₅ larvae. For L5 larvae, the relationship was the second degree equation (r = 0.96). An estimated weight of mossy sorrel leaves consumed by M. dissimilis larvae under natural conditions was 159.3 g, which accounted for about 11% loss of the host biomass.

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