Physiological adaptations of leafhoppers that feed on xylem fluid: Exploring nutrient limitations as a component of integrated crop management

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P. C. Andersen 1, B.V. Brodbeck 1 & R.F. Mizell III 1

1 Univ. of FL. NFREC-Quincy, 30 Research Road, Quincy, FL, 32351-9500, USA

Leafhoppers that feed on xylem fluid have evolved numerous adaptations in order to subsist on this dilute food source. Most of the solutes in xylem fluid (osmolarity = 5 to 50mM) consist of inorganic ions and organic compounds in monomeric form (amino acids, organic acids and sugars). Xylem fluid is characterized by extremely low concentration and diversity of secondary compounds, and amino acid profiles are highly unbalanced with respect to an idealized diet or to insect biomass. The proportion of nitrogen to carbon in xylem fluid of most woody plant species is analogous to that of pure protein, owing to high concentrations of the amides. Adult Homalodisca coagulata [also known as the glassy-winged leafhopper (GWSS)], H. insolita and Cuerna costalis assimilate organic compounds with unprecedented efficiency (>99%). Excess nitrogen is excreted primarily in the form of ammonia, thus conserving 636 and 1926 J/mol of energy compared to urea or uric acid, respectively. Other adaptations of adult leafhoppers includes extremely high feeding rates (up to 100 times body weight per day), diurnal adjustments in feeding rates to maximize nutrient acquisition and polyphagy (or host switching) that coincides with seasonal or phenological changes in plant chemistry. Feeding is not inhibited at high xylem tensions, and is in fact often highest during midday when xylem tensions are maximum (1.5 to 2.5 MPa). The theoretical maximum suction that the cibarial pump is capable of generating is 0.3 to 0.4 MPa; adaptations that allow feeding against high xylem tensions are not currently understood. All facets of GWSS behavior and performance thus far examined have been inexorably linked to xylem fluid chemistry. Seasonal shifts in abundance of adult leafhoppers on different hosts, and leafhopper feeding rates have been correlated with the concentration of the amides in xylem fluid. Artificial diet studies have established that performance of most insects is maximized with balanced profiles of dietary amino acids, yet the performance and behavior of adult GWSS is positively correlated to highly unbalanced amide diets. Adult GWSS in the southeast United States has a host range exceeding 100 plant species; however, immature GWSS are restricted to few host species. Immature GWSS require a more balanced amino acid profile for successful development, and are much less efficient than adults at assimilating high concentrations of amides. These metabolic limitations of immature GWSS, and the concominantly restricted host range, suggest that this life stage is the most vulnerable to control via crop management practices. Index terms: Homalodisca coagulata, amides, amino acids, carbon, nitrogen

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