Bulletin of Insectology 67 (1): 125-130, 2014
ISSN 1721-8861

Sub-lethal exposure to neonicotinoids impaired honey bees
winterization before proceeding to colony collapse disorder
1

2

3

Chensheng LU , Kenneth M. WARCHOL , Richard A. CALLAHAN
1
Department of Environmental Health, Harvard School of Public Health, Landmark Center West, Boston, MA, USA
2
Worcester County Beekeepers Association, Northbridge, MA, USA
3
Worcester County Beekeepers Association, Holden, MA, USA

Abstract
Honey bee (Apis mellifera L.) colony collapse disorder (CCD) that appeared in 2005/2006 still lingers in many parts of the world.
Here we show that sub-lethal exposure of neonicotinoids, imidacloprid or clothianidin, affected the winterization of healthy colonies that subsequently leads to CCD. We found honey bees in both control and neonicotinoid-treated groups progressed almost
identically through the summer and fall seasons and observed no acute morbidity or mortality in either group until the end of winter. Bees from six of the twelve neonicotinoid-treated colonies had abandoned their hives, and were eventually dead with symptoms resembling CCD. However, we observed a complete opposite phenomenon in the control colonies in which instead of abandonment, they were re-populated quickly with new emerging bees. Only one of the six control colonies was lost due to Nosemalike infection. The observations from this study may help to elucidate the mechanisms by which sub-lethal neonicotinoids exposure caused honey bees to vanish from their hives.
Key words: colony collapse disorder, CCD, honey bee, neonicotinoids, imidacloprid, clothianidin.

Introduction
Since its emergence in 2005/2006, the continuing significant losses of honey bees (Apis mellifera L.) colonies
resulting from the symptomatic disease of colony collapse disorder (CCD) has demonstrated our inability toidentify and eradicate the responsible cause(s) of CCD(BBC News, 2013; The New York Times, 2013; vanEngelsdorp et al., 2008). While the prevailing opinions
suggest the linkage of CCD to multi-factorial causes including pathogen infestation, beekeeping practices (including malnutrition), and pesticide exposure in general(Cox-Foster et al., 2007; Blanchard et al., 2008; Higeset al., 2008; vanEngelsdorp et al., 2009; Alaux et al.,2010; de Miranda et al., 2010; Williams et al., 2010; Di
Prisco et al., 2011; Vidau et al., 2011; USDA, 2013),
this notion ignores the differential mortality symptoms;
in particular hive abandonment in CCD vs. diseased colonies. However, recent scientific findings linking CCD
with exposure to neonicotinoids, a group of systemic
insecticides, appear to be gaining traction (Maini et al.,2010; Pareja et al., 2011; Lu et al., 2012; Farooqui,2013; Matsumoto, 2013) and have led to new regulatory
control (Erickson, 2012). In this study, we extend ourprevious study (Lu et al., 2012) showing that sub-lethal
exposure of imidacloprid and clothianidin affected thewinterization of healthy honey bee colonies that subsequently leads to CCD.
Materials and methods
In order to investigate the detrimental effects of sublethal neonicotinoid exposure in healthy honey bee colonies, we utilized the split-plot lifecycle study design inwhich honey bees are fed with pre-determined known
amounts of neonicotinoids and allowed to freely forage

in the environment. We then assessed their hive growthand strength, as well as their mortality and morbidity,
throughout the lifecycle including multiple worker bee
generations. The setup and management of eighteen
study colonies (using 10-frame Langstroth pine hive) inthree apiaries in central Massachusetts was identical tothat previously described (Lu et al., 2012). At eachapiary, we separated six colonies into two groups inwhich honey bees were fed with either sucrose water or
high-fructose corn syrup (HFCS) over the study period.
Each sugar group consisted of two neonicotinoid-treated
and one control colonies replicated in each of the three
apiaries. We purchased sucrose from a local food store
and HFCS from a beverage company. Both sugar waters
made of sucrose and HFCS were analyzed prior to be
used in the experiment and found non-detectable residues of neonicotinoids using a published method (Chenet al., 2013). Starting from July 2nd 2012, we administered 258 µg of imidacloprid (1-(6-chloro-3-pyridinyl)
methyl)-N-nitro-2-imidazolidinimine, CAS# 13826141-3) or clothianidin (1-(2-chloro-1,3-thiazole-5ylmethyl)-3-methyl-2-nitroguanidine, CAS# 21088092-5) in 1.9 liter (0.5 gallon) of sucrose water and
HFCS to the treated colonies each week, respectively,
for thirteen consecutive weeks ending on September
17th 2012. Assuming each colony consisted of 50,000
bees at any given day in spring and summer, we administered 0.74 ng/bee/day of either imidacloprid or clothianidin to treated hives for 13 consecutive weeks. This
dosage is far below the oral LD50 of 3.4 and 118.7
ng/bee for clothianidin and imidacloprid, respectively(Laurino et al., 2013). Control colonies were given neonicotinoid-free sucrose or HFCS throughout the experimental period. Sugar water (both types) was completely
consumed by each colony at the end of each week during the 13-week neonicotinoids administration.
From June 29th to September 24th 2012, we assessedthe brood rearing production of all colonies on a biweekly basis using a modified brood assessment method
as previously described (Lu et al., 2012). In brief, the20-frames in each hive were scored cumulatively for thearea covered by “sealed brood” which is the pupal stageof honey bee development. Brood was estimated by dividing the face of each side of frame into 32 squares(each square containing approximately 100 cells). All
20 frames in each hive were scored by visually estimating the number of squares of capped brood per frame
face. All colonies were treated with Miteaway Quick
strips for controlling Varroa mite on August 13th 2012,
followed by Apistan strips from October 1 st to November 15th 2012. The Varroa mite counts were assessed
twice using the common alcohol wash method on August 13th (pre-Miteaway application) and August 22nd
(post-Miteaway application). In addition, colonies weretreated with Fumagillan-B [9.1 g dissolved in 7.6 liters(two gallons) of sucrose or HFCS] in early October
2012 to control N. apis and N. ceranae, two common
intestinal parasites. Entrance reducers were installed before the hives were ready for winterization.
All colonies were monitored weekly beginning on late
October 2012. Notes were taken on the size of the clusters observed by counting the numbers of frames containing honey bees from the top of the hive in which it
generally took no more than 10 seconds. Starting from
November 2012, hives were supplemented either withcrystallized HFCS or with granular sucrose mixed into a
thick water paste. The food was placed on waxed paper
on top of the frames inside the inner covers. Data wereanalyzed using SPSS Statistics (version 20.0).

Results
We found honey bee colonies in both control and neonicotinoid-treated groups progressed almost identically,
and observed no acute morbidity or mortality in eithergroup until the arrival of winter. In addition, neither thelocations where the hives were set up nor the type ofsugar (high-fructose corn syrup vs. sucrose) fed to honey bees was associated with the brood rearing or theoccurrence of CCD (one-way ANOVA). Therefore datafrom 3 apiary locations and two types of sugar werepooled in the data analysis. As temperatures began todecrease in late October 2012, we observed a steady decrease of bee cluster size in both control and neonicotinoid-treated colonies. While such decline was quickly
reversed in the control colonies in January 2013, theneonicotinoid-treated hives continued to decline (figure1). As shown in table 1, the numbers of frames containing bees were not significantly different among thetreatments from 10/27/2012 to 12/29/2012 (one-way
ANOVA), but became statistically significant different
from 1/5/2013 to 4/4/2013 (one-way ANOVA,
p < 0.0001). At the end of the experiment on 4/4/2013,
there were 5.3, 2.0, and 2.9 frames of bees in the control, imidacloprid, and clothianidin-treated hives, respectively. The diminishing cluster size in the neonicotinoid-treated colonies led to the loss of six of thetwelve (50%) with symptoms resembling CCD, whereas
only 1 of the 6 control colonies was lost exhibiting
Nosema ceranae like symptoms, although we did not
perform any test to confirm Nosema infection in thiscontrol hive. No similar Nosema-like symptoms were

Figure 1. Average numbers of frame (standard deviations shown as error bars) containing honeybees for control-,imidacloprid-, and clothianidin-treated colonies and the corresponding daily average temperature at Worcester regional airport in Worcester MA recorded from October 2012 to April 2013. The daily average temperature readings
were obtained from the NOAA website (http://cdo.ncdc.noaa.gov/qclcd/QCLCD).
126

Table 1. Field recording data from honey bee hives treated with control, imidacloprid, and clothianidin in sucrose
water or high-fructose corn syrup (HFCS) from May 2012 to April 2013.
Treatment
Honey bee hives
Average # of frame with bees (SD)
Recorded from 10/27/2012 to 12/29/2012
Average # of frame with bees (SD)
Recorded from 1/5/2013 to 4/4/2013# of dead colony (%)

Control
Sucrose
HFCS
3
3

Imidacloprid
SucroseHFCS
3
3

Clothianidin
SucroseHFCS
3
3

6.3 (2)

6.8 (2)6.0 (3)6.3 (3)

6.6 (2)

6.3 (2)5.8 (1)4.9 (3)

1.8 (2)2.2 (2)2.9 (2)2.9 (2)0 (0)1 (33.3)

2 (66.7)
1/5/2013
2/9/2013

2 (66.7)
1/5/2013
3/7/2013

1 (33.3)

1 (33.3)

1/5/2013

12/29/12

Date of dead colony observed

3/7/2013

Average Varroa mite counts
Before treatment (SD)10 (6) a
11 (3) a
11 (2) a
10 (3) a
12 (2) a
9 (4) abbbbbAfter treatment (SD)
2 (2)
1 (1)
1 (1)
2 (1)
1 (1)
1 (1) bc
Pooled Data
Honey bee hives
6
6
6
Average # of frame with bees (SD)dd
6.6 (2)6.1 (3)
6.5 (2) dRecorded from 10/27/2012 to 12/29/2012
Average # of frame with bees (SD)
5.3 (2) e
2.0 (2) e
2.9 (2) e
Recorded from 1/5/2013 to 4/4/2013# of dead colony (%)
1 (17)
4 (67)
2 (33)
Average Varroa mite counts
Before treatment (SD)
10 (4) f
12 (2) f
10 (3) fff
After treatment (SD)
2 (1)
1 (1)
1 (1) fa
Varroa mite counts were not significantly different before Miteaway Quick strips treatment between sucrose and
HFCS in control, imidacloprid, and neonicotinoid-treated hives (one-way ANOVA);b
Varroa mite counts were significantly different after Miteaway Quick strips treatment between sucrose and HFCS
in control, imidacloprid, and neonicotinoid-treated hives (one-way ANOVA);c
Data from two sugar treatments were pooled for control, imidacloprid, and neonicotinoid-treated hives;dNumbers of frame containing bees were not significantly different among control, imidacloprid, and neonicotinoidtreated hives during this period of time (one-way ANOVA);e
Numbers of frame containing bees were significantly different among control, imidacloprid, and neonicotinoid-treatedhives during this period of time (one-way ANOVA, p < 0.0001);f
Varroa mite counts were significantly different before and after Miteaway Quick strips treatment in control, imidacloprid, and neonicotinoid-treated hives (paired t-test, p < 0.0001).
observed in the treated hives. Upon close examinationof colonies in early April 2013, we found that the majority of bees in all neonicotinoid-treated colonies, regardless of whether they survived or not, had abandoned
their hives during the course of winter. However, weobserved a complete opposite phenomenon in the control colonies in which instead of abandonment, hives
were re-populated quickly with new emerging bees. The
honey bee clusters in the six surviving neonicotinoidtreated colonies were very small, and were either without queen bees, or had no brood.
We found no significant difference in the degree of
Varroa mite infection between the control and neonicotinoid-treated colonies. The average mite counts were
10-12 per 150 bees in the control and neonicotinoidtreated colonies, respectively, as assessed in mid-August2012 (table 1). We later reduced the mite counts in allcolonies to 1-2 mites per 150 bees after the applicationsof Miteaway Quick strips, a commonly used medicinal
treatments prior to the arrival of winter in which it significantly reduced mite counts from 10-12 to 1-2 mites
per 150 bees, respectively, in control, imidacloprid, andneonicotinoid-treated hives (paired t-test, p < 0.0001).

We also found that neonicotinoids do not appear to affect the quality of brood rearing during summer and fall
(figure 2). The sealed brood counts for both control andneonicotinoid-treated colonies decreased significantly inparallel from July to September 2012 (Pearson, 2-tails,
p < 0.0001). This decreasing (slope = −0.62) trend has
been reported previously (Lu et al., 2012), and is consistent with a dirth of nectar that is common in the New
England area during the summer, and is therefore independent of neonicotinoid exposure.
Discussion
The results from this study not only replicate findings
from the previous study on imidacloprid and extend toclothianidin, but also reinforce the conclusion that sublethal exposure to neonicotinoids is likely the main culprit for the occurrence of CCD (Lu et al., 2012). The
survival of 5 out of 6 control colonies in the same apiaries where the neonicotinoid-treated colonies were set
up augment this conclusion. The observation of wintertemperature modulating the severity of CCD associated127Figure 2. Average numbers of sealed brood count (standard deviations shown as error bars) for control-, imidacloprid-, and clothianidin-treated colonies during the dosing period (from 6/29/2012 to 9/24/2012), and the average
numbers of Varroa mite counts recorded before and after Miteaway Quick strip treatment on 8/13/2012. Sealed
brood counts were neither significantly different between sugars (one-way ANOVA) nor among treatments (oneway ANOVA). However, sealed brood counts were significantly decreased for all colonies from 6/29/2012 to
9/24/2012 (Pearson 2-tails, p < 0.0001).
with sub-lethal neonicotinoid exposure coincides with
reports that CCD often occurs in the winter season. The
modification of the sub-lethal effect of neonicotinoid by
the severity of winter might be significant, and should
not be overlooked in the evaluation of CCD epidemic.
The previous study conducted during a colder winterreported 100% mortality of CCD in colonies treatedwith 0.1 ng/bee/day of imidacloprid (Lu et al., 2012),
one-seventh of the dose used in the present study.
We found that chronic sub-lethal neonicotinoid exposures do not appear to compromise honey bees’ immune
resistance to pathogen infection in this study. This is incontrast to several earlier reports suggesting that the increased CCD mortality of honey bee colonies is due toreduced resistance toward common pathogens, such asincreased susceptibility of Nosema infection, caused byneonicotinoid exposures (vanEngelsdorp et al., 2009;
Alaux et al., 2010; Vidau et al., 2011; Pettis et al.,2012). The similar degree of Varroa mite infection inboth control and neonicotinoid-treated colonies disagrees with the findings that CCD hives are often associated with significantly higher pathogen infestations
than non-CCD hives exposures (vanEngelsdorp et al.,2009; Alaux et al., 2010; Vidau et al., 2011). In addition, a recent re-analysis of genomic data previously
generated from RNA pools of CCD colonies has also
excluded the association of pathogen infection and CCD(Tokarz et al., 2011). It is imperative to emphasize thatwhile pathogen infections are common and serious diseases found in honey bees that often lead to colony
death, the post-mortem examinations of the pathogencaused dead colonies are vastly different to those suf128

fered from CCD (Anderson and East, 2008; Lu et al.,2012). One of the defining symptomatic observations of
CCD colonies is the emptiness of hives in which theamount of dead bees found inside the hives do not account for the total numbers of bees present prior to winter when they were alive (figure 3). On the contrary,when hives die in the winter due to pathogen infection,
like the only control colony that died in the present
study, tens of thousands of dead bees are typically foundinside the hives (figure 4). The absence of dead bees in
the neonicotinoid-treated colonies is remarkable andconsistent with CCD symptoms.
Two critical questions remain to be answered in orderto solve the CCD puzzle. First, why do neonicotinoidtreated colonies lose their ability to renew brood rearing
toward the end of winter when temperatures began torise? Considering that neonicotinoid-treated and control
colonies had identical brood rearing performance prior to
the arrival of winter (figure 1), the failure of neonicotinoid-treated colonies to resume brood rearing, in particular during the transition from winter to spring might bepart of the interplay between sub-lethal neonicotinoid
exposure and CCD. While it is true that the lack of broodrearing might simply be due to smaller surviving clusters
during cold winter months, the surviving neonicotinoidtreated colonies never re-initiated the brood rearing into
warm weather. We found that the severity of CCD
caused by sub-lethal neonicotinoid exposures might bemodulated by winter temperature. A colder and prolonged winter in 2010/2011 in central Massachusetts
rendered a higher CCD mortality rate of 94% (Lu et al.,2012) than the current 50% in 2012/2013. Such disparity

Figure 3. Picture of the bottom board taken from one of the dead neonicotinoid-treated colonies on March 1st, 2013.
The numbers of dead bees in the six dead CCD colonies ranged from 200-600 dead bees.

Figure 4. Picture of the bottom board taken from the only dead control colony on March 1st, 2013. The volume ofdead bees was estimated to be 3.5 l using 1-L graduate cylinder using Atkins (1986) method.
might be due to the fact that the daily average temperature was lower in 63 of 91 days in the winter of
2010/2011 than of 2012/2013. The overall average temperature in the winter months was −3.8 °C (25 °F) in
2010/2011, approximately 2.78 °C (5 °F) lower than in
2012/2013.
Second and perhaps the foremost; why do honey bees
vanish from neonicotinoid-treated colonies during thewinter? It is striking and perplexing to observe the empty neonicotinoid-treated colonies because honey bees
normally do not abandon their hives during the winter.
This observation may suggest the impairment of honeybee neurological functions, specifically memory, cognition, or behavior, as the results from the chronic sublethal neonicotinoid exposure. Although the failure to
initiate brood rearing and the vanishing of the worker

caste in the neonicotinoid-treated colonies might be governed by completely different mechanisms, they suggest the possible involvement of cascading events prior
to the occurrence of CCD. The findings from this study
could be used to elucidate mechanisms by which sublethal neonicotinoid exposure impairs honey bees’ ability to over winter with symptoms consistent with CCD.
We conclude that when honey bees were exposed to either imidacloprid or clothianidin at a dose of 0.73
ng/bee/day for 13 consecutive weeks from July to September 2012, six of twelve previously healthy neonicotinoid-treated colonies died and all progressed to exhibit
CCD symptoms during the winter months. The survivalof control colonies and the absence of CCD-like symptoms in the only dead control colony not only augment
this conclusion but also support the finding that chronic
129sub-lethal neonicotinoid exposure do not appear to compromise honey bees’ immunity toward pathogen infection. The mechanisms by which sub-lethal neonicotinoid
exposure caused honey bees to vanish from their hives
during the winter months needs to be elucidated.
Acknowledgements
This study was generously supported by Wells Fargo
Foundation and the Breck Fund established at the Harvard University Center for the Environment. The views
expressed here are not necessarily those of Wells Fargo
Foundation or the Breck Fund. We thank our friends,
K. Desjardin, F. Jacobs, D. Lewcon, and J. Rogers who
provided space to establish apiaries. We also thankM. Kapp and M. Chen for their assistance in the fieldstudy and the lab analysis.
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Authors’ addresses: Chensheng (Alex) LU (corresponding
author, cslu@hsph.harvard.edu), Department of Environmental Health, Harvard School of Public Health, 401 Park
Drive, Landmark Center West, Boston MA 02215, USA;
Kenneth M. WARCHOL (kenwarchol2@msn.com), Worcester
County Beekeepers Association, 372 Cooper Rd, Northbridge,
MA 01534, USA; Richard A. CALLAHAN (racinc@charter.net),Worcester County Beekeepers Association, 96 Twinbrooke Dr,
Holden, MA 01520, USA.
Received December 21, 2013. Accepted March 27, 2014.