A McPhail trap is a specialized insect trap designed primarily to catch fruit flies in the family Tephritidae, the most economically damaging group of fruit fly pests worldwide. It uses a liquid food bait to lure flies through an opening in the bottom of the trap, where they drown in the solution. Agricultural programs across the globe rely on McPhail traps for surveying, detecting, and monitoring fruit fly populations in orchards, farms, and ports of entry.
How the Trap Works
The McPhail trap has a distinctive two-part design. The bottom portion holds a reservoir of liquid bait and features a central, open invagination, essentially a funnel-shaped opening that points upward into the trap’s interior. Flies are drawn to the bait odors rising from below, fly up through the opening, and become trapped inside. Once inside, they land on the liquid surface and drown. The liquid reservoir serves double duty: it attracts the flies and kills them.
The original McPhail trap was made of glass, but modern versions (often called Multilure traps) are made of plastic. These updated traps typically have a clear top and a bright yellow bottom, a color known to attract fruit flies visually. The clear top also plays a role: once inside, flies tend to move toward the light at the top of the trap rather than back down toward the dark opening they entered through, which keeps them from escaping.
This bottom-entry design is a key advantage. Research comparing McPhail traps to bucket-style traps with side openings found that significantly more Caribbean fruit flies entered McPhail traps. The upward entry path aligns with the natural flight behavior of flies following an odor plume rising from below.
Target Species
McPhail traps are used to monitor several of the world’s most destructive fruit fly species. The Caribbean fruit fly, the Mediterranean fruit fly (commonly called the medfly), the Oriental fruit fly, and the melon fly are all targets of McPhail-based trapping programs. These species belong to the family Tephritidae, and their larvae feed inside fruit, causing massive crop losses in citrus, mango, papaya, stone fruits, and vegetables.
Many countries use McPhail traps as part of official detection and survey programs. Placing traps in areas near ports, airports, and agricultural zones helps authorities catch the first signs of an invasive fruit fly species before it can establish a breeding population. Early detection can mean the difference between a localized eradication effort and a full-blown agricultural crisis.
Bait and Attractants
The standard attractant is an aqueous solution of enzymatic hydrolyzed torula yeast mixed with borax. Torula yeast pellets (typically around 5 grams each) are dissolved in water to create a protein-rich slurry. The borax serves a practical purpose: it slows the decomposition of both the yeast solution and the trapped insects, keeping the trap functional longer and making it easier to identify specimens later.
Fruit flies are not attracted to the yeast itself but to the volatile chemicals released as the protein in the bait breaks down. Ammonia plays a central role. As the yeast decomposes, it produces ammonia and other nitrogen-containing compounds that mimic the smell of rotting fruit and fermenting organic matter, signals that tell a female fruit fly she’s near a good protein source for egg development. Studies have found a direct positive correlation between ammonia release rate and the number of fruit flies captured.
The pH of the bait solution matters more than you might expect. Higher pH increases ammonia release, and higher ammonia release means more flies in the trap. Torula yeast-borax solution naturally maintains a relatively high and stable pH of about 9.0 over a week of field deployment, which is one reason it has remained the go-to bait for decades.
A small amount of propylene glycol (often in the form of RV antifreeze, around a 10% concentration) is commonly added to the solution. This reduces evaporation in hot climates and helps preserve trapped specimens. A typical preparation uses one torula yeast-borax pellet per 100 milliliters of the water-antifreeze mixture.
Reducing Bycatch
One practical challenge with McPhail traps is bycatch: non-target flies and other insects that are also attracted to the protein bait. When traps fill up with miscellaneous species, it becomes harder and more time-consuming to sort through the catch and identify the target fruit flies.
Research on Caribbean fruit fly trapping found that reducing the amount of bait, using two torula yeast pellets instead of six, caught just as many target fruit flies while significantly reducing the number of non-target species contaminating the trap. This is a useful finding for anyone running a survey program, since fewer pellets also means lower cost and less labor during trap servicing.
Servicing and Maintenance
McPhail traps require regular attention. The liquid bait evaporates, decomposes, and loses its attractiveness over time, so traps are typically serviced on a weekly cycle. During servicing, the old bait is poured through a strainer to collect trapped insects for identification, the trap is cleaned, and fresh bait solution is added. In hot, dry conditions, evaporation can be significant enough to require more frequent checks or the addition of propylene glycol to extend the bait’s useful life.
Trap placement also affects performance. Traps are generally hung in the canopy of host trees, positioned in shaded areas where fruit flies are most active. Placement in direct sunlight accelerates bait degradation and evaporation. The height, orientation, and proximity to fruiting trees all influence capture rates, so trapping programs follow standardized protocols to ensure consistent, comparable results across monitoring sites.
McPhail Traps vs. Dry Traps
The liquid bait system is both the McPhail trap’s greatest strength and its biggest limitation. The wet design is highly effective at attracting female fruit flies (which need protein for egg production), making it one of the few trap types that reliably catches both sexes. Many other fruit fly traps use male-specific lures like methyl eugenol or trimedlure, which only attract males of certain species.
The downside is maintenance. Liquid baits are heavy, messy, and need frequent replacement. They also produce foul-smelling waste. Dry trap alternatives using synthetic food-based lures have been developed to address these issues, but the traditional wet McPhail trap remains the standard in many national and international fruit fly monitoring programs because of its proven track record and broad species coverage.