Search for potent attractants of onion flies.

Of various chopped vegetables tested,Allium spp. high in propyl-containing alkyl sulfides (e.g.,cepa group) caught the most onion flies in trapping tests in the field. Fly catches to chopped onion increased with bait quantity. Attractancy of chopped onion changed dramatically during aging in the field; catch increased over the first few days, peaked at ca. fivefold over fresh material by 3-5 days, and then declined sharply. This age-dependent increase in attraction was not seen for garlic (known to have antimicrobial properties) nor with chopped onion mixed with chopped garlic. These data suggested that attraction of onion flies to onions was strongly influenced by microbial activity associated with decomposing onions. The bacteriumKlebsiella pneumoniae was identified as a major colonizer of onions maximally attractive to onion flies. This increased attraction is not due to the previously reported microbially produced volatiles ethyl acetate and tetramethyl pyrazine.