Flies and gnats have a bad reputation - or do they? Based on concerns from residents, the city of Bakersfield, Calif., recently conducted a study to determine whether flies and insects really do breed in green and household waste, and whether this creates potential disease.
In 1993, Bakersfield initiated an automated collection pilot that separated household trash and yard waste, and switched pickups from twice a week to once per week. This pilot was launched to help the city comply with the state's recycling mandate. However, some residents raised concerns that storing waste for a week might lead to public health problems by providing havens for flies and gnats.
Residents believed that insect scavengers would feed on dung or decomposing plant and animal matter, the larva would mature during the week and the situation would breed diseases.
Consequently, Bakersfield's Department of Water and Sanitation began an entomological study to determine whether:
* Insect populations were higher at the pilot project location where there would be complaints vs. at non-pilot or control locations; and
* Insect populations at the pilot location were related to the environmental causes, such as adjacent agricultural land uses, or whether there was a basis for establishing a cause and effect relationship between the insect population and the week-old greenwaste.
To collect data, baited insect traps were placed adjacent to garbage cans in three types of locations - pilot, non-pilot and control. Sweep netting also was used in the adjacent areas to collect insects.
Of the five families of trash- and yard waste-feeding insects that were collected, fungus gnats (Mycetophilidae) and humpback flies (Phoridae) have no potential to cause health problems. House flies (Muscidae), blow flies (Calliphoridae) and flesh flies (Sarcophagidae) have the potential to act as disease vectors.
Following field data collection, the study determined that two major factors contribute to whether flies breed in trash.
First, the higher the temperature, the faster the fly growth. During the study, trash container temperatures were measured and compared to ambient air temperatures (see table). Measurements were taken when trash cans were in full sun and in the shade. The study found that container temperatures in full sun were about 9 percent higher than in ambient conditions because of solar radiation and heat generated by waste biodegradation.
Additionally, for flies to breed, there must be sufficient time for flies to pass through all four life-cycle stages: egg, larva, puparium and adult. Maggots are flies' larval stage.
The study concluded that year-round temperatures were high enough for fly growth, however, flies did not have enough time to develop between residential trash pickups. Although flies developed from trash at the final disposal destination, generally, using daily cover at the landfill was sufficient to prevent flies from breeding.
Of course, the study found that poor housekeeping habits could allow insects to breed around the trash containers. The most significant factor affecting the presence or absence of pests was trash can location and housekeeping habits. If trash cans were kept in an open, dry, sunny area, the number of trapped flies was low. Trash cans kept in shaded, damp areas tended to attract more flies.
Nevertheless, the study determined there wasn't enough time for larval and pupal development between weekly trash pickups. Thus, Bakersfield could continue with its once per week automated collection system without any public health concerns.