The Effect of Plant Nutrition on the Reproductive Rate and Susceptibility to Malathion of Two Strains of the Two-Spotted Spider Mite, Tetranychus telarius (L.)

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Studies were conducted to determine the effect of plant nutrition on the reproductive rate and susceptibility to malathion of two strains of the two-spotted spider mite. The two strains of mites used as test organisms are characterized by their widely different levels of suscept­ibility to phosphate acaricides. Lima bean plants were grown in quartz sand and supplied nutrient solutions containing varying levels of nitrogen, phosphorus and potassium. Total nitrogen and total water soluble carbohydrate in the host plant tissue were determined by standard methods. Correlation analysis with the number of mite progeny and results of leaf analyses were conducted. The effects of plant nutrition on susceptibility to malathion of the two mite strains were determined by comparing per cent mortality data of mites from plants supplied the various nutrient solutions, Dosage-mortality responses of the two mite strains were also determined for mites of both strains from plants supplied nutrient solutions producing the greatest response in susceptibility. The reproductive rate of both strains increased as the supply and absorption of nitrogen increased. However, in studies with the resistant mite, a reduction in the number of progeny occurred on plants supplied the highest nitrogen level. The number of progeny of the malathion non­resistant mite were significantly correlated to increased nitrogen absorption. Increased reproductive rate of both strains was correlated to the carbohydrate present in the leaf tissue. When carbohydrate content in the leaf tissue was increased along with an increase in nitrogen supply and absorption the mite reproductive rate also increased. At still higher levels of nitrogen supply and absorption, carbohydrate content of leaf tissue decreased and the mite reproductive rate was depressed or failed to increase, The malathion non-resistant mite appeared more responsive to plants deficient in nitrogen than the resistant mite, On plants supplied the low nitrogen nutrient solutions, non-resistant mites failed to subsist, while the resistant mites maintained relatively high populations. The malathion non-resistant mite also produced more progeny on plants when phosphorus supply was increased. Populations of the resistant mite increased at higher levels of potassium. Phosphorus and potassium supply alone and in combination with certain levels of nitrogen affected nitrogen absorption and the relative amounts of carbohydrate present in the leaf tissue. Increasing the nitrogen supply to host plants resulted in increased susceptibility of the resistant mite to malathion. The non-resistant mite was more responsive to variations in phosphorus supply of host plants. A decrease in susceptibility followed an increase in phosphorus supply.