western flower thrips ( wft ) , frankliniella occidentalis , is a highly polyphagous insect that causes both direct and indirect effects on plant development and health .
the adults and larvae feed on epidermal and subepidermal cells of both meristematic and mature leaf and flower tissues , inhibiting plant growth and development and causing necrotic or light - reflective blotches on the tissue .
furthermore , they indirectly damage plants by transmitting tospoviruses such as tomato spotted wilt virus ( reitz , 2009 ) . as a result
, wft has become the most serious pest in several vegetable and flower crops world - wide ( daughtrey et al . , 1997 ; reitz , 2009 ) .
the widespread use of chemical insecticides to control wft has led to increasing resistance against the major classes of synthetic insecticides ( broadbent and pree , 1997 ; flanders et al . , 2000 ; broughton and herron , 2009 ) . the growing awareness and demand for insecticides that are not environmentally hazardous has stimulated the study of plant - derived compounds for pest control ( boeke et al . , 2004 ) .
such compounds could be used as natural pesticides , and in theory , genes responsible for the biosynthesis of those compounds could be isolated and transferred to crops to improve plant defense against wft ( annadana et al . , 2002 ;
among the sources of botanical pesticides , pyrethrins from pyrethrum plants ( tanacetum cinerariifolium ) represent the economically most important class of compounds with broad usage both in homes and organic agriculture ( casida , 1973 ) .
pyrethrins are neurotoxins that bind to voltage - gated sodium channels of neuronal cells , causing the channels to remain open ( davies et al . , 2007 ) .
pyrethrins comprise a group of six closely related esters , named pyrethrin i and ii , cinerin i and ii , and jasmolin i and ii . they are found in all aboveground parts of the pyrethrum plants , but predominantly in the ovaries of the flower heads ( brewer , 1973 ) . on average , the concentration of pyrethrins is about 0.1  % ( dry weight ) in leaves and 1 - 2  % ( dry weight ) in flowers ( baldwin et al . , 1993 ) . assuming a water content of 90  % , pyrethrins account for around 0.01  % of the fresh weight of leaves and 0.1 - 0.2  % of the fresh weight of flowers .
pyrethrins are effective against a broad spectrum of insects , while their toxicity for mammals is very low , allowing their use as a preharvest spray ( casida and quistad , 1995 ; schoenig , 1995 ) .
western flower thrips are sensitive to synthetic pyrethroids ( thalavaisundaram et al . , 2008 ) , but there is no report on the effect of natural pyrethrins against wft .
pyrethrins might provide pyrethrum with a broad range protection against many different insect pests , but the role of pyrethrins in pyrethrum defense has not been studied . in initial experiments , we observed that wft adults died within one day when fed pyrethrum leaves , but that they were abundant in open flowers . here , we tested the hypothesis that pyrethrins are responsible for protecting pyrethrum leaves against wft by assessing adult and embryo toxicity , and by examining feeding and oviposition deterrence both in vitro and in planta .
a pyrethrum field close to luxi , yunnan province , china , was used for surveying thrips populations ( 2427'10.34"n-10332'21.01"e ) .
the field was 0.5  ha in size , and the presence of insect species was monitored during the flowering period of spring 2010 , when the flowers were predominantly in developmental stages 25 [ numbered according to casida ( 1973 ) ] . to assess populations of small resident insects including thrips , flowers at each developmental stage
each flower was taken by the stem and turned upside down into a jar containing 75  % alcohol .
the procedure was repeated until each jar contained the insects from 100 flowers from a single block and at a particular stage .
the number of insects of each species for each stage was scored . in the case of thrips ,
the number of adults and larvae were scored separately . among all collected thrips , 30 were randomly picked and identified , where possible to the species level .
a population of wft was mass - reared on flowering chrysanthemum ( chrysanthemum morifolium ramat . )
sunny casa in a greenhouse under a photoperiod of l16:d8 at 25    2c . in this study ,
the chrysanthemum plants used for bioassays were from the same cultivar , but were grown in an insect - free compartment of the greenhouse under the same light and temperature conditions .
all bioassays were conducted in a climate room at 2022c with a l16:d8 photo regime .
pyrethrum oil ( 70  % w / w ) had been extracted from dried and ground pyrethrum flower heads with liquid co2 leaving no solvent residue ( honghe senju biological co. ltd . , yunnan , china ) .
butylated hydroxytoluene ( bht ) had been added to the oil ( 1  % ) to prevent oxidation .
we confirmed the concentration and composition of the oil by gas chromatography  mass spectrometry comparison to a pyrethrin standard ( nguyen et al . , 1998 ) . since the major insecticidal compounds in pyrethrum
have long been known as pyrethrins ( casida , 1973 ) , the effect of pyrethrum oil was considered to be the effect of pyrethrins . when calculating the concentrations of pyrethrins in different solutions ,
the percentage of pyrethrins in the oil ( 70  % ) was taken into account .
for example , 1  % ( w / v ) pyrethrins was prepared by dissolving 14.3  mg pyrethrum oil in 1  ml solvent . the toxicity of pyrethrins was evaluated by topical application to thrips ( robb et al . , 1995 ) .
pyrethrum oil was dissolved in acetone to achieve a concentration range of 1 to 30  mg pyrethrins per ml , and the solutions were applied to the thorax with a 10-l glass syringe at 1  l per thrips .
the droplet briefly covered the thorax of the insect and also the paper support before evaporating in a few seconds , leaving a residue both on the insect and the support .
after treatment , all thrips were transferred to petri dishes containing a piece of chrysanthemum leaf embedded in an agar substrate .
mortality was assayed after 24  h by counting the number of insects that did not respond to prodding with a fine brush .
six replicates were used for each concentration , and 10 thrips were used per replicate .
percent mortality was corrected for mortality observed in acetone control using schneider - orelli s formula ( schneider - orelli , 1947 ) .
a dual - choice leaf disk assay was used to determine the deterrent effect of pyrethrins on wft .
all leaf disks ( diam 1.6  cm ) were punched from chrysanthemum leaves of similar leaf age .
pyrethrum oil was dissolved in 0.2  % ( v / v ) aqueous tween-80 to achieve 3 concentrations of pyrethrins : 0.01 , 0.1 , and 1  % ( w / v ) .
control leaf disks were sprayed with solvent solution ( 0.2  % tween-80 ) , and test leaf disks were sprayed with the pyrethrin solutions using a potter precision laboratory spray tower , which produced a uniform deposit ( 3  l / cm ) of solution on the leaf disks .
groups of 10 wft were positioned between a control and a test leaf disk placed abaxial side up and 2  cm apart on a 1.5  % ( w / v ) agar - bed in a petri dish ( 7  cm diam ) .
after positioning the thrips , the petri dish was covered by a 120  m mesh size nylon mesh lid to prevent condensation .
the number of wft on each leaf disk was recorded 0.25 , 1 , 2 , 4 , 20 , and 28  h after the release of the wft .
each concentration was replicated with 12 leaf disks . at each time point , a student 's paired t - test was used to assess the significance of the differences in the mean number of wft between test and control .
oviposition - deterrent effects were assayed with a non - choice method slightly modified from annadana et al .
the assay was conducted in perspex ring cages ( 3  cm in length and 3.5  cm diam ) , which were closed with a nylon mesh at the bottom .
pollen of scotch pine ( pinus sylvestris l. ) was supplied in a small open tube as food source for wft . after placing 10 wft in a cage ,
the top was sealed with two layers of stretched parafilm , with 300  l aqueous solution in between the layers .
the solutions used were water , 0.2  % tween-80 , or pyrethrins at 0.01 , 0.1 , or 1  % dissolved in 0.2  % tween-80 .
wft were allowed to adapt to the diet ( pollen and water ) for 3  d , and then every day for 5  d fresh test solution was provided .
all eggs were deposited in the solutions , and were counted daily under a binocular microscope .
data were analyzed by a one - way anova and a mean separation test was conducted using lsd (   =  0.05 ) . around 200 wft were kept in a perspex ring cage ( 7  cm in length and 9  cm diam ) to allow oviposition in a water solution as described above .
eggs laid on the same day were collected with a fine brush under a binocular microscope and then transferred to 2 layers of filter paper in petri dishes ( 3.5  cm diam ) .
the filter papers were drenched in 300  l of assay solution ( water , 0.2  % tween-80 or pyrethrins at 0.01 , 0.1 , or 1  % in 0.2  % tween-80 ) so that each paper was fully wetted but had no excess solution . after transferring the eggs ,
the developmental status of eggs was monitored every day for 6  d. to facilitate the observations , the bottoms of the petri dishes were marked with lines that could be seen through the filter paper from the top , and the eggs were placed on filter paper along these lines .
this facilitated finding the eggs under the microscope , and the viability of hatched larvae was assessed in terms of their ability to move away ( > 0.5  cm ) from the hatch position .
data were analyzed by a one - way anova and mean separation test was conducted using lsd (   =  0.05 ) .
mature pyrethrum leaves were harvested in november from a field in the netherlands when they were still flowering ( 5159'22.08"n-539'44.75"e , wageningen ) .
two or three pieces of leaves were placed , abaxial side up , on 1  % ( w / v ) agar in a petri dish ( 7  cm diam ) . after transferring 10 wft to each petri dish ,
petri dishes with two leaf disks ( 1.6  cm diam ) of chrysanthemum leaves , with a total mass similar to the mass of the pyrethrum leaf samples , or with only agar were used as controls .
the mortality of wft was recorded daily for 3  d. to test the in planta activity of pyrethrins against wft , pyrethrins were infiltrated into whole chrysanthemum leaves as described by ratcliff et al .
leaf disks ( diam 1.6  cm ) were punched from the infiltrated leaves , avoiding the infiltration points so that wft would not contact pyrethrins directly except at the edge of the disk . in the initial experiments , we infiltrated only water into the leaves and determined that on average 29.1  mg (  2.1  mg ) water could be infiltrated into each leaf disk ( 6 replicates ) .
as the fresh weight of each leaf disk was on average 45.3  mg (  1.2  mg ) , we infiltrated 0.025 , 0.25 , or 2.5  % pyrethrins solution to bring the concentrations to 0.01 , 0.1 , or 1  % pyrethrins .
the assay and data analysis were conducted as described above for the choice assays with topically applied pyrethrins .
the number of wft on each leaf disk was recorded 0.25 , 1 , 2 , 4 , 20 , and 28  h after the release of the wft . to test the effects of pyrethrins on oviposition and hatching of larvae
, wft were assayed with chrysanthemum leaf disks as described by de kogel et al .
leaf disks were punched from untreated leaves , from leaves infiltrated with 0.2  % tween-80 , or from leaves containing 0.01 , 0.1 , or 1  % pyrethrins in tween solution .
wft were placed on leaf disks ( 1.2  cm diam , 2 wft / disk ) , which were embedded , abaxial side up , on agar in wells of 24-well greiner plates .
plates were covered with parafilm , and every well was carefully sealed by pressing the parafilm on the edge of each well .
wft were allowed to oviposit for 48  h and were then removed , with simultaneous assessment of mortality . subsequently , half of the leaf disks from each plate were used to determine the number of eggs , and the other half of the leaf disks were used to determine the number of hatched larvae . to determine the number of eggs
, the leaf disks were boiled in water for 3  min so that the eggs were clearly visible under a binocular microscope with transmitting light . to determine the number of hatched larvae , the leaf disks were transferred to petri dishes containing water and incubated in a climate chamber ( 25c , l16:d8 ) for 5  d to allow the larvae to hatch .
data were analyzed by a one - way anova and mean separation test was conducted using lsd (   =  0.05 ) .
our field survey in china showed that several thrips species were the most abundant ( 98  % ) insects on pyrethrum flowers ( table  1 ) .
in addition , a few nysius species ( heteroptera : lygaeidae ) ( 1.9  % ) and lacewing larvae ( neuroptera ) ( 0.05  % ) were found .
a total of 30 individuals were identified to species level ; the thrips species found were mainly thrips tabaci ( 44  % ) , frankliniella occidentalis ( western flower thrips , or wft , 25  % ) , and thrips flavus ( 22  % ) .
the number of thrips in flowers was dependent on the flower s developmental stage ( fig .  1 ) .
the number of thrips increased until stage 3 ( the first row of disk florets are open ) , and then decreased in later stages .
larvae accounted for 7 - 26  % of the total number of thrips per flower , depending on the flower developmental stage ( fig . 
1).table 1frequencies of small insect species living on pyrethrum flowers in the fieldinsectsfrequency ( % ) speciesfrequency ( % ) thripidae ( thrips)98thrips tabaci43frankliniella occidentalis25thrips flavus21thrips palmi3other species6nysius sp.1.9n.d.1.9chrysoperla / chrysopa sp .
( lacewing larva)0.05n.d.0.05a total of 1200 insects were collected to count the frequencies of different insects .  
1distribution of thrips adults and larvae across different developemental stages of pyrethrum flowers in the field .
error bars indicate se ( n  =  300 per stage ) . stage 2 , vertical ray florets ; stage 3 , horizontal ray florets and first row of disk florets open ; stage 4 , 3 rows of disk florets open ; stage 5 , all disk florets open frequencies of small insect species living on pyrethrum flowers in the field a total of 1200 insects were collected to count the frequencies of different insects .  
, not determined distribution of thrips adults and larvae across different developemental stages of pyrethrum flowers in the field .
stage 2 , vertical ray florets ; stage 3 , horizontal ray florets and first row of disk florets open ; stage 4 , 3 rows of disk florets open ; stage 5 , all disk florets open we assayed the suitability of pyrethrum leaves as a food substrate for wft .
mortality could be as high as 80  % within 3  days , although the degree of mortality depended on the plant source ( data not shown ) .
when only water and agar were provided , with no plant - based food , only 20 - 30  % wft died in 3  days .
this showed that the mortality of wft was caused by a toxic principle of pyrethrum leaves rather than deterrence or starvation .
the toxic principle of pyrethrum plants against insects has long been known to be a group of 6 pyrethrin esters ( casida , 1973 ) .
we were , therefore , interested in specifically testing the effect of pyrethrins against wft . to determine the effects of pyrethrins against wft
, pyrethrins were tested in vitro at different concentrations on adult mortality , feeding , oviposition , and embryo development .
the mortality of wft female adults increased with the concentration of topically applied pyrethrins in the range of 1 to 30  mg / ml . probit analysis showed that the lc50 and lc90 of pyrethrins was 12.9  mg / ml ( with 95  % confidence limit of 10.9 - 14.8  mg / ml ) and 39.0  mg / ml ( with 95  % confidence limit from 30.7 to 50.4  mg / ml ) , respectively .
thrips were significantly deterred from feeding by 0.1  % and 1  % pyrethrins ( fig .  2 ) .
when given a choice between chrysanthemum leaf disks coated with 0.2  % tween ( control ) or 0.1  % added pyrethrins , after 2  h significantly more ( 61 - 77  % of thrips ) settled on control leaf disks .
within 1  h , 72 - 90  % of thrips chose control leaf disks .
for both concentrations of pyrethrins , the maximum deterrent effect was reached at 4  h. application of 0.01  % pyrethrins on leaf disks did not show significant deterrent effects except at the 4  h time point ( fig .  2).fig .
2dual choice assays of western flower thrips on chrysanthemum leaf disks sprayed with 0.2  % tween ( control ) or 0.2  % tween with 0.01  % , 0.1  % or 1  % pyrethrins .
the presence on either leaf disk was visually recorded 0.25 , 1 , 2 , 4 , 20 and 28  h after wft release .
asterisks indicate significant differences to the control ( * : p  <  0.05 ; * * : p  <  0.01 ) .
error bars indicate se ( n  =  120 per treatment ) dual choice assays of western flower thrips on chrysanthemum leaf disks sprayed with 0.2  % tween ( control ) or 0.2  % tween with 0.01  % , 0.1  % or 1  % pyrethrins .
the presence on either leaf disk was visually recorded 0.25 , 1 , 2 , 4 , 20 and 28  h after wft release .
asterisks indicate significant differences to the control ( * : p  <  0.05 ; * * : p  <  0.01 ) .
error bars indicate se ( n  =  120 per treatment ) pyrethrins negatively affected oviposition by wft ( fig .  3 ) .
the carrier , 0.2  % tween-80 , did not affect the oviposition of thrips compared to water throughout the experiment , but wft oviposited significantly fewer eggs with increasing pyrethrin concentrations during the 5-day experiment ( fig . 
3the number of eggs deposited by western flower thrips when supplied with different concentrations of pyrethrins starting on day 1 .
data points with the same letter within days are not significantly different , p  <  0.05 .
error bars indicate se ( n  =  60 per treatment ) the number of eggs deposited by western flower thrips when supplied with different concentrations of pyrethrins starting on day 1 .
data points with the same letter within days are not significantly different , p  <  0.05 .
error bars indicate se ( n  =  60 per treatment ) the development of eggs was inhibited by 0.1  % and 1  % pyrethrins .
about 80  % of larvae hatched when the eggs were incubated with water , 0.2  % tween , or 0.01  % pyrethrins , while only 28  % or 6  % of the larvae hatched when the eggs were incubated with 0.1  % or 1  % pyrethrins , respectively ( fig . 
the embryos that did not develop into larvae had severely stunted and abnormal shapes ( fig . 
4percentage of larvae hatching from western flower thrips eggs during incubation with different concentrations of pyrethrins .
data points with the same letter within days are not significantly different , p  <  0.05 .
( a ) , larva hatched in solutions of water , 0.2  % tween or 0.01  % pyrethrins at day 5 ; ( b ) , abnormally developed embryos in solutions of 0.1  % and 1  % pyrethrins at day 5 ; ( c ) and ( d ) , embryos before treatment percentage of larvae hatching from western flower thrips eggs during incubation with different concentrations of pyrethrins .
data points with the same letter within days are not significantly different , p  <  0.05 .
error bars indicate se ( n  =  40 per treatment ) effects of pyrethrins on embryo development of western flower thrips at day 5 .
( a ) , larva hatched in solutions of water , 0.2  % tween or 0.01  % pyrethrins at day 5 ; ( b ) , abnormally developed embryos in solutions of 0.1  % and 1  % pyrethrins at day 5 ; ( c ) and ( d ) , embryos before treatment to study in planta activity of pyrethrins against wft , thrips were assayed with chrysanthemum leaves that had been infiltrated with pyrethrins to contain 0.01 , 0.1 , or 1  % pyrethrins . in this experiment ,
the pyrethrins could not be contacted directly by thrips except by feeding , and the source of nutrition consisted of leaves instead of pollen . in the reproduction assay ,
thrips fed with chrysanthemum leaf disks containing pyrethrins exhibited higher mortality and lower reproduction rates compared to those fed with untreated leaf disks or leaf disks containing 0.2  % tween ( table  2).table 2mortality , number of eggs and hatched western flower thrips larvae per leaf disk on chrysanthemum leaf disks infiltrated with different concentrations of pyrethrinstreatment of leaf disksmortality ( % ) eggshatched larvaeuntreated leaf disks0 a2.0    0.4 a1.4    0.3 aleaf disks containing 0.2  % tween0 a1.7    0.5 a1.3    0.3 ableaf disks containing 0.01  % pyrethrins25.0    6.7 b1.3    0.3 ab0.8    0.2 bleaf disks containing 0.1  % pyrethrins29.2    7.9 b0.7    0.2 bc0.1    0.1 cleaf disks containing 1  % pyrethrins68.8    9.3 c0 c0 cvalues ( mean    se , n  =  48 per treatment ) followed by the same letter within a column are not significantly different ( anova : p  >  0.05 ) mortality , number of eggs and hatched western flower thrips larvae per leaf disk on chrysanthemum leaf disks infiltrated with different concentrations of pyrethrins values ( mean    se , n  =  48 per treatment ) followed by the same letter within a column are not significantly different ( anova : p  >  0.05 ) in the dual - choice assay , chrysanthemum leaves containing 0.1  % and 1  % pyrethrins showed significant deterrent effects on thrips within 15  min of release ( fig . 
a total of 74 - 93  % of the thrips settled on the control leaf disk when the other leaf disk contained 0.1  % pyrethrins , and 85 - 95  % thrips settled on the control leaf disk when the other leaf disk contained 1  % pyrethrins .
6percentage of western flower thrips settled on the control chrysanthemum leaf disk in dual choice assays of leaf disks containing 0.2  % tween with or without 0.01  % , 0.1  % , or 1  % pyrethrins .
the choices were recorded 0.25 , 1 , 2 , 4 , 20 , and 28  h after wft release .
asterisks indicate significant differences to the control ( * : p  <  0.05 ; * * : p  <  0.01 ) .
error bars indicate se ( n  =  120 per treatment ) percentage of western flower thrips settled on the control chrysanthemum leaf disk in dual choice assays of leaf disks containing 0.2  % tween with or without 0.01  % , 0.1  % , or 1  % pyrethrins .
the choices were recorded 0.25 , 1 , 2 , 4 , 20 , and 28  h after wft release .
asterisks indicate significant differences to the control ( * : p  <  0.05 ; * * : p  <  0.01 ) .
pyrethrins , well - known natural insecticidal compounds , are found exclusively in and extracted from the composite flowers of pyrethrum ( tanacetum cinerariifolium ) , which belongs to anthemideae tribe within the astaraceae family ( casida and quistad , 1995 ) . remarkably , the potential role of pyrethrins in pyrethrum plant defense has not been studied . here
, we report that western flower thrips ( wft ) adults thrive on pyrethrum flowers , but die within a few days on pyrethrum leaves . the hypothesis that pyrethrins are responsible for protecting pyrethrum leaves against wft was tested by spraying or infiltrating pyrethrins to leaves of chrysanthemum , a related pyrethrins - free species belonging to the same tribe .
we assessed toxicity to the adult and embryo stages of wft , and negative effects on feeding and oviposition both in vitro and in planta , and found that the natural concentrations of pyrethrins present in leaves have strong negative effects on wft .
we speculate that the thrips found on pyrethrum flowers survive on pollen that is devoid of pyrethrins ( t. yang , unpublished data ) . for many populations of wft
, resistance has been reported for some synthetic insecticides ( espinosa et al . , 2005 ) .
previously , several other plant - derived compounds were tested for their insecticidal effects against wft adults .
for example , carvacrol at 1  % and thymol at 0.1  % and 1  % significantly reduced the oviposition rate of wft when these compounds were sprayed on leaf disks , but neither compound affected the feeding activity of wft ( sedy and kosehier , 2003 ) .
salicylaldehyde ( 0.1  % and 1  % ) and methyl salicylate ( 0.1  % and 1  % ) were also tested . within 24  h of applying 1  % methyl salicylate to bean or cucumber , the feeding and oviposition activities of thrips females
the insect could be a result of changes in the plant induced by methyl salicylate , since it is a plant hormone involved in induced resistance ( pieterse et al . , 2009 ) .
a series of commercially available plant - derived essential oils tested at recommended concentrations ( 0.02 - 0.5  % ) , including neem oil , rosemary oil , peppermint oil , garlic oil , and cottonseed oil , caused less than 30  % mortality within 7  days ( cloyd et al . , 2009 ) .
our results showed that 0.1  % and 1  % pyrethrin solutions sprayed on leaf disks significantly deterred wft at 4  h , and topically applied pyrethrins were toxic to adults at an lc50 value of 12.9  mg / ml ( 1.3  % ) . by mimicking the natural site of pyrethrin accumulation by infiltration of leaves
, we found that 1  % pyrethrins caused 69  % mortality and completely inhibited oviposition .
furthermore , 0.1  % pyrethrins was strongly deterrent and resulted in abortion of 95  % of the embryos , while as little as 0.01  % pyrethrins caused 25  % mortality in 2  days .
we propose , therefore , that the natural concentrations of pyrethrin in pyrethrum leaves , around 0.01  % by fresh weight , accounts for the observed high mortality of thrips adults on this plant .
wft eggs are embedded in plant tissues ( childers , 1997 ) , and as a result they are unlikely to be affected by non - systemic chemicals that are applied on the surface of plants .
however , pyrethrins naturally accumulate inside pyrethrum tissues , stored in what appear to be unstructured intercellular cavities ( m.a .
jongsma , unpublished observations ) . therefore , besides feeding and oviposition deterrence , the embryo - toxic effect of pyrethrins is a third component that contributes to their effect for plant defense against wft ( figs .  4 and 5 ) .
compared to some synthetic insecticides , the toxicity of natural pyrethrins against wft in the absence of synergists was not high . in previous studies using topical application methods , the lc50 values of insecticides tested against susceptible wft strains ranged from 10 to 83  g / ml for pyrethroids , 20 to 960  g / ml for carbamates , and 49 to 522  g / ml for organophosphates ( espinosa et al . , 2005
the lc50 value of pyrethrins against wft by topical application was determined as 12.9  mg / ml , and the action of pyrethrins was , therefore , 10 to 1000-fold weaker than for these synthetic pesticides . on the other hand
, pyrethrins did show much stronger negative effects on feeding behavior and reproduction , which may be explained by the action of pyrethrins on the nervous system , resulting in disordered function of excitable ( nerve and muscle ) cells ( bradberry et al . ,
pyrethrins not only caused mortality of adults and embryos , but also significantly reduced oviposition ( table  2 ) .
all these factors together cumulatively affect the life history parameters . as a result wft damage on pyrethrin - containing leaves
we hypothesize that if plants such as closely related chrysanthemum species , which do not contain any pyrethrins , were genetically engineered to produce pyrethrins , their resistance to wft in leaves could be significantly improved .
 pyrethrins are active ingredients extracted from pyrethrum flowers ( tanacetum cinerariifolium ) , and are the most widely used botanical insecticide . however , several thrips species are commonly found on pyrethrum flowers in the field , and are the dominant insects found inside the flowers . up to 80  % of western flower thrips ( wft , frankliniella occidentalis ) adults died within 3  days of initiating feeding on leaves of pyrethrum , leading us to evaluate the role of pyrethrins in the defense of pyrethrum leaves against wft . 
 the effects of pyrethrins on wft survival , feeding behavior , and reproduction were measured both in vitro and in planta ( infiltrated leaves ) . 
 the lethal concentration value ( lc50 ) for pyrethrins against wft adults was 12.9  mg / ml , and pyrethrins at 0.1  % ( w / v ) and 1  % ( w / v ) had significantly negative effects on feeding , embryo development , and oviposition . about 20 - 70  % of wft were killed within 2  days when they were fed chrysanthemum leaves containing 0.01 - 1  % pyrethrins . 
 chrysanthemum leaves containing 0.1  % or 1  % pyrethrins were significantly deterrent to wft . in a no - choice assay , 
 the reproduction of wft was reduced significantly when the insects were fed leaves containing 0.1  % pyrethrins , and no eggs were found in leaves containing 1  % pyrethrins . 
 our results suggest that the natural concentrations of pyrethrins in the leaves may be responsible for the observed high mortality of wft on pyrethrum . 
Introduction
Methods and Materials
Results
Discussion