Protecting Food Crops From Contamination with Filth Fly Transmitted Human Pathogens

Recent outbreaks of human illness due to Salmonella and E. coli associated with human food crops have received significant media attention and resulted in costly industry recalls.  However, the source of the contaminant bacteria in many of these outbreaks remains unclear.  We are investigating the possibility that filth flies may be responsible for acquiring these pathogenic bacteria from domestic animals and moving them into human food crops. Some species of filth flies are commonly associated with animal feces and recent work by our group has shown that filth flies contaminated with E. coli O157:H7 can be found resting on human food crops such as lettuce.  Furthermore, contaminated flies can deposit these pathogenic bacteria on visited plants.  House flies exposed to E. coli O157:H7 were shown to regurgitate this bacterium onto spinach plants, with SEM revealing that bacteria adhered to the spinach phylloplane and increased in number after 7 days.   The bacteria also adhered to the fly labellae and tarsi in low to moderately high numbers.  Flies are known to disperse distances > 15 km, but probably commonly disperse less than 2-3 km from developmental sites associated with domestic animals.  Placing food crop production facilities at distances over 2-3 km from domestic animal facilities will likely reduce the opportunity for contaminated flies to reach the food crop.  Current research is evaluating the role of homopteran pests to attract filth flies to food crops as a result of honeydew production and deposition on the plant.  Field collected flies offered a choice of attractant materials including aged honeydew colonized with sooty mold, commercial fly bait, and a milk-sugar-yeast liquid attractant, were significantly more attracted to the aged honeydew than to any other material tested.  It is thought that flies and other insects are attracted to honeydew in the field as a source of sugars to support flight energy needs.  Identification of the specific volatiles that are attractive to flies is one component of an ongoing research project funded by a nearly 1 million dollar research grant to lead PI Astri Wayadande at Oklahoma State University.