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Title: SPATIAL AND TEMPORAL DISTRIBUTION OF IMIDACLOPRID AND THIAMETHOXAM IN CITRUS AND IMPACT ON HOMALODISCA COAGULATA POPULATIONS

Author
item Castle, Steven
item BYRNE, F - U CA, RIVERSIDE
item BI, J - U CA, RIVERSIDE
item TOSCANO, N - U CA, RIVERSIDE

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2004
Publication Date: 1/1/2005
Citation: Castle, S.J., Byrne, F.J., Bi, J.L., Toscano, N.C. 2005. Spatial and temporal distribution of imidacloprid and thiamethoxam in citrus and impact on homalodisca coagulata populations. Pest Management Science 61:75-84.

Interpretive Summary: The duration that an insecticide is active in a crop in terms of lethal and sub-lethal effects on insect populations is a critical concept in pest management that has been poorly developed. Information on the persistence of an insecticide is often broadly generalized and tailored for health and environmental concerns rather than for pest management purposes. Therefore, this study was undertaken to develop a better understanding of the residual activity of 2 key systemic insecticides, imidacloprid and thiamethoxam, in citrus following their application through mini-sprinkler systems common to citrus orchards in California. Imidacloprid was shown to have much longer persistence than thiamethoxam as it suppressed glassy-winged sharpshooter populations for as long as 5 months following application. Both compounds were found to distribute relatively uniformly throughout mature citrus trees. This information will provide citrus growers with better guidelines for understanding how long a single treatement of either imidacloprid or thiamethoxam can be expected to protected their citrus orchards from glassy-winged sharpshooter.

Technical Abstract: Titres of two systemic neonicotinoid insecticides in citrus trees were measured in conjunction with conventional evaluations of their impact on glassy-winged sharpshooter (GWSS) populations. Xylem fluid samples were collected at regular intervals and from multiple locations within field-grown citrus trees to determine imidacloprid and thiamethoxam concentrations using commercial ELISA kits. Uptake profiles varied considerably with peak mean titres of imidacloprid occurring 6-8 weeks after application compared to 2 weeks for thiamethoxam. The persistence of each compound also varied as near-peak levels of imidacloprid were sustained for another 6-10 weeks before gradually declining. In contrast, thiamethoxam titres declined more rapidly after the initial peak, possibly reflecting an application rate only ¼ of that used for imidacloprid. Within-tree distributions were more similar for the 2 compounds with no significant effect due to height of the sample (upper or lower half) or to the quadrant location within the tree, with the exception of one quadrant in the thiamethoxam-treated trees. Substantial reductions in GWSS nymphs and adults were observed in imidacloprid-treated trees during the 2001 trial and sustained for 4-5 months after treatment. Treatment effects on nymphs were not as well pronounced in the 2002 trial when overall GWSS infestations were much reduced from the previous year. However, consistently lower adult infestations were still observed in 2002 for both treatments compared to untreated trees. Information on the spatial and temporal profiles in citrus trees was obtained for both compounds to compliment field impact data and improve understanding of their pest management potential.