Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2005
Publication Date: 10/1/2005
Citation: Bi, J.L., Castle, S.J., Byrne, F.J., Tuan, S.J., Toscano, N.C. 2005. Influence of seasonal nitrogen nutrition fluctuations in orange and lemon trees on population dynamics of the glassy-winged sharpshooter (homalodisca coagulata).. Journal of Chemical Ecology 31(10): 2289-2308. Interpretive Summary: The glassy-winged sharpshooter (GWSS) Homalodisca coagulata (Say) is an exotic insect in California and important vector of Xylella fastidiosa, a xylem limited, plant pathogenic bacterium that causes Pierce’s disease (PD) in grapes. During the 1990s, populations expanded throughout much of southern California and became conspicuous in citrus orchards and vineyards as well as in much of the urban landscape. In Temecula valley, a major wine grape production area in southern California, wineries have lost 20-30% of their vines to PD since 1997. Dietary nitrogen is an important nutritional index impacting survival, growth, and reproduction of phytophagous insects. Nitrogen nutrients are particularly limited for xylophagous insects, such as GWSS, because xylem fluid consists of over 95% water and is the most dilute food source for herbivores. Adult GWSS prefers and performs best on host plants containing high contents of amides in the xylem fluid, whereas immatures require lower levels of amides and higher levels of many other amino acids. As a year-round host in southern California, citrus plays a critical role in the feeding and reproductive ecology of GWSS. The present research was initiated to determine the influence of seasonal nitrogen nutrition fluctuations in xylem fluid on GWSS population dynamics on lemon and orange trees.
Technical Abstract: The glassy-winged sharpshooter (GWSS) is a xylem feeder that develops conspicuous, year-round populations in many citrus-producing regions of California. Field studies were conducted in a combined lemon and orange orchard to determine the influence of changing amino acid concentrations on relative densities of GWSS. Nineteen protein amino acids were detected in xylem fluid of both lemon and orange trees. Although annual profiles of total and essential amino acids were similar for each citrus species, mean concentrations were consistently higher in lemons than in oranges for most of the year, except for one critical period in late winter/early spring when concentrations were higher in oranges. Principal component analysis followed by factor analysis was performed individually on lemon and orange data sets to identify a reduced number of orthogonal factors composed of amino acids having similar seasonal profiles. Four factors were identified for each citrus species that accounted for 85% and 79% of the total variation in the orange and lemon analyses, respectively. These were then examined with respect to shifts in GWSS numbers that occurred asynchronously in lemons and oranges over the annual population cycle. Three distinctive number shifts were identified that included a peak in adult numbers in lemons during August 2001, significantly higher numbers in lemons relative to oranges during midwinter, and finally an increase in oranges of both adults and nymphs during spring 2002. Various groups of amino acids, i.e., factors, displayed peak annual or elevated levels during the intervals when shifts in GWSS numbers were occurring. Soluble protein levels in oranges and lemons did not correspond to shifting GWSS numbers as certain amino acids. However, soluble protein levels were higher in oranges during late winter/early spring when GWSS adults were sexually active. Potential roles of these amino acids and proteins in GWSS host selection are discussed.