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andFitness differences between phosphine-resistantsusceptible strains of Tribolium castaneum

Cao Yang", Wang Dianxuarr' and Pat. J. Collins2

Abstract

The hfe histones of four strains of Tribolium castaneumwere compared to test if there was a lmk between phosphineresistance and fitness. Two of the test strains weresusceptible to phosphine and one strain had a low-levelresistance while the fourth strain had relatively highresistance to this fumigant. Detailed populationdemographics showed that there were biologically Importantdifferences between the highly resistant msects comparedwith the reference susceptible strains. The most importantof these was the lower survival rate of the resistant strain.However, this strain also had a shorter mean genera nontime giving It an innate capacity for mcrease equal to thesusceptible msects. It IS concluded that, although therewere fitness differences between resistant and susceptibleinsects, they were not hkely to be useful for resistancemanagement.

Introduction

Currently and for the foreseeable future, both the Chineseand Australian grain industries will rely heavily on phosphinefumigant to control insects in storage. Expenence withmany other insecticides used by the grain mdustry suggeststhat the future use of phosphine may be jeopardised orlmuted by the development of resistance in target pestspecies. For tlus reason it is important that ways were foundto manage resistance. Among the tactics suggested ascomponents of a resistance management strategy, animportant and practical potential tactic is the rotation ofalternative treatments. In this tactic individual insects areexposed to only one compound but the populationexpenences more than one insecticide over time Thesuccess of such a tactic depends on the assumption that thefrequency of individuals resistant to one insecticide willdecline dunng application of the alternative. Such a decline

1 Zhengzhou Gram College, 140 , Songsan road, Zhengzhou, 450052,P R Chma

2 Queensland Department of Pnmary Industnes, 80 Meners Road,Indooroopilly Qld 4068, Australia

will occur when there is negative cross-resistance, a fitnessdifference associated with resistance, or Immigration ofsusceptible individuals (Tabashnik 1990).The aim of this study was to investigate the possibility

that differences in fitness between phosphine resistant andsusceptible strains occur and if so, their significance. Manyaspects of insect biology can be important in determiningoverall fitness. In the current study an age-specific life tablemethod was used to compare detailed demographics ofphosphine resistant and susceptible insects

Materials and Methods

Insect strains

Four strains of Tribolium castaneum were compared inthese experiments. They included two phosphine-susceptiblestrains labelled ZZTC and JSTC from China, a referencestrain from Australia with weak resistance to phosphine,ABTC, and a Chinese a strain with relatively high resistanceto phosphine. The resistance status of these strains wasestablished by using the recommended FAG bioassay methodfor phosphme resistance (FAG 1982). With this methodadult test insects are exposed for 20 h to a graded series ofconcentrations of phosphine. Mortality is counted 14 afterthe exposure period. The probit method described by Finney(1971) was used to analyse results of multi-dose assaysusing an in-house computer program (M. Bengston,unpublished). Resistance factors (RF) were obtained as theratio of the LCso of phosphine against the test strain and theSstram. Resistance to phosphine of the four experimentalstrains IS summarised in Table 1.

Life history studies

Survival and duration of development of immaturestagesAdult Tribolium. castaneum were randomly allocated to

Jars of whole-wheat meal for an oviposition period of 24h.Eggs were gently SIeved from each jar and 100 to 150 wereplaced individually into glass containers with a small amountof wheat flour and yeast. Each msect was examined dally atthe same time and in the same sequence until adult eclosionto determine the duration of the immature stages and torecord mortality. The proportion of insects that died withineach stage of development was calculated relative to the

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Proceedings of the 7th International Worktn,g Conference an Stored-product Protection - Volume 1

numbers surviving the previous stageAnalysis of duration of developmentAn analysis of variance was used to test for differences

between strains. Separate analyses were performed for eachdevelopmental stage and an overall analysis was performedfor total time of development.

Table 1. Phosphine resistance characteristics of the four experim ental strains of Tribolium castaneum.

Stram Resistance LCso(mglL) L~9 9(mglL) Regression equation

JSTC Susceptible 0.008 0.017 Y = 10 . 23 x + 26.29

ZZTC Susceptible 0.01 0.03 y =6.42x + 17.85

ABTC Low resistance 0.05 0.133 Y = 6 . 63 x + 13.88

AYTC High resistance 1.5 7.6 Y = 4.38x + 4.23

Adult female surmval and fertilityFor each stram ten cohorts, each compnsing 10 msects

less than 24h old, were added to a glass jar containing 500gwhole-wheat meal and bread yeast. Adult mortahty wasrecorded at weekly intervals for 20 weeks and survivingbeetles were transferred to new Jars of medium. Jars fromwhich adults had been removed were retained under theoriginal conditions until all progeny had emerged.Life table statsstic«Biological data were summarised by hfe table statistics

(Southwood 1978). Information obtained included lx' theproportion of females ahve during age interval x; mx, thenumber of living female progeny produced per female duringthe age interval x; and Ro, the net reproductive rate that isobtained from the sum of the products Lx mx.The innate capacity for increase rrn' and the finite rate of

natural increase, A. (the natural antilog of r In ), werecalculated from Lx and ni; tables by iterative substitution oftrial values of r rn in the expression:lxmx exp ( - r x) = 1. Mean generation time, T, was

derived from the relationship:T = LogeRo/rrn (Andrewartha and Birch 1954).

Adult weightFour to seven samples of 100 adult insects were randomly

sieved from culture jars of each strain and weighed. Thebeetles were then starved for 24 h and weighed again.

Results

Survival of immature stages

Under the experimental conditions used there was nomortality in the egg stage of any strain (Table 2). Losses inthe larval stage were relatively low at 17% in thesusceptible strain ZZTC compared with about 23% mortahtyin JSTC and AYTC, and about 22% m ABTC. More severelosses occurred III the pupae. Mortality was highest in the

highly resistant stram AYTC at about 43% wlule losses inother strains ranged from about 40% in ZZTC to about 35%in ABTC. Overall survrval to adulthood was low with highestsurvival recorded for ABTC and ZZTC at about 50% andlowest for AYTC at about 44%.

Table 2. Survival of immature stages of four strains ofTribolium castaneum.

Insect Survival of Immature stages

stram Egg Larva Pupa Total

JSTC 1.000 0.766 0.619 0.474

ZZTC 1.000 0.830 0.602 0.500

ABTC 1.000 0.872 0.651 0.509

AYTC 1.000 0.767 0.576 0.442

* The proportion surviving III each stage was calculated relative tothe number of survivors form the previous development stage.

Duration of development of immature stages

There were highly significant (P < 0.01) differencesbetween strains at all stages of development (Table 3). Thehighly resistant strain, AYTC, hatched at least one dayearlier than the other three strains wlule the weakresistance strain, ABTC, was slowest taking 6.8 days toemerge. Agam, larval development time was quickest in thehighly resistant AYTC and slowest m the ABTC strain. Pupadevelopment time was slowest at about 11 d in the weeklyresistant ABTC and susceptible ZZTC, and significantly (P<0.01) quicker in AYTC and JSTC.Total development time was sigmficantly (P < 0.01 )

faster in the highly phosphine resistant stram, AYTC, thanm the other three strains. Detailed study of the duration oflarval instars (Fig. 1) revealed that the highly resistantstrain, AYTC, developed through seven mstars while theother strains developed through eight.

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Proceedings of the 7th International Working Conference on Stored-prcduct Protection - Volume 1

Fig. 1. Duration of development of larval instars of four strains of Tribolium castaneum.

Adult female survival and fertility

Adult female survival schedules (Table 2) showed that thehighly resistant strain, AYTC, had lowest survival rate at0.375 after 20 weeks compared with 0.461 and 0.490 forthe two susceptible strains, JSTC and ZZTC, respectively,and 0.455 for the weakly resistant strain ABTC. Fertility

curves are shown in Fig. 2. There was little obviousdifference between strains except for strain JSTC thatshowed a strong peak in production of progeny betweenweeks 6 to 9. In general, fertility decreased markedly in allstrains after about week 12 and was lowest between weeks15 and 20.

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Proceedings of the 7 th International Worktng Conference on Stored-product Protection - Volume 1

45C 40i 35G)

!~~5' 20co~ 15~ 105 5f.J.c 01-+---I---4---I---+--+--+-+-+--+--+---1-1--+--+--+--+--+--+--+----1

--+-JSfC-8-zzrc-b-ABTC--*-AYTC

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Fig. 2. Fertility curves of four strams of Trtboltum castaneum.

Population statistics

The net reproductive rate was the highest in strain JSTC,about 25% higher than in the two resistant strains ABTCand AYTC. In contrast, the innate capacity for mcrease rm'and its antilog, the finite rate of natural increase, A, werehighest m the highly resistant strain AYTC. This stram alsohad the shortest mean generation time, T, ranging from 79to 155% faster than the other strains investigated.

Table 4. The net reproductive rate (Ro), innate capacityfor increase ( rm), fimte rate of increase (A), andthe mean length of a generation (T) of fourpopulations of Tribolium castaneum with variousresistance to phosphine.

Strain Ro rm A T

JSTC 470.3 0.48 1.616 12.82ZZTC 394.4 0.44 1.553 13.58ABTC 354.5 0.44 1.553 13.34AYTC 363.5 0.49 1.632 12.03

Adult weights

The mean weights of both starved and non-starved adultinsects were lower in the two resistant strains than in thesusceptible insects, the most resistant stram, AYTC, beingthe lightest (Table 5). Detailed weight data are listed inTable 1.

Table 5. Mean weight (rng ) of starved ( 24h) and non-starved adults of four strains of Triboliumcastane.

Strains JSTC ZZTC ABTC AYTC

Starved 0.2398Non-starved 0 .2455

0.26600.2718

0.23490.2396

0.23020.2330

weeks

Discussion

Knowledge of any differences m the biology of resistantinsects compared with susceptible may be exploited whendesigning resistance management strategies. For example,strategies such as alternatmg insecticides require that thefrequency of resistance to a particular chemical declme whenit is not in use. That is, the frequency of resistance genes inthe population must decrease. The most importantmechanisms that will allow this to occur are either 1) thefrequency of resistance genes is diluted by an influx ofsusceptible insects into the population or 2) resistant insectsproduce less progeny than susceptible i.e. they are less fitthan susceptibles. In a Situation, such is the case with manygrain storages, where unrrugration may not be a major factorand any fitness differences between genotypes can be verysignificantIn these experiments the life histories two reference

susceptible Tribolium castaneum strains were comparedWith those of representative resistant strains Overall, therewere only small differences between the life histories of thesusceptible strains and the low-level resistant strain, ABTCAlthough this stram showed lowest immature developmenttime and a lower adult survival in addition to the lowestreproductive rate of any stram, its innate capacity forincrease, rm' was equal to the susceptible strain, ZZTC,but less than that of the other susceptible stram, JSTC. Incontrast, there were important differences between thehighly resistant strain, AYTC, and the reference susceptiblestrains. This stram was charactensed by havmg the lowestimmature survival rate and fastest immature developmenttime (with one less larval instar ). In addition, adults ofAYTC were the smallest of any strain and had the lowestSurvival rate. Nevertheless, although this stram also had alower net reproductive rate than either of the susceptible

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Proceedings of the 7th International Working Conference on Stored-product Protection - Volume 1

strains, it had the highest innate capacity for increase, rm ,

of any strain. This was due to its significantly shortergeneration time.The results of these experiments show that there are

fitness differences between phosphine-resistant andsusceptible strains of Tribolium castaneum. Thesedifferences, however, are small and It is likely that theycould not be exploited in a resistance management strategy

Acknowledgements

The work was financially supported by the StateAdministration of Grain Reserve, Ministry of InternalTrade, Beijmg, Chma, and Australian Centre forInternational Research (ACIAR). ThIS support IS gratefullyacknowledged.

References

Andrewartha, H. G. and Birch, L. C. 1954. Thedistribution and abundance of animals. University of

Chicago Press, Chicago.Collins, P. J, Mulder, J. C., and Wilson, D. 1989.Variation in hie history parameters of Oryzaephilussurinamensis (L.) (Coleoptera: Silvanidae). J. StoredProd Res. 4, 193-199.FAG. 1982. Recommended methods for detection andmeasurement of agricultural pests to pesticide. Tentativemethod for adults of some major pest species of storedcereals, with methyl bromide and phosphine. FAGMethodNo. 16, FAGPlant Protection Bulletin 23, 12 - 26.Finney, D. J. 1971. Probit analysis, 3rd ed. CambridgeUniversity Press, London.Southwood T. R. E. 1978. Ecological methods, 2nd ed.Chapman and Hall, London.Tabashnik B. E. 1990. Modeling and evaluation ofresistance management tactics, In: Roush, R. T. andTabashmk, B. E. ed. Pesticide Resistance in Arthropods,Chapman and Hall, New York. pp.153 -182.White, G. G. 1984. Variation between FIeld and LaboratoryPopulation of Tribolium castaneum (Herbst) (Coleoptera:Tenebnonidae). Aust. J. Ecol 9, 153- 155.

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