Collecting Blow Flies

Blow flies are fast fliers and can be very elusive if you attempt to catch them with a net. One exception is netting around carrion where flies may gather in great numbers and may be sluggish. They also can be found in sunny areas which warm up first on cool mornings, especially on east or south facing walls of structures. Some blow flies, can be found congregating on isolated peaks at high elevation, including some rare species although more common species of blow flies may also be found there. Blow flies may also be caught with Malaise traps, this tactic is especially useful to collect species like Protocalliphora which are rarely attracted to carrion.

Bait traps using various types of carrion can be very effective attracting many species of blow flies and is the preferred tactic for sampling for species when studying blow flies in a new area. In my current studies, I use butterfly bait traps purchased online from the “Bug Dorm Store,” which are also very effective for blow flies and Mesembrinellidae. See photo of trap in photo tab. These economical traps fold up to a small size making them easily portable, yet they are very effective. BioQuip makes a much larger and more expensive butterfly trap which is more durable for high volume collecting. With both these traps, I spray the top and upper sides of the netting with a residual pesticide like permethrin or cyfluthrin to kill flies quickly once they enter the trap. If this is not done, flies beat themselves up trying to escape which ruins specimens. Most residual roach and ant sprays are effective for this purpose. Sprays must be repeated every few weeks or more frequently if there is significant rainfall. Spraying must be done carefully, avoid treating the lower part of the trap, especially where the bait sits. Most sprays are repellent and if you spray that area, the number of flies entering the trap will be reduced. If you want to avoid use of pesticides, traps should be emptied daily to get good specimens. Traps with live flies can be placed in a cold area (like a freezer) to immobilize flies, and then they can be killed in a kill jar. Spraying a little pesticide in a glass jar and allowing the spray to dry can make an effective kill jar that is low toxicity and low odor.

See the material and methods sections of Whitworth 2010 and 2012 for details on using bait traps. For the budget minded collector, an effective trap can be created from a water bottle, using mice for bait (see below for details). One important issue is traps must be hung out of reach of animals which will attempt to scavenge on the carrion bait. Dogs and cats are an obvious threat if you are near residences, but I have also had, bears, small mammals, scavenger birds and even iguanas (in the tropics) steal bait. I have had the best luck avoiding bait loss by hanging traps as high as possible on the end of a thin branch. In bear country they must be very high (10 feet or more); normally hanging them so the bait is about 6 feet above the ground is adequate. I have noted in some areas, traps hung too high collect far fewer flies. This is not surprising as most dead animals will likely be encountered at ground level. You may have to risk a lower trap set to get some species or good numbers of specimens. To make traps easy to hang, I buy commercial plant hangers made of several wires twisted together and hang the trap with it. See photo of hanger in photo tab. I bend one of the wires into a downward hook and use a forked stick to raise and lower the trap to and from a branch. I usually can find a dead branch in the area which allows me to lift the trap to hang it on a horizontal branch. If no good sticks can be found to hang the trap, you may need to make one from a long wooden dowel or broom handle. On the BioQuip trap I also add an easily removable tray to hold the bait, which makes emptying the trap easier and allows me to wash the bait holder which can become very foul smelling from rotting carrion. The Bug Dorm trap already comes with an easily removable tray.

Using the right bait is very important. Normally the stinkier the better, though in the tropics with high heat and humidity, carrion deteriorates very rapidly and needs to be “refreshed” every few days. In cooler climates, you may need to accelerate spoiling, by placing bait in an enclosed container in the sun or even by warming it in a microwave, don’t cook it however. Dead fish, chickens or small mammals with “guts and blood in them” work best. I usually make cuts in the carcass to cause bleeding to attract flies. I have used whole animals like dead rodents, rabbits and various types of “road kill” which can be long lasting and effective, once they start spoiling. I do not find beef or pork organ meat particularly effective; it dries up rapidly and normally has been cleaned of blood. I also do not like using cleaned fish, they work better if they have not been cleaned, although eventually the flesh will spoil enough to attract flies. I have collected from the far north to the tropics, and available baits vary by region. I have bought chickens from farmers, fish from fish vendors and even begged meat scraps or spoiled meat from restaurants for bait. My wife usually accompanies me on these trips and she has helped me get bait or find sites to hang traps, by mentioning the various CSI type TV programs which virtually everyone can relate to. The words CSI now seem to be universally understood. Once most people hear that you are studying forensically important insects, they are often happy to help you get bait or allow you to hang a trap on their property.

The water bottle bait trap

(Adapted from a trap described by Ermakov (2010) in Russian. Translation provided by Mikhail Koslov, Ecology Section, University of Turku, Turku, Finland). This trap is an inexpensive alternative to the traps discussed above.

Trap construction:

• Trap (see figure) is made of two transparent (colorless) plastic water bottles, 1.5 L + 0.5 L. cut along the dotted line with a sharp knife or box cutter, make these cuts on opposite sides of the bottle (Fig. A). The size of the cut depends on the size of the bottle. The horizontal cut needs to be long enough that when you push the flaps in (#2) so there will be a gap just big enough for flies to squeeze into the bottle (Fig. B). If the gap is too big, they can fly back out too easily, too small, they can’t get in. Bottles are connected with plastic rings made of each bottle cap; drill the center out of each cap, then connect them on the outside with duct or electricians’ tape. To hold the caps firmly together, the tape should overlap to the inside of the lower cap, so when the bottle with liquid is installed in the cap, the weight won’t pull the caps apart.
• Place about 200 ml of 0.5-1% water solvent of formaldehyde in the small bottle. Formaldehyde solution evaporates slowly and leaves adult blow flies in good condition, but damages DNA. You can use other liquids like propylene glycol, or alcohol also. Any solution you use needs to kill the specimen quickly and preserve it.
• Drill a small hole in the bottom (top of the trap) of the larger bottle (#4) and thread a fine wire through it (#1). On the top you form a hook to hang the trap, inside you have a hook to hang the rodent. Wrap thin wire or string around the neck of the rodent before it is completely thawed (#3) and hang it on the inside hook. Make sure the rodent is completely inside the bottle.

Bait -I have used house mice, rats, and voles for bait, they all work. You can buy small frozen rodents from pet stores or you can trap them like I do. Whole, undamaged rodents spoil slowly compared to most bait, rate of decay is temperature dependent. If you want immediate attractiveness, thaw the bait and leave it in a warm spot so it starts to smell before baiting the trap. If I’m in a hurry and have access to a microwave, I thaw the bait and warm it a little (don’t cook it) so it will attract blow flies faster. You could use a variety of other small animals or their parts for bait, like fish, fish heads, etc. also.

Trap placement -Traps need to be high enough that dogs or wild animals can’t reach and destroy them. They also need to be on the end of slender branches so climbing animals can’t reach them.

Effectiveness of Trap -This trap has proved to be exceptionally effective in trapping species of blow flies that are rarely collected in bait traps, including Calliphora genarum, C. stelviana, and Lucilia magnicornis. In a prior study (Kozlov and Whitworth 2002) on the Kola Peninsula, Russia, conventional bait traps collected very few of these rare species, but in a subsequent study with this new trap, many specimens of the rare species were collected. These traps do not attract large numbers of flies but can be a way to survey large and remote areas where traps may be destroyed or lost. 

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Terry Whitworth, Ph.D.
Entomologist
3707 96th ST E
Tacoma, WA98446
Phone 253-845-1818
email: twhitworth@blowflies.net