Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/26/2008
Publication Date: 9/1/2008
Citation: Sethi, A., Mcauslane, H.J., Alborn, H.T., Nagata, R.T., Nuessly, G.S. 2008. Romaine lettuce latex deters feeding of banded cucumber beetle: a vehicle for deployment of biochemical defenses. Entomologia Experimentalis et Applicata. 128:410-420 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 pest in a lettuce cultivar. It is known that lettuce cultivars can have different levels of resistance towards the banded cucumber beetle as well as other pest insects, but previous research has failed to clarify the nature behind this resistance. By studying insect feeding behavior and development on a resistant versus a susceptible romaine lettuce cultivar scientists at the university of Florida, department of Entomology and Nematology and USDA ARS Center for medial, Agricultural and Veterinary Entomology discovered that latex, that lettuce typically excrete when damaged, was the source of the resistance factors. Although the milky looking latex is sticky and can function as a physical barrier to herbivory, that quality of the latex was not the source of the resistance. Furthermore there didn’t seem to be a direct correlation between insect resistance and bitter taste of the lettuce. Bioassay driven solvent extractions of latex and repeated chromatographic separations resulted in a fraction that deterred the beetle form feeding when applied to the surface of artificial diet. The aim for ongoing and 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: Latex is widely found among plant species and is known to play a defensive role against certain herbivores. Two romaine lettuce cultivars, ‘Valmaine’ (resistant) and ‘Tall Guzmaine’ (susceptible) were selected to study the potential of latex as a defense mechanism against the banded cucumber beetle, Diabrotica balteata LeConte. Latex from ‘Valmaine’ strongly inhibited feeding of adult D. balteata compared to ‘Tall Guzmaine’ when applied to the surface of artificial diet. Beetles consumed significantly less diet from disks treated with ’Valmaine’ latex than they consumed from diet treated with ‘Tall Guzmaine’ latex, in both choice and no-choice tests. In a choice test involving diet disks treated with ‘Valmaine’ latex from young leaves versus disks treated with latex from mature leaves, the beetles consumed significantly less diet treated with latex from young leaves. No significance difference in diet consumption was found between diets treated with latex from young and mature leaves of ‘Tall Guzmaine’ in choice tests. Three solvents of differing polarity (water, methanol and methylene chloride) were tested to extract deterrent compounds from latex; ‘Valmaine’ latex extracted with water:methanol (20:80) strongly inhibited beetle feeding when applied to the surface of artificial diet. In no-choice tests, fewer beetles were observed feeding on diet treated with water:methanol (20:80) extract of ‘Valmaine’ latex than on diet treated with water:methanol (20:80) extracts of ‘Tall Guzmaine’ latex, resulting in significantly less consumption of the diet treated with the ‘Valmaine’ latex extract. These studies suggest that moderately polar chemicals within latex may account for resistance in ‘Valmaine’ to D. balteata.