Twospotted Spider Mite (Acari: Tetranychidae) Infestations on Soybeans: Effect on Composition and Growth of Susceptible and Resistant Cultivars

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Abstract
Effects of feeding of the twospotted spider mite, Tetranychus urticae Koch, on leaf and seed composition and dry matter accumulation in two soybean, Glycine max (L.) Merr., cultivars previously found to respond differently to mite infestation were investigated to gain a greater understanding of resistance mechanisms in soybeans to this wide host range plant pest. The two soybean cultivars, ‘Bonus’ and ‘Williams’, were infested with mites at the V3, V6, and R2 stages of development with 0, 10, and 200 mites per first fully expanded trifoliolate leaf. A relatively greater reduction in dry matter accumulation was seen in ‘Williams’ than in ‘Bonus’ at the same mite population level on the foliage. However, mite population increase was significantly lower on ‘Williams’ than on ‘Bonus’. Increasing mite infestation levels caused a slight increase in total soluble sugars and starch and a small decrease in total nitrogen and phosphorus levels of the leaves of both cultivars. Mite infestation had no effect on composition of seeds. Increasing mite damage caused a large reduction (55.26%) in leaf chlorophyll content and an even more dramatic loss (79.3%) in leaf carotenoid content with a greater loss in ‘Williams’ at the low mite infestation level. Results of these studies indicated that at least part of the increased resistance of ‘Williams’ to damage from T. urticae was due to a greater hypersensitive response of ‘Williams’ to mite attack. This response increased oxidizing conditions in the leaves, resulting in an extensive loss of carotenoids and chlorophyll. Increased oxidizing conditions in ‘Williams’ decreased mite population increases to a greater extent than it decreased net photosynthesis (dry matter accumulation).