Until recently mite resistance from grooming/chewing has
received little attention in the national bee magazines. I had previously used Purdue Mite Biter
stock with this trait. Last fall I
became aware of the availability of feral bee stock that also exhibited this
trait. I find it fascinating that
wild/feral bees had naturally developed a resistance to mites. NOTE: All honeybees exhibit chewing/grooming
to a limited extent. The PMB line and
this feral stock show enhanced chewing/grooming.
In 2020 I purchased two breeder queens from Dorothey Morgan
in Kentucky. One was of Purdue Miter
Biter (PMB) stock and the other of feral stock from mountainous areas in
Kentucky. Both stocks are known for
“chewing” on varroa mites. Dorothey has
not treated her bees for the past seven years since converting her entire
apiary to PMB stock. The feral stock
came from members of the Kentucky Queen Breeders Association. Testing by Dorothey and Dwight Wells had shown
this feral stock met or exceeded the level of “chewing” exhibited by the PMB
stock. Dwight is knowledgeable about the subject of
feral bees. NOTE: In 2020 Purdue
University did not produce PMB breeder queens for distribution due to the
COVID19 shutdown of their campus.
Dorothey also credits some of her overwintering success to not purchasing packaged bees and thus avoiding importation of any viruses floating around the general bee population (read that as from bees that were held in the California holding yards prior to almond pollination). Also, PMB and feral bee lines do not eliminate the mites, but reduce their numbers sufficiently so that hives can overwinter without need for mite treatments. Reaching an equilibrium between the parasite and host is how the Apis Cerana (the Asian honeybee) evolved to live with the varroa mite, although not through the same mechanism.
I wanted to assess how these two breeder queen hives were doing in
relation to mites. You could say I am
from Missouri, the “show me” state. I
wanted those two hives to show me that they had reduced mite counts. Utilizing witness boards was intended to
fulfill that need. Near the end of June,
I began daily inspection of the witness boards.
Initially I simply counted and recorded the number of mites on each
board. During the rapid growth stage of
the hives, I did not want to be removing 300 bees to perform alcohol-wash mite
checks. I was already opening the hives
on a weekly basis to take brood for queen rearing. Too much stress on a hive can result in queen
supercedure.
I also tracked mite counts in two overwintered mixed-breed
hives. One hive’s mite drop was rising
rapidly by the end of July (already greater than 20 mites/day), and
consequently I treated both mixed breed hives with FormicPro. Due to the treatments I did not include their
data here. The worst hive needed a second treatment at
the end of August when the mite drop again built up to 20 per day. (As a side note I requeened the hive after
the second treatment with a f1 PMB queen).
Here is the mite drop from the two breeder queen hives. I am presenting the data as 7-day moving
averages to smooth the curves. The
maximum daily mite drop for the feral queened hive was 10 mites and for the PMB
queened hive 23 mites. The maximum drop
for both hives occurred on the same day after the nighttime temperature got
down to the low 30s (lowest so far this fall).
A few comments/observations.
1)
It must be remembered that these were not
overwintered hives which would have a longer time frame for the mite population
to build.
2)
Recommended sticky board mite drop limits are
all over the place. I have found some
recommendations that mite drops of more than 10 or 12 mites per day indicate
the hive is in trouble and needs treatment. Other sources indicate daily drops
of up to 40 or 60 per day in the fall are the treatment threshold. I put the 12 mites per day limit in my
graph.
-BetterBee recommended a 12/day limit in August
-Brushy Mountain recommended a 60/day limit in fall
-PerfectBee recommended a 60/day limit in fall
-Virginia Cooperative Extension recommended a 40/day limit
-Ontario Ministry of Ag recommends a 12/day limit in August
3)
I performed two alcohol-washes (Aug. 21st
and Sept 25th) which yielded either 0 or 1 mite. These values are good for that time of year. Limits of 3% or 9 mites per 300 bees are normally
deemed acceptable. These two alcohol
wash checks show these chewing/grooming bees were controlling varroa mites as
desired.
4)
I haven’t
found any literature stating how to compare alcohol-wash results versus mite
drop counts.
5
6)
This uncontrolled test was a small sample of
only two hives. I would not jump to the
conclusion that “feral” queens, in general, are better than “PMB” queens. It may be true, but the small size of my test
prevents me from stating this is so. I
can’t determine whether the feral hive or PMB hive is doing a better job of
removing mites. Is the lower daily drop
of the feral hive a sign of better grooming performance or is the higher spike
during the cold spells a sign the PMB hive is doing a better job. I can’t wheedle the answer out of my limited
data. The IMPORTANT thing is that
the varroa levels of both hives are staying below both the alcohol-wash and
sticky board limits.
7)
My initial thought was that the two spikes in
the mite drop at the end of the summer were attributable to the end of summer
decline in brood rearing and cooler nighttime temperatures resulting in the
bees clustering. But after looking at
the nighttime temperatures it seems that nighttime temperature was the primary
factor in both spikes. The first spike occurred when the temperature
dipped to the high 30s (Sept 19th, 39F). The second bigger spike occurred when
nighttime temperature dropped to the low 30s (Oct 2nd, 32F). Mite
drop rose following each cold spell, usually several days in length, and began
declining as the nighttime temperatures warmed again. The maximum height of the
bump up occurred a few days after the cold night due to the damping effect of
the 7-day moving average used to smooth the curve. I
assume the colder temperatures are causing the bees to cluster and results in
enhanced grooming/chewing. A third cold snap (Oct 19th, 25F)
did not result in a mite drop spike. Why
the difference?
At the end of July, I started gathering all mites from the
witness boards and examining their legs for evidence of chewing. I have had no formal training in this
process, but simply looked for severed legs or missing edges on their carapace
(shell). I was using a low cost/low magnification
digital microscope which may have hindered the accuracy of my
observations. In addition, I did not
think of looking for severed antenna that are located between the eight
legs. About 5% of the dropped mites were
alive and moving their legs, 45% were
dead, and 50% had chewed legs and dead.
Both the PMB and feral hive drops exhibited roughly the same 50% chewing
rate. If I assume all of the dead mites
were due to natural mortality (this assumption is highly questionable), then I
could surmise that the “chewing behavior” had roughly doubled the mite
mortality.
The onset of colder nights (~ <40F) also resulted in finding no live mites on the witness board the following morning. When nighttime temperatures warmed back up live mites again appeared on the witness board. My thought is that the live mites that are on the witness board die of exposure during the night without the warmth of the bee cluster. Are the live mites falling to the witness board simply because they lost their footing or due to grooming? I can’t answer that question. So if mites are dislodged by either “chewing” or grooming or simply losing their footing they will probably die during the winter if they drop to the floor of the hive.
Now playing with the data a little. The graph shows a daily mite drop of roughly
8 mites per day (average of the two hives).
Taking away the 50% of mites showing chewing that leaves 4 mites per day
which would be considered natural mite drop do to mortality. Natural mite mortality runs about 0.5%. Therefore, there is about 800 mites in each
hive (4/0.005=800). Based on Randy
Oliver’s mite program a hive with 800 mites in mid-October should NOT have a
varroa induced crash. That makes me very
hopeful about this potential solution and confirms in my limited way that these
bees can survive with no treatment.
Overall, I am happy with the performance of both hives/queens
and will be taking these two hives through winter untreated. Assuming they survive (and with these low
mites loads they should survive), I will track mite drop through another year
to see how they cope with the longer period for mite reproduction.
I would say both the PMB and feral queened hives have
controlled the mite build up as advertised.
My only reservation of my observations is that they were not of
overwintered hives. Given the fact that
Purdue’s and Dorothey’s apiaries have been untreated for years puts that issue to bed and these queens will continue to be my focus for mite control next year.
From these breeder queens I have put about 50 queens into winter nucs. Next year I will monitor a few of these f1 queens (first generation) to see if they control the mites to the same extent. I would expect them to be not as good because this year’s drone population in my apiary was not all of PMB/feral genetics and therefore there would probably be a dilution of the “chewing trait”. I expect it will take several years to improve my entire apiary to equivalent performance and for me to be able to stop mite treatments. This problem of dilution also occurs with all improved bee lines (VSH, Minnesota Hygenic, Russians, etc.). Once you make the decision to improve your apiary you need to stick with it for several years to counter the genetic dilution effect that occurs due to the honeybee queen’s mating behavior. Purchase of package bees/queens also adds to the dilution. Its everything or nothing.
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