Green bottle flies eat different things depending on their life stage. Adults feed on sugary liquids like flower nectar and the juices from decaying organic matter, while larvae feed almost exclusively on dead animal tissue. This split diet makes green bottle flies both pollinators and decomposers, playing a surprisingly important ecological role.
What Adults Eat
Adult green bottle flies have sponging mouthparts, not biting ones. They can only consume liquids or food that has been liquefied first. When they land on something solid, they regurgitate digestive fluid onto the surface to dissolve it, then sponge the resulting liquid back up. This limits their diet to things that are already wet or can be broken down quickly.
Carbohydrates are the primary fuel for daily activity. Adults get sugar from flower nectar, honeydew (the sticky residue left by aphids), overripe fruit, and any sugary liquid they encounter. They’re commonly spotted on composite flowers like oxeye daisies, which offer both nectar and a large pollen disc. Flower pollen, which can contain up to 80% protein, provides the amino acids female flies need to develop their eggs. When flowers aren’t available, adults readily turn to animal feces, garbage, and the fluid seeping from decaying meat or carcasses.
This flexibility is key to their survival. Green bottle flies can shift to being near-obligate flower foragers when carrion and feces are scarce, essentially functioning as pollinators. But when decomposing animal material is available, they’ll choose it, especially females preparing to lay eggs.
What Larvae Eat
Larvae, commonly called maggots, are strictly necrophagous. They feed on the soft tissue of dead vertebrate animals. A female green bottle fly lays her eggs directly on a carcass or in a wound, and the hatching larvae immediately begin consuming the surrounding dead tissue. They don’t chew. Instead, they release a cocktail of digestive enzymes directly onto the tissue surface. These enzymes, primarily protein-breaking compounds, liquefy the dead material so the larvae can ingest it.
One remarkable feature of green bottle fly larvae is their selectivity. They strongly prefer dead or decaying tissue over healthy, living tissue. This trait is so reliable that doctors have used these larvae for centuries in a practice called maggot debridement therapy, placing sterilized larvae into chronic wounds to clean away infected or dead skin while leaving healthy tissue intact. The larvae secrete antimicrobial substances alongside their digestive enzymes, which helps disinfect the wound as they feed.
How They Find Food
Green bottle flies are remarkably good at locating food from a distance, and they rely heavily on smell. Their antennae detect airborne chemical compounds that signal the presence of a meal. A sulfur compound called dimethyl trisulfide, released by fresh carrion, is one of the strongest attractants for egg-laying females. For locating feces as a food source, flies respond to a blend of chemicals including indole and several alcohols like phenol and cresol. Indole appears to signal “food” rather than “egg-laying site,” helping flies distinguish between the two resources.
Interestingly, a fly’s preferences shift with age and reproductive status. Younger females that still need protein to mature their eggs respond strongly to the combination of floral color (especially yellow) and floral scent, which together signal the presence of pollen and nectar. Once females are ready to lay eggs, their chemical preferences shift toward the sulfur compounds associated with carrion.
Larvae as Decomposition Engines
The feeding behavior of green bottle fly larvae has an outsized effect on how quickly dead animals break down in nature. When enough larvae gather on a carcass, they form dense aggregations called maggot masses. The collective metabolic activity of hundreds or thousands of feeding larvae generates heat, raising the temperature inside the mass 10 to 20 degrees Celsius above the surrounding air. Internal temperatures can reach 40°C or higher. This warmth accelerates both larval growth and the rate of tissue breakdown, creating a feedback loop that speeds decomposition dramatically.
A mass needs to reach roughly 50 grams, about 1,450 larvae, before this self-heating kicks in. The larvae regulate the temperature through behavior, moving to the edges of the mass when it gets too hot, preventing themselves from cooking. This rapid, efficient consumption of dead tissue recycles nutrients back into the soil and removes decaying material from the environment, reducing the risk of disease spread from rotting carcasses.
The Downside of Their Diet
Because adult green bottle flies feed on feces, garbage, and decomposing flesh before landing on food in your kitchen or on a picnic table, they’re effective carriers of bacteria. Their bodies and digestive tracts harbor a range of microorganisms, including species of Salmonella, E. coli, Pseudomonas, and Staphylococcus aureus. Every time a fly lands on a surface, regurgitates digestive fluid, and sponges up a meal, it potentially deposits bacteria picked up from its last feeding site. This mechanical transfer is the primary way green bottle flies contribute to the spread of foodborne illness, making them more than just a nuisance when they show up around food.