Abstract Key factors are those causes that are most responsible for the observed changes in population density between generations. Stage-frequency of alfalfa weevil was modeled using Manly-1997 model. Multiple decrement life table parameters from an 8 year-field study were analyzed using traditional andkcontribution methods. In traditional method, the key factor was determined askLII, the death of period-two larvae from all mortality factors, whereas ink contribution method, the key factor was determined as b2&3, oviposition rate of females per square meter. These differences result directly from the sensitivity of the pop-ulation growth rate, k, to variation of both k and b in various stages of the weevil. Among all mortality factors, only kLI-Z, the death of period-one larvae from the entomopathogenic fungus,Zoophthora phytonomi(Arthur) Batko (Zygomycetes: Entomophthoraceae),kLIIandkLII-Z, the death of period-two larvae from Z. phytonomi, acted in a density dependent fashion. Remaining factors were independent of population densities affected. Using k contribution method, life table approach still remains a major way of studying the dynamics of field populations for applied ecologists and population managers.
