Flies, including common species such as the house fly (Musca domestica) and the fruit fly (Drosophila melanogaster), navigate their environment using a complex chemosensory system. This sensory apparatus allows them to detect minute quantities of airborne chemicals, guiding them directly toward food sources, mates, and suitable locations for laying eggs. Understanding the specific attractants that trigger this behavior is the most effective way to implement targeted control and prevention strategies against these common pests.
Chemical Signatures of Decay
Flies are highly attuned to the chemical signals released during the decomposition of organic matter. These volatile organic compounds (VOCs) act as long-distance beacon signals, indicating both a rich feeding opportunity and a perfect substrate for the next generation.
The decay of high-protein substances, such as animal carcasses or feces, produces attractants like ammonia and various sulfur-containing molecules. Ammonia is a byproduct of amino acid breakdown, signaling a nitrogen-rich environment beneficial for larval development. House flies are strongly drawn to these nitrogenous odors, which indicate decaying fecal matter—a preferred site for oviposition.
Hydrogen sulfide and other reduced sulfur compounds, such as methanethiol, are characteristic products of putrefaction and are attractants for many fly species, including blow flies and flesh flies. These chemicals inform the insect that the organic material is in an advanced stage of decay, making it soft and easily digestible for their maggots.
Specific chemical blends are also released from decaying plant material, though these often overlap with the protein decay signatures. For example, cadaverine and putrescine are diamines produced when amino acids lysine and ornithine break down in both animal and plant tissues. These malodorous molecules are particularly attractive to species that utilize moist, rotting vegetation as a nursery.
The precise detection of these decay VOCs is managed by specialized olfactory receptors located on the fly’s antennae. Female flies use this olfactory precision to ensure her eggs are deposited in an environment where the larvae will have immediate access to nutrients upon hatching. This chemical profile essentially serves as an environmental suitability map for reproduction, minimizing the risk of offspring starvation.
Attraction to Sugars and Fermentation
In contrast to the odors of putrefaction, many smaller fly species, particularly fruit flies (Drosophila), are primarily drawn to the sweeter and more acidic chemical profiles associated with fermentation. This process, driven by yeasts and bacteria acting on carbohydrates, signals a readily available source of high-energy food. They are often found near spilled sugary liquids, old beer, or molasses.
The primary attractants in this category are ethanol (alcohol) and acetic acid (vinegar), both byproducts of yeast metabolism in overripe fruits. Ethanol provides a readily metabolized energy source for the adult fly, while the presence of acetic acid confirms that the substrate is actively fermenting. Fruit flies possess highly sensitive olfactory neurons specifically tuned to detect these alcohol and acid molecules.
While low levels of ethanol are attractive, high concentrations can become repellent, indicating an environment that is too toxic for the fly or its larvae. This allows flies to home in on the optimal stage of ripeness or decay for feeding. They also target the yeast itself, which is a rich source of protein and B vitamins often lacking in the fruit pulp.
Yeast colonies, whether on bread dough or fermenting fruit, release carbon dioxide, which can also act as a secondary attractant. The combination of carbon dioxide, ethanol, and acetic acid creates a specific plume that guides the fruit fly to its preferred location. This attraction is primarily a search for adult sustenance and a moist, soft nursery for their eggs.
The attraction to sugars extends beyond fermentation, as many flies feed directly on nectar, honeydew, and other simple carbohydrates for quick energy. House flies readily consume spilled juice or soda, using their sponging mouthparts to ingest the sweet residue. The presence of a simple sugar signals an energy reward.
Visual and Thermal Signals
Flies do not rely solely on their sense of smell, as they also possess highly developed visual systems that guide their movements over short and long distances. They perceive light across a much broader spectrum than humans, including ultraviolet (UV) light, which is invisible to the human eye. This ability allows them to navigate by polarized light and spot resources that reflect UV.
The attraction to light, particularly UV light, is often exploited in commercial insect traps. Many species exhibit a phototactic response, meaning they instinctively move towards a light source. The intensity and wavelength of the light are more significant than just the presence of illumination.
Specific colors also play a substantial role in visual attraction, as certain hues mimic natural food sources or favorable habitats. Yellow is widely used in sticky traps because it strongly reflects light in a way that mimics flowers or foliage favored by many flying insects. Conversely, certain shades of blue are effective for trapping tsetse flies because they resemble large host animals.
Beyond light and color, flies are also highly sensitive to environmental factors like warmth and moisture, which serve as physical cues. A plume of warm, moist air can indicate the presence of a large mammal, a fresh pile of dung, or a sheltered, humid area suitable for resting and breeding. This thermal sensitivity helps flies quickly locate a host or a patch of fresh decay.
The combination of these non-chemical signals allows flies to confirm a target once they are in close range, complementing the long-distance guidance provided by chemical plumes. Movement is another strong visual attractant, as flies are highly sensitive to motion, often interpreting it as a potential host or a competitor.