Review: Aliano and Ellis. (2008). Apidologie, 39: 481-487
Written: November 14, 2008
Posted: 11/15/08
Word count: 696
Question: What is the mode of action for oxalic acid control of varroa?
Answer: Direct contact, not fumigation
Our continuing troubles with
varroa mites can sometimes seem daunting, with near ubiquitous infestations,
increasing virulence, and developing resistance to chemical controls.
In the face of these challenges, some beekeepers have been tempted
to use non-labeled alternative controls, which cannot be condoned.
These issues do, however, necessitate the exploration into as many safe alternatives for control as possible.
Fortunately, there are several
relatively new alternatives to Apistan (fluvalinate) and Checkmite+
(coumaphos), as both have mites developing resistance to them. Products
such as MiteAway II (formic acid) and ApiLife VAR (thymol) have been
on the market for several years and show high levels of mite control
when used properly. Their mode of action is to turn the hive into a
fumigation chamber; the chemicals are soaked in pads that release them
into the air, and the volatiles then contact the mites and kill them.
One newer alternative, which
has been used in Europe and Canada for many years, is the use of oxalic
acid (OA). Like the formic acid and thymol products, OA is considered
a “softer” chemical than the synthetic acaracides used
more widely in this country, yet it has shown to have high efficacy
(>90%) in varroa control. It can be readily introduced into a beehive
by applying it in several ways, but the most popular has been to drip
about a half cup full of 3.5% solution directly onto the bees. But
we need to understand more about its mode of action before we can be
assured that it is both effective and safe for the bees.
A recent study by Nick Aliano
and Marion Ellis from the University of Nebraska tested the mode of
action for OA in a simple yet elegant study. At issue was whether OA
kills mites through fumigation (like formic acid and thymol), by making
the bees somehow
“distasteful” to the mites (by ingesting the chemical), or direct
contact among the bees (and therefore to the mites). They established several
experimental hives in single-story hives, which they then divided in half with
one of three types of division boards. The first was a double screen, which
permitted volatiles and other odors to pass from one side to the other but
did not allow any interactions among workers between the two sides. The second
was a single screen, which also allowed volatiles to be exchanged but also
trophallaxis among workers from the two sides (but not direct contact). The
third was a queen excluder, which allowed volatiles and workers to interact
with each other directly.
The researchers then administered
oxalic acid treatment to one side of each double-hive, measuring mite
levels before and after treatment on both sides. They then treated
all units with coumaphos to kill the remaining mites, which enabled
them to calculate the efficacy of the OA treatment. More importantly,
they were then able to compare the mite control on the non-treated
halves of each hive to determine the mode of action of OA. If it kills
mites through fumigation, then all three treatments should show equal
levels of control; if it kills mites through ingestion, then the single
screen and queen excluder treatments should show higher levels of control
than the double screen; and if it kills mites through direct bee-to-bee
contact, then only the queen-excluder treatment should show effective
control.
It is this latter result that
they found, strongly suggesting that direct worker interaction is necessary
for oxalic acid to control varroa mites. Thus OA does not turn a hive
into a fumigation chamber, like some of the other newer products, but
rather needs to be passed from bee to bee, and from bee to mite. This
study should give us hope for yet another effect control method for
varroa. But let’s wait until it is registered before we start
using it, which hopefully should be soon.
Reference
Aliano, N. P. and M. D. Ellis. (2008). Bee-to-bee contact
drives oxalic acid distribution in honey bee colonies. Apidologie, 39: 481-487.
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