Can Robbing Screens Reduce Mite Immigration?: Part 1
Can Robbing Screens Reduce Mite Immigration?
Part 1
Randy Oliver
ScientificBeekeeping.com
First published in ABJ July 2024
After writing about my investigations into bee drift and mite immigration, I was asked whether the use of robber guards (aka robbing screens) could decrease the amount of mite immigration into hives. So I ran some controlled field trials to find out.
Mite Immigration and Bee Drift
In order for any parasite to survive as a species, it must not only reproduce, but also transmit itself or its offspring to fresh hosts before the original host dies. Not only that, a successful parasite must also spatially disperse to new locations or host populations.
The varroa mite enjoys vertical transmission from a mother hive to a daughter hive when that colony swarms (for the record, it’s the parent queen and a portion of her colony’s adult workers that swarm, leaving behind what will become a daughter colony if one of her daughter queens successfully mates).
Mites can also engage in horizontal transmission when some individuals hitch a ride on a flying bee to a different colony. Such horizontal transmission also results in spatial dispersal of a mite bloodline to different locations than where it was genetically created or the mite was born (hence the rapid spread of varroa after introduction to an area, or the dispersal of miticide-resistant mite genotypes). These rides take place during the mite’s dispersal phase, which occurs between its reproductive phase(es), during which it is trapped beneath the cell capping.
Terminology change: We originally incorrectly used the term “phoretic” to describe mites riding on adult bees. But that term only applies to parasite using a temporary host solely for transportation, without feeding upon it. We’re now using the more accurate term “dispersal” phase.
During this dispersal phase, mites not surprisingly exhibit a preference for riding on nurse bees [[1]], since they not only need to feed upon the well-developed fat bodies of nurses in order to effectively reproduce [[2]], but it is only nurses that stick their heads into the cells of the 5th-instar larvae that a mite must transfer to in order to reproduce.
I find it fascinating that Nolan and Delaplane [[3]] found that mites that have not yet reproduced to exhibit a stronger preference for nurse bees over foragers, than those that have already reproduced at least once. Riding on a forager increases a mite’s risk of death, but may result in them catching a ride to another hive. It appears that evolution has rewarded this risky behavior. In addition, Cervo [[4]] found that the differences in the cuticular odor profiles of nurse vs. forager bees disappeared when their colonies suffered from a high varroa infestation rate –– which increased the proportion of mites riding on workers that exit the hive.
Practical application: Due to the above changes in cuticle odor and behaviors, colonies suffering from a high mite infestation rate become “mite diffusing varroa factories” [[5]], which results in the out-of-hive emigration of mites on the foragers, and immigration (via drifting) of some of those mites into surrounding colonies (into some colonies more than others).
Side Note: An Observation of Potential Varroa Transfer Host?
Varroa destructor relatively recently jumped host species when humans brought Apis mellifera into contact with its native host, Apis cerana. Apis mellifera could now be considered as varroa’s preferred host — at least until European honey bees evolve more resistance to this nasty parasite. Honey bee pupae are the obligate host of varroa –– the only host upon which the mite can reproduce. But the mite also uses adult honey bees as transfer hosts for dispersal, with the advantage that the mite can benefit from being able to feed on its ride.
However, the mite could conceivably use a different animal species as a temporary transfer host, upon which the mite would be called a phoretic, since that host would function solely for mechanical transfer, rather than providing any nourishment to the mite.
Varroa (being blind) choose their appropriate honey bee hosts (whether nurse bee, 5th-instar larva, or forager bee) by smell. So to abandon its host honey bee and jump onto a different species for phoretic transport, a mite would need to find that other species to be more olfactory-attractive.
I may have observed an example of this very (and previously unreported) behavior last July, when a bee landed on the arm of my assistant Rose Pasetes (not an unusual occurrence). I happened to notice that a varroa mite immediately abandoned that host bee in preference for a human! I whipped out my cell phone to document this surprising phenomenon (Figures 1 & 2).
Fig. 1 I snapped this photo moments after the mite hopped off the bee and onto Rose’s skin. Could we have been witnessing an attempt by a mite to use a human as a transfer host?
Practical application: Paenibacillus larvae, the causative parasite for American Foulbrood disease routinely passively benefits from humans transferring their spores to other hives. Could varroa benefit by actively using us to do the same?
Fig. 2 The mite (visible at 7:00 below the bee) made no effort to feed upon Rose, but also no inclination to return to the bee. Fearful that we may have been witnessing the evolution of a novel means of varroa phoretic behavior, we euthanized the mite. We didn’t have a cup large enough to perform a mite wash, but Rose later confirmed that she was not hosting any additional phoretic mites.
Practical application: I was briefly involved in a mosquito-breeding laboratory in Brazil, where there were numerous female technicians. Some of the gals were clearly mosquito magnets, whereas others were unattractive to the bloodsuckers, again presumably due to their particular skin scent. We need to pay attention to determine whether varroa is evolving to take advantage of beekeepers as hosts for phoretic transmission!
On to robbing screens
OK, potential varroa transmission averted, let’s move onto how it might help to use robbing screens to decrease the immigration of mites into hives. I’ve previously written extensively on the subjects of bee drift and mite immigration [[6]].
Bee behavior: Scouts and foragers do use vision to identify potential floral sources of nectar, but it’s even easier for them to simply scout the neighborhood for a particular scent that they’ve associated with a nectar reward. And that scent may be emanating from a hive. As far as honey bees are concerned, undefended nectar or honey is fair game. “Robbers” are simply foragers following a floral scent that led them to a hive rather than to a flower (we must remember not to equate a honey bee’s exploitation of a potential food source with the morally repugnant plundering of one group of humans by another; a honey bee is incapable of evil intent).
If you put a window screen in front of a hive entrance, the resident bees will quickly learn to fly around it, whereas scout bees from other hives will show up on the screen where the stream of exhaust air from the hive passes through it –– investigating the source of a floral odor emanating from that hive [[7]].
Using that concept, robbing screens use a screen to separate the path of a hive’s exhaust stream of aromatic air from the location of the hive entrance. This makes it more difficult for scout bees, following the scent of ripening nectar, to find the intentionally offset and reduced entrance of the protected hive. The hope is that potential robbers will just give up trying to get in.
The design of robbing screens
All robbing screens that I’ve seen have similar designs (see some examples in Figures 3-5). All can also generally serve as mouse or shrew guards, and according to some beekeepers (I’d appreciate any observations), reduce predation by yellow jackets.
Fig. 3 Country Rubes sells a “traditional” wood-frame robbing screen using low-air-resistance hardware cloth for its screen (which minimizes any exhaust air being diverted through the entrance hole), and a step that separates the screen from the adjustable top entrance hole. It’s designed to be used on bottom boards that have the beeways flush with the hive body (a handy beekeeper can modify any type of entrance guard to fit their particular hive).
Fig. 4 BeeSmart produces a plastic robbing screen with two adjustable top entrances. This design will fit between extended beeways or on 8-frame hives.
Fig. 5 BeequipNZ sells a stainless steel Robber Guard, which can be flipped upside down to offer a full-width opening across the top (the block below the guard in this photo was only necessary because I raised the entrance for a stickyboard). Since they were designed for the narrower New Zealand hive width, I did need to use a wedge at one side.
Although there may be some congestion when using a small entrance hole, even strong colonies tolerate them in hot weather in my environment.
Using entrance guards to minimize Mite Immigration during Overt Robbing
If a colony gets severely weakened by the varroa/virus complex while there is any fresh nectar inside, it may be quickly mobbed and robbed by other colonies. Like rats leaving a sinking ship, some of the mites in the collapsing colony may hitchhike rides on invading bees and be taken back to the robbers’ home colony (or colonies) [[8]].
An observation: Last year I was taking regular mite washes from a “mite donor” colony that I was nursing, and then one morning watched it get robbed out in a few hours. Once robbing subsided later in the day, I inspected the colony, now completely emptied of every drop of honey. There were only a few dead bees in front of the entrance (presumably guards that died fighting the robbers). On the emptied combs, there was a softball-sized cluster of presumably young (and hungry) workers that had apparently not engaged in fighting. Curious, I washed a half cup of them. The mite wash count taken two days earlier had been 44; it had now jumped to 117, which suggested that many of the mites preferred to remain on the young resident workers, rather than hitching a ride on a robber.
Practical application: Installing a robbing screen once robbing has begun is futile –– the robbers will quickly figure out how to get in.
Overt vs. Covert Robbing
“Overt robbing” of weak or collapsing colonies is one thing. But low-level “covert” robbing (aka “surreptitious” or “progressive” [[9]] robbing), is when robbers enter a hive without being stopped by the guards, help themselves to some stored nectar (or perhaps beg it from a returning forager), and then take it back home, later returning for more. Covert robbing can occur at a low level. On the other hand, I’ve watched yards of nucleus hives get robbed dry during a strong nectar flow by robbers from (presumably stronger) colonies elsewhere, with no signs of fighting taking place, despite dozens of robbers entering every minute.
Anyway, my data suggest that covert robbing may contribute to mite “diffusion” from high-mite hives to neighboring colonies. Robbing screens can reduce either covert or overt robbing from taking place. So could it be of benefit to install robbing screens on your own hives?
Practical application: A robbing screen does not prevent bees from exiting and reentering their own guarded hive and robbing other hives. Thus, it would not reduce immigration of mites brought back by its own robbers.
However, installing robbing screens on your own high-mite hives may help reduce mite diffusion from them to your low-mite hives. But if you’ve got varroa under control in all your hives, there would be little or no benefit to installing robbing screens, other than to keep your strong hives from robbing any weak ones.
But what if you have neighboring beekeepers or feral colonies with high mite levels?
Practical application: Installing robbing screens on your own hives would not prevent your colonies from robbing neighboring high-mite hives. If you have neighboring beekeepers who do not control their mites, you might benefit from giving your neighbors robbing screens for Christmas.
That’s not to say that robbing screens Couldn’t be of Benefit
Despite the fact that placing a robbing screen on your hive wouldn’t prevent your bees from bringing back mites from other colonies, it is still plausible that installing the devices might reduce mite immigration from drifting bees. (It could also conceivably reduce mite-carrying covert robbers from high mite hives from entering your low-mite hives and leaving their mites behind, but it’s not clear how often that actually takes place).
As detailed in my previous articles, late in the season when mite levels may be high in neighboring colonies, some hives may experience substantial mite immigration on days when the weather is favorable for flight (independent of overt robbing taking place). I found that a substantial amount of bee drift (and thus mite dispersal) can take place between high-mite donor hives and bee-attractive receiver hives up to a half mile away (not all hives are attractive to drifting bees, and some hives in a yard do not experience any mite immigration).
Practical application: I found that there can be a high correlation between incoming drifted bees and mite immigration [[10]], with some hives being far more attractive to drifting bees than others. So mite immigration into at least some of your hives could plausibly be reduced by using robbing guards –– provided that the guards actually prevent drifted bees from entering.
Testing whether Robbing Guards Reduce Mite Immigration
To determine whether installing entrance guards would reduce mite immigration, my helpers and I ran three different trials in 2022 and 2023, involving over 200 hives in total, and eight different apiaries. It wasn’t as easy a question to answer as I hoped it might be. Data collection involved some 300 mite washes and over a thousand stickyboard counts (taken on average twice a week) from mite-zeroed hives [[11]].
Trial A –– Crossover trial, No Mite Donor Hives
In this trial, we tracked mite immigration (via stickyboard counts) in 24 hives over 49 days, half with guards on, half without, swapping the guards back and forth from time to time (crossover design), so that each hive was tested with guards on or off (to account for the expected large degree in hive-to-hive variation in mite immigration, based upon our previous research).
Trial B –– Multiple Apiary Comparative Trial
Concurrent with Trial A, but in six different yards, we took starting and ending mite wash counts from 133 hives, then treated them all with a low-dose OAE (extended-release oxalic acid) pad to reduce mite buildup. We installed robbing screens on half of them (randomly blocked by starting mite count) to see whether having guards on made a difference in their final infestation rates 51 days later.
Trial C –– Crossover trial, With Mite Donor Hives
In this trial, we tracked mite immigration in 28 hives over 95 days, again alternating guards being on or off the test hives. But in this trial, we placed high-mite donor hives adjacent to the test hives to increase the amount of mite dispersal.
I’m out of space, so will continue with our findings next month.
Citations and notes
[1] Xie, X, et al (2016) Why do Varroa mites prefer nurse bees? Scientific Reports 6, 28228
[2] Ramsey, S, et al (2019) Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. Proceedings of the National Academy of Sciences, 116(5): 1792-1801.
[3] Nolan IV, M & K Delaplane (2017) Parasite dispersal risk tolerance is mediated by its reproductive value. Animal Behaviour 132: 247-252.
[4] Cervo, R, et al (2014) High Varroa mite abundance influences chemical profiles of worker bees and mite–host preferences. Journal of Experimental Biology, 217(17), 2998-3001.
[5] A Study on Bee Drift and Mite Immigration: Part 1
[6] A Study on Bee Drift and Mite Immigration: Parts 1-6. ABJ February through July 2023
[7] A Survey on Robbing at Collapse. ABJ February 2023
[8] Peck DT & TD Seeley (2019) Mite bombs or robber lures? The roles of drifting and robbing in Varroa destructor transmission from collapsing honey bee colonies to their neighbors. PLoS ONE 14(6): e0218392.
[9] Mangum, W (2012) Robbing: Part 2: Progressive Robbing. ABJ 152(8): 761-764.
[10] A Study on Bee Drift and Mite Immigration Part 5. ABJ June 2023
[11] https://scientificbeekeeping.com/suggested-protocol-to-determine-amount-of-mite-immigration/