.
ROBYN M. UNDERWOOD AND ROBERT W. CURRIE
Department of Entomology, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2
J. Econ. Entomol. 98(6): 1802Ð1809 (2005)
ABSTRACT The combination of the concentration of formic acid and the duration of fumigation
(CT product) during indoor treatments of honey bee, Apis mellifera L., colonies to control the varroa
mite, Varroa destructor Anderson & Trueman, determines the efÞcacy of the treatment. Because high
concentrations can cause queen mortality, we hypothesized that a high CT product given as a low
concentration over a long exposure time rather than as a high concentration over a short exposure
time would allow effective control of varroa mites without the detrimental effects on queens. The
objective of this study was to assess different combinations of formic acid concentration and exposure
time with similar CT products in controlling varroa mites while minimizing the effect on worker and
queen honey bees. Treated colonies were exposed to a low, medium, or high concentration of formic
acid until a mean CT product of 471 ppm*d in room air was realized. The treatments consisted of a
long-term low concentration of 19 ppm for 27 d, a medium-term medium concentration of 42 ppm for
10 d, a short-term high concentration of 53 ppm for 9 d, and an untreated control. Both short-term
high-concentration and medium-term medium-concentration fumigation with formic acid killed
varroa mites, with averages of 93 and 83% mortality, respectively, but both treatments also were
associated with an increase in mortality of worker bees, queen bees, or both. Long-term lowconcentration
fumigation had lower efÞcacy (60% varroa mite mortality), but it did not increase
worker or queen bee mortality. This trend differed slightly in colonies from two different beekeepers.
Varroa mite mean abundance was signiÞcantly decreased in all three acid treatments relative to the
control. Daily worker mortality was signiÞcantly increased by the short-term high concentration
treatment, which was reßected by a decrease in the size of the worker population, but not an increase
in colony mortality. Queen mortality was signiÞcantly greater under the medium-term medium
concentration and the short-term high concentration treatments than in controls.
Queen Bee and Colony Mortality. The proportion
of queens killed during fumigation differed between
treatment rooms (P 0.001) with zero of 20 queens
dying in the control room, zero of 21 dying in the
long-term low-concentration treatment, Þve of 21
queens dying in the medium-term medium-concentration
treatment, and seven of 21 queens dying in the
short-term high-concentration treatment (Fig. 4). SigniÞcantly
more queens were killed in the short-term
high concentration (P 0.01) and medium-term medium
concentration (P 0.05) than in the control.
Two queens were killed in the control room; one
before the start of fumigation and another several
weeks after fumigation ceased.
Treatment room did not signiÞcantly affect colony
survival as estimated on 17 April, 9 d after the colonies
were moved outdoors (P 0.05). However, there was
a signiÞcant effect of treatment room on the queen
state of surviving colonies (P0.01; Fig. 4). Queenless
colonies were successfully requeened in the spring
after treatment.
Varroa Mite Mean