The presence of Rickettsia is associated with increased susceptibility of Bemisia tabaci (Homoptera: Aleyrodidae) to insecticides.
ABSTRACT The presence of certain symbiotic microorganisms may be associated with insecticide resistance in insects. The authors compared the susceptibility of two isofemale lines, Rickettsia-plus and Rickettsia-free, of the sweet potato whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) to major insecticides from different chemical groups, including imidacloprid, acetamiprid, thiamethoxam, pyriproxyfen, spiromesifen and diafenthiuron.
While the Rickettsia-plus and Rickettsia-free lines showed no differences in their susceptibility to imidacloprid and diafenthiuron, higher susceptibility of the Rickettsia-plus line to acetamiprid, thiamethoxam, spiromesifen and especially pyriproxyfen was observed. LC(90) values indicated that the Rickettsia-free line was 15-fold more resistant to pyriproxyfen than the Rickettsia-plus line.
Findings indicate that the infection status of B. tabaci populations by Rickettsia is an important consideration that should be taken into account when performing resistance monitoring studies, and may help in understanding the dynamics of B. tabaci resistance, symbiont-pest associations in agricultural systems and the biological impact of Rickettsia on whitefly biology.
- SourceAvailable from: Mylène Weill[show abstract] [hide abstract]
ABSTRACT: In the mosquito Culex pipiens, insecticide resistance genes alter many life-history traits and incur a fitness cost. Resistance to organophosphate insecticides involves two loci, with each locus coding for a different mechanism of resistance (degradation vs. insensitivity to insecticides). The density of intracellular Wolbachia bacteria has been found to be higher in resistant mosquitoes, regardless of the mechanism involved. To discriminate between costs of resistance due to resistance genes from those associated with elevated Wolbachia densities, we compared strains of mosquito sharing the same genetic background but differing in their resistance alleles and Wolbachia infection status. Life-history traits measured included strength of insecticide resistance, larval mortality, adult female size, fecundity, predation avoidance, mating competition, and strength of cytoplasmic incompatibility (CI). We found that: (1) when Wolbachia are removed, insecticide resistance genes still affect some life-history traits; (2) Wolbachia are capable of modifying the cost of resistance; (3) the cost of Wolbachia infections increases with their density; (4) different interactions occurred depending on the resistance alleles involved; and (5) high densities of Wolbachia do not increase the strength of CI or maternal transmission efficiency relative to low Wolbachia densities. Insecticide resistance genes generated variation in the costs of Wolbachia infections and provided an interesting opportunity to study how these costs evolve, a process generally operating when Wolbachia colonizes a new host.Evolution 03/2006; 60(2):303-14. · 4.86 Impact Factor
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ABSTRACT: The sweet potato whitefly, Bemisia tabaci, harbors Portiera aleyrodidarum, an obligatory symbiotic bacterium, as well as several secondary symbionts including Rickettsia, Hamiltonella, Wolbachia, Arsenophonus, Cardinium and Fritschea, the function of which is unknown. Bemisia tabaci is a species complex composed of numerous biotypes, which may differ from each other both genetically and biologically. Only the B and Q biotypes have been reported from Israel. Secondary symbiont infection frequencies of Israeli laboratory and field populations of B. tabaci from various host plants were determined by PCR, in order to test for correlation between bacterial composition to biotype and host plant. Hamiltonella was detected only in populations of the B biotype, while Wolbachia and Arsenophonus were found only in the Q biotype (33% and 87% infection, respectively). Rickettsia was abundant in both biotypes. Cardinium and Fritschea were not found in any of the populations. No differences in secondary symbionts were found among host plants within the B biotype; but within the Q biotype, all whiteflies collected from sage harboured both Rickettsia and Arsenophonus, an infection frequency which was significantly higher than those found in association with all other host plants. The association found between whitefly biotypes and secondary symbionts suggests a possible contribution of these bacteria to host characteristics such as insecticide resistance, host range, virus transmission and speciation.Bulletin of Entomological Research 09/2007; 97(4):407-13. · 1.99 Impact Factor
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ABSTRACT: Wolbachia are obligate intracellular bacteria present in reproductive tissues of many arthropod species. It has been reported that few silverleafing populations of Bemisia tabaci were positive for Wolbachia, whereas non-silverleafing populations were more likely infected with Wolbachia and all that infect B. tabaci are Wolbachia belonging to supergroup B. However, current detection methods were shown to be not sensitive enough to uncover all infections. Herein, a protocol based on polymerase chain reaction-restriction fragment length polymorphism analysis of Wolbachia 16S ribosomal DNA is presented. A systematic survey for the prevalence of Wolbachia infection in natural populations of B. tabaci using this method revealed that (1) all populations of B. tabaci tested positive for Wolbachia and the overall infection rate reached 80.5% (293 positives in 364 tests); (2) both single infection and superinfection existed within individual whiteflies tested; and (3) silverleafing populations of B. tabaci most likely harbored A Wolbachia as single infection, whereas non-silverleafing populations tend to carry B Wolbachia as superinfection. It is clear that the Wolbachia infection pattern is closely related to the genetic races of B. tabaci, and the infection frequencies are apparently much higher than those described previously. This study shows that detection methods can significantly influence estimation of Wolbachia infection. It is supposed that Wolbachia may be acting as a biotic agent promoting rapid differentiation and speciation of B. tabaci. This is the most systematic survey of Wolbachia infection within B. tabaci.Current Microbiology 07/2007; 54(6):467-71. · 1.52 Impact Factor
Pest Management SciencePest Manag Sci (2008)
The presence of Rickettsia is associated
with increased susceptibility of Bemisia tabaci
(Homoptera: Aleyrodidae) to insecticides
Svetlana Kontsedalov,1Einat Zchori-Fein,2Elad Chiel,2,3Yuval Gottlieb,1
Moshe Inbar3and Murad Ghanim1∗
1Department of Entomology, Institute of Plant Protection, the Volcani Centre, Bet Dagan 50250, Israel
2Department of Entomology, Institute of Plant Protection, Newe-Ya’ar Research Centre, Ramat-Yishai 30095, Israel
3Department of Evolutionary and Environmental Biology, University of Haifa, Haifa 31905, Israel
BACKGROUND: The presence of certain symbiotic microorganisms may be associated with insecticide resistance
in insects. The authors compared the susceptibility of two isofemale lines, Rickettsia-plus and Rickettsia-free,
of the sweet potato whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) to major insecticides from
different chemical groups, including imidacloprid, acetamiprid, thiamethoxam, pyriproxyfen, spiromesifen and
RESULTS: While the Rickettsia-plus and Rickettsia-free lines showed no differences in their susceptibility to
imidacloprid and diafenthiuron, higher susceptibility of the Rickettsia-plus line to acetamiprid, thiamethoxam,
spiromesifen and especially pyriproxyfen was observed. LC90 values indicated that the Rickettsia-free line was
15-fold more resistant to pyriproxyfen than the Rickettsia-plus line.
CONCLUSION: Findings indicate that the infection status of B. tabaci populations by Rickettsia is an important
consideration that should be taken into account when performing resistance monitoring studies, and may help in
understanding the dynamics of B. tabaci resistance, symbiont-pest associations in agricultural systems and the
biological impact of Rickettsia on whitefly biology.
2008 Society of Chemical Industry
Keywords: insecticide resistance; symbiont; whitefly; Bemisia tabaci; Rickettsia
Symbiotic relationships with bacteria are quite com-
mon within the Arthropoda, where the interactions are
known to have a substantial influence on the biology
of both partners.1Such associations can be obligatory
or facultative for the host, and, to date, they have
been reported to be involved in nutrition, host plant
utilization, reproductive manipulation and ability to
cope with environmental factors.1However, the role
of symbiotic bacteria in pesticide resistance is largely
The sweet potato whitefly Bemisia tabaci (Genna-
dius) (Homoptera: Aleyrodidae) is a cosmopolitan,
polyphagous phloem-feeder that inflicts damage in
many crops owing to direct feeding and the vectoring
of plant viruses.2,3Bemisia tabaci is a complex of bio-
types that vary greatly with respect to characteristics
such as host range, fecundity, insecticide resistance,
ability to transmit plant viruses and induction of plant
Allwhiteflyspecies are known
obligatory bacterium Portiera aleyrodidarum Thao &
Baumann, which supplements their unbalanced sap
diet. In addition, different B. tabaci populations may
harbour a diverse array of bacterial tenants, including
Hamiltonella, Arsenophonus, Cardinium, Wolbachia,
Rickettsia and Fritschea.6–9Although these bacteria
are known from other arthropods, virtually nothing
is known about their effects on the whitefly host.
Interestingly, acorrelation hasbeen foundbetweenthe
insect biotype and the bacteria it carries: all B-biotype
B. tabaci host Hamiltonella, but they have not been
found to carry either Wolbachia or Arsenophonus.10In
contrast, the Q biotype has a frequent association with
Arsenophonus and Wolbachia, and in Israel was never
associated with Hamiltonella. Although the presence
of Rickettsia may vary among populations, sites and
∗Correspondence to: Murad Ghanim, Department of Entomology, Institute of Plant Protection, the Volcani Centre, Bet Dagan 50250, Israel
(Received 4 December 2007; revised version received 22 January 2008; accepted 27 January 2008)
2008 Society of Chemical Industry. Pest Manag Sci 1526–498X/2008/$30.00
S Kontsedalov et al.
crops, it is the only symbiont that is commonly
detected in both biotypes. Therefore, this bacterium
was used to test the hypothesis that its presence in
B. tabaci influencesthepest’ssusceptibility todifferent
Two strains of B. tabaci were established by setting
isofemale Rickettsia-plus and Rickettsia-free lines
derived from the same population. This was done
by allowing 30 females to lay eggs in individual
leaf cages. After egg lay, the females were tested for
the presence or absence of Rickettsia using a genus-
specific amplification of an rDNA PCR fragment as
previously described.7,10Adults that developed from
eggs laid by females that tested positive for Rickettsia
were pooled to establish the Rickettsia-plus population.
Adults that developed from eggs laid by females that
tested negative for Rickettsia were pooled, and this
population was called Rickettsia-free. The B-biotype
population used to establish these isofemale lines was
obtained from the laboratory culture of Professor Dan
Gerling at Tel Aviv University and carries Portiera,
Hamiltonella and Rickettsia. All whiteflies were reared
on cotton seedlings (Gossypium hirsutum L. cv. Acala)
under standard laboratory conditions of 26 ± 2◦C and
a photoperiod of 14:10 h light:dark.
MATERIALS AND METHODS
Based on their mode of action and usage, six
insecticides were chosen (Table 1). The applied
concentration of each insecticide was based on LC50
values previously determined for the susceptible B-
biotype B. tabaci strain.11
Because the insecticides differ in their modes of
action, application and targeted insect stage, different
bioassays were employed.The effectof thiamethoxam,
acetamiprid, imidacloprid and diafenthiuron was
scored on adults. The assay for thiamethoxam
and acetamiprid was performed by dipping cotton
seedlings (20–25cm high, with two true leaves) for
20s in an aqueous dispersion of the test formulation
and then allowing the plant to air dry for 2h.
Adult whiteflies (15–20 per replicate) were then
confined on the treated seedlings using clip-on leaf
cages for 48h, after which their mortality was
scored. For diafenthiuron, cotton seedlings were
treated as described above, and leaf discs were
punched out and placed into petri dishes filled with
1.5% agar. Adult whiteflies (15–20 per replicate)
were exposed to the treated discs for 72h and
their mortality was scored. For imidacloprid, cotton
stems with two true leaves were inserted in plastic
vials containing the formulation dispersion for 24h,
and petri dishes with leaf discs were prepared as
described above. Adult mortality was determined
after 48h of whitefly exposure to the treated leaf
discs. Pyriproxyfen is transovarially active against
eggs, and thus egg mortality was scored. After foliar
application, adult whiteflies were exposed to treated
leaves as described above. They were removed after
a 48h egg-laying period, and the number of hatched
eggs was counted after 8days. Spiromesifen is an
ovolarvicidal compound, i.e. it is mostly active against
the larvae when eggs are treated. Cotton seedlings
infested with 0- to 2-day-old eggs were dipped in
the formulation dispersion, and the cumulative larval
mortality (expressed as suppression of pupation) was
determined 18days post-application. Deionized water
was performed with a minimum of five replicates.
Comparisons of mortality and egg hatch were
performed using Student’s t-test with unequal sample
sizes, and percentage data were transformed (angular
transformation) before analysis. Probit analyses of
the concentration-dependent mortality data were
performed using POLO-PC,12after correction with
Abbott’s formula13which takes into account the
Table 1. Description and dosage of the tested insecticides
commercial nameproducer active ingredient
(ppm)mode of action
Tiger 10EC (100gL−1
Syngenta, Switzerlandthiamethoxam4Targets nAchRs1
Nippon Soda Co.,
acetamiprid0.25Targets nAchRsAll stages
imidacloprid5Targets nAchRsAll stages
Inhibits lipid synthesis
Sumitomo Co., Japanpyriproxyfen0.04Eggs
spiromesifen3Eggs and larvae
1nicotinic acetylcholine receptors
Pest Manag Sci (2008)
Effect of Rickettsia on susceptibility of B. tabaci to insecticides
mortality in the control experiment run in parallel with
the treatment. Control experiments were performed
independently for each insecticide tested. Failure of
95% LC to overlap at a particular lethal concentration
indicated a significant difference.
A comparison using Student’s t-test with unequal
sample sizes between the two B. tabaci populations
revealed a clear trend of higher insecticide suscepti-
bility to five out of the six tested compounds in the
presence of Rickettsia (Fig. 1). Non-significant trends
in adult mortality of the Rickettsia-plus and Rickettsia-
free lines were found for imidacloprid (81 and 65%
respectively; t = 1.37, P = 0.2) and diafenthiuron (30
and 33% respectively; t = 0.04, P = 0.96). Mortal-
ity rates of the Rickettsia-plus strain were significantly
higher compared with the Rickettsia-free line when
treated with acetamiprid (95 versus 79% adult mortal-
ity respectively; t = 2.86, P = 0.021), thiamethoxam
(87 versus 63% adult mortality respectively; t = 2.45,
P = 0.025) and spiromesifen (68 versus 47% larval
mortality respectively; t = 1.9, P = 0.046) (Fig. 1).
The most striking difference was observed when the
two B. tabaci strains were treated with pyriproxyfen:
while 43%eggmortality wasobservedinthe Rickettsia-
free strain, the Rickettsia-plus strain exhibited 90%
mortality (t = 9.33, P = 0.00024) (Fig. 1). A log-
response pyriproxyfen concentration curve was built
(on a probit scale) for the two strains. All four
concentrations tested resulted in a significantly higher
susceptibility of the Rickettsia-plus strain relative to the
Rickettsia-free one (Table 2 and Fig. 2).
Figure 2. Log concentration–response curves (probit scale) of the
effect of pyriproxyfen on Rickettsia-plus and Rickettsia-free strains.
Effect is on egg hatch in the two Bemisia tabaci strains. Bars
represent mean standard errors of mortality in each concentration
The assumption tested in this study was that the
presence of Rickettsia alters the response of B. tabaci
to pesticide application. It was found that, in the
presence of Rickettsia, the whitefly’s susceptibility to
five out of the six insecticides tested was increased, in
spite of their variable mode of action and target stages
are associated with insecticide resistance in insects.
For example, the cigarette beetle Lasioderna serricorne
(F.) (Coleoptera: Anobiidae) contains a symbiotic gut
yeast Symbiotaphrina kochii Jurzitza ex Gams & v. Arx,
Figure 1. Mortality rates caused by application of various insecticides (Table 1) of Bemisia tabaci B biotype from Rickettsia-plus and Rickettsia-free
lines. Asterisk indicates a statistically significant difference (see text).
Table 2. Susceptibility of Rickettsia-plus and Rickettsia-free strains to pyriproxyfen (LC Ratio = LC Rickettsia-free/LC Rickettsia-plus)
Rickettsia-plus7611.31 ± 0.14
Rickettsia-free871 1.18 ± 0.10
Pest Manag Sci (2008)
S Kontsedalov et al.
which is involved in the detoxification of natural
and synthetic poisons.14Similarly, the ability of the
apple maggot fly Rhagoletis pomonella Walsh (Diptera:
Tephritidae) to degrade and detoxify the phytotoxin
phloridzin depends on the presence of the bacterial
gut symbiont Enterobacter agglomerans (Beijerinck)
Ewing & Fife.15Furthermore, a positive correlation
has been found between the presence of insecticide
Culex pipiens L. (Diptera: Culicidae).16Although the
presence of Wolbachia does not directly affect the
insect’s susceptibility, the density of the symbiont
increases the cost of resistance.17These authors
hypothesized that, in the presence of resistant genes,
the mosquitoes suffer a physiological cost that, in turn,
reduces their ability to control Wolbachia densities.
The consequent increase in Wolbachia infection levels
has deleterious effects on the host, thus increasing the
cost of insecticide resistance.
The number of Rickettsia in B. tabaci nymphs
and adults varies greatly among individuals and
may reach very high densities.7This high bacterial
load may partially explain the significant fitness
The increased mortality exhibited by the population
carrying the Rickettsia in response to pesticide
applications may restrict the proliferation of infected
individuals and thereby contribute to the fact that
the presence of the symbiont is not fixed in field
populations.10Another possibility is that Rickettsia
possesses an as yet undiscovered fitness disadvantage
for its whitefly host. This disadvantage might weaken
the whitefly and make it vulnerable to environmental
stresses, including pesticide pressure.
In spite of the insecticide resistance burden, most
B. tabaci in the field carry Rickettsia.10Either frequent
horizontal transmission of the symbiont or a fitness
advantage that has yet to be discovered in this system
maybehelpfulinmaintainingthe Rickettsiain different
populations of the pest.
The two B. tabaci lines compared in this study
were established by setting isofemale lines with
individuals originating from one population that had
been kept in the laboratory for many years; thus,
both lines are B biotype that harbour Portiera and
Hamiltonella, and they differ only in the presence or
absence of Rickettsia. Crosses between the two lines
showed that they successfully mate and produce fertile
offspring (Chiel E, unpublished data). These facts,
together with the very straight line obtained on the
concentration–response curves to pyriproxyfen for the
two lines (Fig. 2), suggest that they differ only in the
presence or absenceof Rickettsia, andthat the presence
of Rickettsia is responsible for the differences observed.
More experiments on lines with and without
Rickettsia are required in order to understand the
mechanisms by which the symbiont affects the biology
of B. tabaci in general, and insecticide resistance in
This research was supported by Grant No. 2004416
from the US–Israel Binational Science Foundation
(BSF) to EZ. This is Contribution No. 507/07 from
the ARO, The Volcani Centre, Bet Dagan, Israel.
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Pest Manag Sci (2008)