Pollen trap problems are mostly due to technique.  If you put a pollen trap on in the middle of a day,  the foragers will have a hard time figuring it out.  Worst case are the entrance/front mounted traps, where inexperienced returning bees may pile up and can heat prostrate the colony - I've see it happen.

Traps with more screen surface area such as those mounted under the hive or front mounted towers help eliminate this problem.

Bees will always find any gap and dodge the trap, so you need tight hives or a lot of duct tape.

Bees will drift to hives without traps IF they have problems figuring out how to navigate the trap which suddenly appeared.

As per closing off all gaps - the absence of pollen in a trap and bee flight in and out of their alternate entrance(s) will show you where you need to seal things up.

The KEY to success and to reducing problems is to put the trap in place at night after everyone has come back home, but before anyone has left the hive in the morning.  If you can, leave the screens out or open for a day or two so that the bees get used to their new entrance - in essence, their hive just got a lobby.  Then, when you want to collect pollen, insert or close the screen before the bees leave in the morning to forage.  They will want/need to get out.  They'll stop long enough to figure it out, and its presence when they return won't be a surprise - they'll scoot right  in.   After a day or so with screens in place, the foragers will know what to do if the screen is open or closed, and you won't have to do this a crack of dawn - unless you've got long intervals (days/weeks) between collections.  In that case you will likely have new foragers who haven't learned about pollen screens.

One more thing to remember; the first time a pollen gatherer returns through a pollen trap screen, she Is likely to stop when her pollen pellets hit the screen, back up, try again.  She doesn't want to lose them.  Eventually, bees give in and charge through.  I've seen some that lose the pellet, and if it's still visible, the bee will come back out to try to collect it.  If bees have expressions, one can imagine she's mad.  However, they soon learn to just run through, often twisting, so they avoid losing both pellets.  Studies have shown that most traps only collect about 50% of the pollen.  I can vouch for the actual number, but bees get artful in how they get through; and  they are fast learners.

I recommend not leaving screen on or closed for days at a time.  You run the risk of either shorting them of pollen needed for brood or of them shifting nectar gatherers into pollen gathering mode to make up for the shortfall.  We usually alternate periods of open and closed traps.

I've tried lots of screens.  Metal ones can shed zinc and iron into the product.  Plastic ones can sag or become brittle with heat.  Anything thick walled (I've made quick traps out of PVC pipe with drilled holes) - and the hole size becomes more critical since to a bee, a thick material with holes is like running down a pipe  - too small, and some bees from some hives will be able to go through, but others won't, and some hives will have an entire population that can't get through.

Round, triangular, square holes - each has a trade off in terms of efficiency and ease of passage.  Square holes are easier to dodge by twisting diagonally, reason many hardware cloth type screens are doubled up in bottom mounted traps.  The most effective that I found are hard to get in the U.S.  In the 90s, I ordered them out of Europe.  These have screens made of flat plastic sheets with star-shaped holes.  There are points aimed inward that act like springs or fingers.  Bees easily push through, but it's difficult to avoid contact with the fingers, lose the pollen pellet.  Perhaps one of Bee-L members can point us to a current supplier.  At one time, I was able to buy rolls of the material to make custom traps.

Finally, the little plastic traps first mentioned in this thread are great for sampling for pesticide analysis.  They weren't designed for the bottom boards most of us use in the USA, and the drone escapes soon become by pass points, so we tape them up.  The screen falling down is easily corrected by using binder clips (found in most office supply stores in the US).  We just snap one onto the screen to keep it up.  And, when dropped, be sure to press inward on each bottom corner of the screen, so it pops over a little plastic bump - otherwise it won't lock in place and bees from a strong hive will actually push it open.  These traps are cheap, easy to install, lv open for day or two, and then close.  But, they need a bigger roof if you're in a climate with heavy rain or near sprinkler, and they get brittle with UV light and heat, so they're only good for one season, then they start to break and warp.

Bottom mounted traps are easier to use, once installed, but you get hive debris in them.  The best traps are large boxes with big screen areas mounted vertically over the entrance and the tray in front of the landing board.  Properly designed, you can install without lifting the hive, have sufficient screen area so bees won't impede air flow, and get cleaner samples.  That said, I've not found commercial ones that I like, so we design and build our own for our research.   Cedar boxes, screens, and plastic collection trays, with a big overhanging roof to keep the pollen dry.


 
J.J. Bromenshenk
Bee Alert
Missoula, Mt
 
 


 

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