The attraction of flies to cannabis is complex, depending on the fly type and the plant’s development stage. While some flies are not drawn to healthy cannabis plants, others are specifically attracted to the conditions created during cultivation, harvest, or storage. The powerful chemicals produced by the plant, along with the moisture and organic matter associated with the growing process, create distinct signals that various fly species seek out. Understanding these varying attractions is necessary for successfully managing the presence of flies around cannabis.
The Primary Attractants in Cannabis
Flies are primarily drawn to cannabis not by the cannabinoids, but by a combination of volatile organic compounds and environmental conditions. The most significant chemical attractants are the terpenes, which are the aromatic molecules responsible for the plant’s distinctive scent profile. Cannabis can contain over 100 different terpenes, and these compounds function as chemical signals that can either attract or repel various insects depending on the specific strain and fly species.
Certain blood-sucking flies, such as phlebotomine sand flies, show a strong preference for Cannabis sativa, suggesting attraction to some of its volatile compounds. Beyond chemical signals, the cultivation environment itself attracts many common fly pests. Flies are heavily drawn to moisture, high humidity, and decaying organic matter, which are often byproducts of the growing process.
The rich soil and decomposition of fallen leaves or plant debris offer a breeding ground and food source for fly larvae. Overwatering, a common cultivation error, keeps the topsoil consistently damp. This dampness provides the perfect condition for certain fly species to lay their eggs, explaining the presence of flying insects throughout the plant’s lifecycle.
Specific Flies and Their Preferred Stages
The type of fly attracted to cannabis depends largely on the plant’s stage and environment, creating distinct pest management issues. Fungus gnats (Sciarid flies) are the most common fly pest during the vegetative and flowering stages. These small, dark flies are strongly attracted to wet, organic-rich soil, where females lay eggs in the top inch of the growing medium. The larvae primarily consume fungus and decaying organic matter, but they can damage tender root hairs and young roots, especially in seedlings.
Fruit flies (Drosophila melanogaster), often mistaken for fungus gnats, are attracted during the post-harvest and storage phases. Their primary attractants are fermentation, sugars, and decaying plant material, which results from improperly cured buds or accumulated waste. These flies seek out sweet, fermenting scents, making them a nuisance in trimming or curing rooms if sanitation is poor.
House flies (Musca domestica) are not typically plant pests but are attracted to the overall environment, especially in outdoor or soil-based operations. House flies are drawn to decaying organic matter, animal waste, and fertilizers present in the growing medium. While they do not feed directly on the plant, their presence is undesirable as they can leave waste on leaves and buds, potentially acting as vectors for bacteria.
Prevention and Mitigation Strategies
Preventing fly infestations requires tailoring strategies to the specific environment and fly species. During cultivation, the primary focus is controlling soil moisture to deter fungus gnats. Growers should allow the top one to two inches of soil to dry out completely between watering sessions. This interrupts the gnat life cycle by eliminating the moist conditions necessary for eggs to hatch.
Cultivation Controls
Mechanical controls, such as yellow sticky traps, should be placed near plants to capture adult fungus gnats and monitor infestation severity. Applying a layer of diatomaceous earth to the soil surface also helps. The fine powder cuts the exoskeleton of flies and larvae as they crawl through it, causing dehydration.
For heavy infestations, growers can use biological controls. This includes introducing beneficial nematodes or applying products containing Bacillus thuringiensis israelensis. This bacterium specifically targets and kills the larvae in the soil.
Post-Harvest Mitigation
In the post-harvest and storage stage, mitigation centers on sanitation and environmental control to deter fruit flies and house flies. Proper curing practices and quickly removing all plant waste, trimmings, and debris eliminate potential food sources. Storing dried cannabis in airtight containers, such as glass jars, prevents the escape of residual odors that attract flies and physically blocks their entry. Maintaining a clean, dry environment with good airflow significantly reduces the presence of flies by removing the moisture and decaying matter they rely on.