Location: Chemistry ResearchTitle: Banded cucumber beetle (Coleoptera: Chrysomelidae) resistance in romaine lettuce: understanding latex chemistry Author
Submitted to: Arthropod-Plant Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2011
Publication Date: 6/1/2012
Citation: Sethi, A., Alborn, H.T., Mcauslane, H.J., Nuessly, G.S., Nagata, R.T. 2012. Banded cucumber beetle (Coleoptera: Chrysomelidae) resistance in romaine lettuce: understanding latex chemistry. Arthropod-Plant Interactions. 6:269-281. Interpretive Summary: Lettuce, Lactuca sativa L., is one of the most important vegetable crops grown throughout the world. It is vulnerable to attack by various insect pests including the banded cucumber beetle, Diabrotica balteata LeConte. In southern Florida, foliar feeding by D. balteata adults leads to significant economic damage. Growers currently are highly dependent on pesticides which increase production cost and pose a threat to the environment, farm workers and natural beneficial enemies of insect pests. Host plant resistance is therefore a valuable alternative strategy for the management of this economic insect. It is known that lettuce cultivars have different levels of resistance towards the banded cucumber beetle as well as other pest insects. In a previous study, it was discovered that latex, that lettuce typically excrete when damaged, was the source of the resistance factors but that neither stickiness nor bitter taste could be correlation with resistance. Solvent extractions and a crude purification step resulted in a fraction that deterred the beetle form feeding when applied to the surface of artificial diet. In this study scientists at the University of Florida, department of Entomology and Nematology and USDA ARS Center for Medical, Agricultural and Veterinary Entomology describe a series of bioassay driven chromatographic separations that resulted in a feeding deterrent fraction with non-protein amino acid characteristics. The aim for future research will be to identify the feeding deterrents and investigate how these compounds affect the feeding behavior of the banded cucumber beetle as well as other pest insects, a knowledge that will contribute to the development of new cultivars expressing insect resistance along with superior horticultural traits through conventional and transgenic approaches.
Technical Abstract: Many plants subjected to herbivore damage exude latex, a rich source of biochemicals, which play important roles in host plant resistance. Our previous studies showed that fresh latex from Valmaine, a resistant cultivar of romaine lettuce Lactuca sativa L., applied to artificial diet is highly deterrent to feeding by banded cucumber beetle, Diabrotica balteata LeConte, compared to the latex of a closely related susceptible cultivar Tall Guzmaine. The deterrent factor(s) could be extracted from Valmaine latex with water:methanol (20:80). In this study, further fractionation of the methanolic crude extract of Valmaine latex was performed using reverse phase and ion exchange solid-phase extraction to isolate the deterrent compounds. Retention of deterrent compounds on anion and cation exchange resin suggested the presence of highly polar compounds with both carboxylic and amine groups in Valmaine latex. Further bioassay-directed fractionation of cation exchange extract using LC/MS indicated the presence of at least 3 major, and an unknown number of minor compounds in the bioactive fraction between 3 and 4 min. The m/z 210 out of the 3 major compounds showed strong amino acid characteristics (similarities with glutamine and glutamic acid) when subjected to further MSn degradation. Our studies suggest that nitrogenous ingredients of latex play a key role in Valmaine resistance to D. balteata, and latex may be a source of bioactive compounds with a potential use in pest management.