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Over a period of 18 months the bacteria associated with approximately 70 adult flies of four Dacus species were isolated and identified. The flies were D. tryoni (Froggatt) and D. neohumeralis Hardy from guava (Psidium guajava L.), mulberry (Morus nigra L.) and peach [Prunus persica (L.) Batsch], D. cacuminatus (Hering) from wild tobacco (Solanum mauritianum Scop.), and D. musae (Tryon) from banana (Musa paradisiaca L.), and were collected in the field when these host plants were fruiting. All flies examined were surface-sterilized prior to aseptic dissection in which crop and mid-gut (stomach) or oesophageal bulbs were removed for culturing. Bacteria were also isolated from faeces of field-collected flies, as well as from host fruit surfaces, oviposition sites and larvae-infested tissue in host fruit. The predominant bacteria found in the alimentary tract of flies and in associated fruit specimens were members of the family Enterobacteriaceae. Klebsiella oxytoca, Erwinia herbicola and Enterobacter cloacae were the most frequently isolated species, with Serratia spp., Citrobacter jreundii, Proteus spp., Providencia rettgeri and Escherichia coli, being found less frequently. No one bacterial species was found to be consistently associated with anyone fly species. The bacterial species found most frequently in the alimentary tract were also found in large numbers on the surfaces of host fruit and in stung fruit.

Using caged host trees on which we manipulated food and oviposition sites, we investigated the foraging behavior of individually-releasedBactrocera tryoni (Diptera: Tephritidae) females in relation to state of fly hunger for protein, presence or absence of bacteria as a source of protein, degree of prior experience with host fruit, and quality of host fruit for oviposition. One aim was to evaluate whether it is immature or matureB. tryoni females that are responsible for initially inoculating host fruit surfaces with "fruit-fly-type" bacteria, the odor of which is known to attractB. tryoni females. We found that 3-week-old immature females provided with sucrose but deprived of protein from eclosion had a much greater propensity than 3-week-old protein-fed mature females to visit vials containing fruit-fly-type bacteria, irrespective of whether vials were associated with adjacent host fruit or not. In the absence of associated bacteria in vials, immature females had a much lower propensity than mature females to visit host fruit. In the presence of bacteria in vials, however, propensity of immature and mature females to visit fruit was about equal. Mature (but not immature) females were more inclined to visit fruit that ranked higher for oviposition (nectarines) than fruit that ranked lower (sweet oranges). Mature females that attempted oviposition during a single 3-min exposure period to a nectarine prior to release were much more likely to find a nectarine than were mature females naive to fruit or immature females with or without prior contact with fruit. Exposure to a nectarine before release did not affect the propensity of either mature or immature females to alight on an odorless visual model of a nectarine, however. As judged by numbers of leaves visited, protein-deprived immature females were more active than protein-fed mature females, irrespective of the sorts of resources on a tree. Together, our findings lead us to conclude that (1) the firstB. tryoni females to arrive on the fruit of a host tree and therefore inoculate the fruit with fruit-fly-type bacteria are unlikely to be sexually immature, but to be mature as a result of having earlier acquired protein elsewhere, (2) the odor of colonies of fruit-fly-type bacteria when associated with host fruit will attract protein-hungry but not protein-fed females, and (3) the odor of the fruit itself will attract mature females (especially experienced ones) but not immature females. These findings illustrate the value of considering jointly the state of a resource patch together with the physiological and experiential state of the individual when investigating the foraging behavior of an insect.

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