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Crop Protection 29 (2010) 178–182

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Crop Protection

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Effect of different host plants on the fitness of diamond-back moth,Plutella xylostella (Lepidoptera: Plutellidae)

Rabia Saeed a, Ali H. Sayyed b,*, Sarfraz A. Shad c, Syed Muhammad Zaka c

a Department of Entomology, Central Cotton Research Institute, Multan, Pakistanb Institute of Biotechnology, Bahauddin Zakariya University, Multan, Pakistanc Department of Entomology, University College of Agriculture, Bahaudin Zakariya University, Multan, Pakistan

a r t i c l e i n f o

Article history:Received 23 March 2009Received in revised form13 September 2009Accepted 27 September 2009

Keywords:Plutella xylostellaCruciferous cropsInsect growthIntrinsic rate of population increaseDevelopment time

* Corresponding author.E-mail address: ali_sayyed@hotmail.com (A.H. Say

0261-2194/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.cropro.2009.09.012

a b s t r a c t

The diamond-back moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae) is a destructive cosmopolitanpest of cruciferous crops. The pest is present wherever its host plants exist and is considered to be one ofthe most widely distributed of all the Lepidoptera. We investigated the effect of various host plants onthe fitness of P. xylostella and tested the hypothesis by studying development time, growth, fecundity andsurvival on cabbage (Brassica oleracea capitata), cauliflower (Brassica oleracea botrytis), radish (Raphanussativus), turnip (Brassica rapa), mustard (Brassica compestris) and canola (Brassica napus var. canola). Thedevelopmental time from eggs to adult eclosion was the shortest (10 days) on canola and the longest(13 days) on turnip. Fecundity was greatest on canola (350) followed by cauliflower (268 eggs) byfemales eclosed from the pupae reared on canola and cauliflower, respectively, while the minimumnumbers of eggs (184) were observed on cabbage. The number of eggs hatched was the highest (80%)when larvae fed on cauliflower. Survival to the adult stage was the highest (94%) on mustard followed bycauliflower and lowest (64%) on turnip. The net replacement rate was lowest for populations reared oncabbage (32.3), which was also reflected by the lowest intrinsic rate of population increase (0.20). Thecorrelation between the intrinsic rate of population increase (rm) and the mean relative growth rate wassignificant (t¼ 20.02 d.f.¼ 4, P< 0.05). Canola and mustard proved to be the most suitable hosts forP. xylostella because of shorter developmental period, higher percentage of survival and higher number ofeggs. The data point to the role of host plants in increasing local P. xylostella populations.

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1. Introduction

Plutella xylostella is one of the most destructive and cosmopol-itan pests of cruciferous crops causing over US $1 billion worthdamage globally per annum (Talekar and Shelton, 1993). Outbreaksof P. xylostella in Southeast Asia sometimes cause more than 90%crop loss (Verkerk and Wright, 1996). It may have originated fromEurope (Hardy, 1938) or possibly South Africa (Kfir, 1998). Whateverthe origin of this pest, it has now been recorded from 128 countriesof the world including Pakistan (Talekar and Shelton, 1993). Thepest is present wherever its host plants exist and is considered to bethe most widely distributed of all the Lepidoptera (Shelton, 2004).In Pakistan, P. xylostella is a serious pest of cruciferous crops (Abroet al., 1994; Khaliq et al., 2007) and sometimes infestation forcedgrowers to plough up the standing crops in spite of multipleinsecticides application (Abro et al., 1994). This exceptional pest

yed).

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status is due to the diversity and abundance of host plants, lack ordisruption of its natural enemies, its high reproductive potential,with up to 20 generations per year and its proven ability to rapidlyevolve resistance to insecticides (Shelton, 2004).

Interest in behavioural manipulation as an insect pest manage-ment tool has increased in the last 30 years, largely due to the desireto reduce reliance on broad-spectrum insecticides (Foster and Har-ris, 1997). Push-pull strategies, which combine repellent andattractant crops to manipulate pests and natural enemy populationsare a typical example (Agelopoulos et al., 1999) but exploitation ofthese strategies requires detailed knowledge of how diverse crophost plants alter herbivore life history and behaviour (Umbanhowarand Hastings, 2002; Awmack and Leather, 2002). For instance,variation in host-plant quality may affect the body size of herbivo-rous insects, which, in turn, can determine life-history parameterssuch as fecundity, longevity, and survival (Stern and Smith, 1960;Sequiera and Dixon, 1996; Awmack and Leather, 2002).

Fitness costs include reduced survival, low pupal weight andincreased developmental time or reduced fecundity (Sayyed et al.,2008a). In the current study, we were interested in investigating

R. Saeed et al. / Crop Protection 29 (2010) 178–182 179

the effect of various host plants on the fitness of P. xylostella. Thegoal of this study was to determine the performance of P. xylostellacollected from cauliflower and reared on to other cruciferous crops.

2. Materials and methods

2.1. Host plants

The test cultivars used in these experiments were red cabbage(Brassica oleracea var capitata), cauliflower (China ka Moti) (Brassicaoleracea var botrytis), Chinese radish (Raphanus sativus), turnip(Neela shaljum) (Brassica rapa), mustard (Prickly heat) (Brassicacompestris) and canola (Abaseen) (Brassica napus var. canola). Thetest plants were grown in plastic pots (6�10� 20 cm) and used inexperiments at four to six weeks old. We used the most susceptibleeconomically available varieties after consultation with localfarmers.

2.2. Insects and general rearing

The larvae of P. xylostella were collected from cauliflower fieldsfrom Multan, Pakistan and brought to the laboratory. The area isused for cotton crops in summer but in winter farmers growcruciferous vegetables (cauliflowers and cabbage) and farmers haveseveral groups of insecticides available to control P. xylostella.Previously we have shown that different populations of P. xylostellafrom Multan have developed resistance to organophosphates,pyrethroids and new chemistry insecticides (Khaliq et al., 2007;Sayyed et al., 2008b). Further studies with P. xylostella from the areaindicated that in the absence of selection pressure resistance wasstable (Sayyed et al., 2008a). The larvae were reared at a tempera-ture of 25� 2 �C; 60–70% relative humidity and a L: D (16:8 h) oncauliflower until pupation. Newly emerged adults were transferredin to glass jars (16 cm diameter and 23 cm height) for mating andegg laying on cabbage leaves. The cabbage leaves were provided foreggs, which were replaced daily. To keep the leaves turgid, wetcotton wool was wrapped around the petiole of the leaves. Adultswere fed on cotton wool impregnated with 10% honey solution.

2.3. Development and survival of immature P. xylostella

To test the effect of host plants on the duration of developmentaltime, insects were taken from the field-collected population. Thepopulation was reared for a generation on each host plant toeliminate the effect of food reserves in the eggs and its effect on theemerging larvae, before starting the experiment. Twenty five leafdiscs (5 cm diameter) of four to six weeks old Brassica oleracea var.capitata, B. oleracea var. botrytis, B. rapa, B. compestris, B. napus var,canola and Raphanus sativus respectively were cut and placedindividually in 25 Petri dishes, each dish containing one leaf discand one moistened filter paper. On each leaf disc, five neonatelarvae were released and each Petri dish was designated asa replicate. Larval food was changed every day and the duration ofdevelopment of different larval instars, total larval developmentperiod, pupal period, total developmental period (first instar toadult emergence) and survival were recorded. The P. xylostellainstars were differentiated by head capsule width, which is ca0.16 mm, 0.26, 0.38 and 0.63 mm for first to fourth instar larvaerespectively (Sayyed et al., 2002).

2.4. P. xylostella reproduction on different host plants

Six pairs of adults from each host plant were kept in separateglass jars (16 cm diameter and 23 cm height) with a leaf asdescribed above and fed on cotton wool saturated 10% (w/v) honey

solution. The leaves were changed daily and the numbers of eggs laidwere counted. The eggs were kept at 25� 2 �C and 65� 2% RH undera L:D 16:8 h cycle. Hatching usually occurred within 4–5 days but theeggs were given up to 7 days to hatch before the number ofunhatched eggs was counted. The data on all biological parameterswere analysed statistically using SAS (2008) with PROC GLM and thedifferences between means were compared using LSD (P< 0.05).

2.5. Growth rate

Twenty randomly selected third-instar larvae were selectedfrom the populations reared on each host plant (see 2.3), weighedand batches of five larvae were placed in separate Petri dishes (9 cmdiameter) with a fresh host plant leaf disc on a filter paper saturatedwith water to keep the leaf turgid. We chose third-instar larvae forgrowth rate measurements since it is difficult to accurately recordthe initial weight of neonates. The leaf discs were replaced everytwo days but pupation was recorded daily. Pupae were removed,weighed, and placed in a new, empty Petri dish.

The mean relative growth rate (MRGR) was calculated using theformula:

MRGR ¼ ½ln W2ðmgÞ � ln W1ðmgÞ�=T

where W1 and W2 are the initial larval (L3) and pupal weightsrespectively and T is the time (days) from L3 to the pupal stage(Radford, 1967).

2.6. Intrinsic rate of population increase

The net replacement rate (R0), the average number of femaleoffspring produced by each female during its entire lifetime (Birch,1948; Sayyed and Wright, 2001), was calculated for each of the hostplants. P. xylostella had 50:50 sex ratio on cabbage, cauliflower andcanola but 48:52 on turnip and radish, which did not differsignificantly from 50:50 (data not shown). To limit the results tofemale progeny only, the mean number of progeny produced wascalculated as the mean number of eggs laid per female multipliedby the proportion of eggs gave rise to surviving adults, divided by 2:

R0 ¼ ðn� Ie � IaÞ=2

where n was the mean number of eggs per female, Ie was thefraction of fertile eggs, Ia was the fraction of eclosing adults, and 2was the sex ratio coefficient.

The net replacement rate was then used to calculate the intrinsicrate of population increase, rm (Birch, 1948; Sayyed and Wright,2001):

rm ¼ ðln R0Þ=T

where T was the development time from egg to adult eclosion.

3. Results

3.1. Development of immature stages, survival andoviposition period

There were significant differences in the developmental timesfrom egg to pupation among the different host plants (Table 1,F¼ 48.48; d.f.¼ 5,120; P< 0.001). The larvae fed on canola andmustard developed faster (10.20 and 10.80 days respectively) thanthose on any other host plants (P< 0.05) but the larvae reared onturnip had a longer total development time (13.0 days) than thosereared on other host plants (Table 1). There was no significantdifference in development time on cauliflower and cabbage or

Table 1The effect of different hosts on the duration of larval development of Plutella xylostella.

Hosts 1st larvalinstar (days)

Second larvalinstar (days)

Third larvalinstar (days)

Fourth Larvalinstar (days)

Pupal developmentalperiod (days)

Total developmentalperiod (days)

Meane SE Meane SE Meane SE Meane SE Meane SE Meane SE

Cauliflower 2.52bc 0.65 1.36d 0.57 1.08c 0.28 1.68ab 0.49 4.32a 0.56 11.0bc 1.0Cabbage 2.92a 0.27 1.48cd 0.51 1.36b 0.49 1.24c 0.44 4.36a 0.49 11.4b 0.70Mustard 2.80ab 0.50 1.48cd 0.59 1.08c 0.28 1.08cd 0.28 4.36a 0.70 10.8c 0.96Canola 2.28c 0.54 1.76bc 0.44 1.08a 0.28 1.00d 0.00 4.08a 0.49 10.2d 0.58Turnip 2.68ab 0.48 2.00b 0.50 1.88a 0.53 1.84a 0.37 4.60a 0.50 13.0a 0.87Radish 2.76ab 0.72 2.96a 0.73 1.24bc 0.44 1.60b 0.50 4.28a 0.54 12.8a 0.90LSD 5% 0.31 0.32 0.22 0.24 0.32 0.46

e Means sharing same letters in a column are not significantly different from each other (P> 0.05).

R. Saeed et al. / Crop Protection 29 (2010) 178–182180

turnip and radish (Table 1). The P. xylostella survival on cabbage wassignificantly lower than mustard (P< 0.05; Table 2). The survival offirst instar larvae to adults differed significantly on the six hostplants (F¼ 62.98; d.f.¼ 5,10; P< 0.05). The lowest survival was onturnip while the highest rate of survival was on mustard (Table 3).Similarly the females that emerged from larvae reared on turnipand raddish had the longest oviposition period (9 days) whichdiffered significantly (P< 0.05) from the females reared on cabbage(6 days; Table 2).

There were significant differences in body weight among larvaereared on different hosts (Table 2). The third-instar larvae reared oncanola and turnip were the heaviest, while larvae reared oncabbage and raddish were the lightest.

3.2. Fecundity and egg viability

The mean number of eggs laid by females emerged from larvaereared on canola 350 was significantly higher (F¼ 72.5, d.f.¼ 5,10,P< 0.01) than the numbers from females from larvae reared onother host plants (Table 2). Fecundity on cabbage and mustard wasreduced relative to the fecundity of females on cauliflower.

Egg viability from females from larvae reared on cauliflower wassignificantly higher than the viability of eggs from females fromcabbage, mustard, canola and turnip (F¼ 101.5, d.f.¼ 5,10, P> 0.01Table 2).

3.3. Intrinsic rate of population increase

The net replacement rate was the lowest for population rearedon cabbage, which also had the lowest intrinsic rate of populationincrease (Table 3). It was also observed that the mean relativegrowth rate of the larvae reared on cabbage was lower (Table 3).The relationship between the intrinsic rate of population increase(rm) and the mean relative growth rate was significant (t¼ 20.02d.f.¼ 4, P< 0.05; Table 3). Similarly the correlation betweenintrinsic rate of population increase and mean relative growth ratewas significant with correlation coefficient 0.95 (d.f.¼ 4, P< 0.01).

Table 2The effect of different host plants on different life traits of Plutella xylostella.

Host plants % Survival Oviposition period Gr

Meane SE Meane SE M

Cauliflower 82a 0.89 7.33ab 0.22 14Cabbage 78a 0.88 6.00ad 0.44 9Mustard 90b 1.90 8.30bc 0.24 15Canola 94b 0.67 7.93bc 0.13 17Turnip 64c 0.33 8.93c 0.13 12Radish 86a 0.88 8.93c 0.35 11

e Means sharing same letters in a column are not significantly different from each oth

4. Discussion

The development, survival and reproductive rate of specificinsects can vary on various host plants, the factors determine plantsuitability for herbivorous hosts. Shorter development times andhigher rates of reproduction of insects on a host indicate greatersuitability of a host plant (Awmack and Leather, 2002). Althoughdevelopmental rates and reproduction provide important cluesconcerning the ability of the host to support a complete insect lifecycle, these data should be linked to other parameters such asmortality before definitive conclusions are drawn concerning hostsuitability. In the present study the developmental period ofP. xylostella varied on different hosts. For example, it was longest onturnip and radish but was the shortest on cabbage (Table 1).Similarly, Talekar and Shelton (1993) suggested that host plantinfluences insect development rates. This variation in developmentrate could be due to variation in nutritional and phago-stimulantfactors (Syed and Abro, 2003). The canola, mustard and cauliflowerwere found to be better hosts for P. xylostella as the larval periodswere shorter compared with the other hosts tested. A fasterdevelopmental time on a particular host may allow a shorter lifecycle, high reproductive productivity, and more rapid populationgrowth (Singh and Parihar, 1988). It may also reduce generationtime. In Pakistan, P. xylostella has 12–14 generations per year so thedifferences in generation time may accumulate over the cropgrowing season. This would be reflected in the ultimate populationsize. In a growing season, all of these six host plants are available atany one time, and we do not yet know how many generationsmight be spent on each host in the field. Therefore, a further studyis required to establish the number of generations that P. xylostellacan complete in the field on the six host plants tested.

Low body weights on cabbage and mustard were associatedwith rapid development time, this reinforces the suggestion thatcabbage and mustard are more suitable hosts for P. xylostella thancauliflower, canola, turnip or radish. However the relationshipbetween body weight and fecundity may not be as simple as itseems, especially in the case of lepidoptera and aphids. The bodyweight- fecundity relationship may become unreliable if the steps

owth (mg/ml) Fecundity % Eggs viability

eane SE Meane SE Meane SE

.0a 0.11 268a 6.0 56.6a 0.52

.00b 0.13 184b 3.84 45.0b 0.33

.0a 0.24 226c 2.65 62.4c 0.54

.0c 0.18 350c 5.0 80.0d 1.11

.0d 0.12 248ac 1.02 70.1c 0.87

.0d 0.83 234d 0.89 39.5c 1.39

er (P> 0.05).

Table 3Intrinsic rate of population increase (rm) of Plutella xylostella on various host plants.

Host Plants Net reproductive rate(R0 per generation)

Mean relativegrowth rate

Intrinsic rate of naturalincrease (rm, per day)

Cauliflower 96.48 0.44 0.30Cabbage 32.29 0.34 0.20Mustard 65.86 0.40 0.26Canola 81.80 0.46 0.29Turnip 64.51 0.37 0.22Radish 40.25 0.35 0.19

R. Saeed et al. / Crop Protection 29 (2010) 178–182 181

between independent and dependent variables are increased(Leather, 1988). In addition to differences in development andweight, there were marked differences in survival of larvaebetween the different host plants because of differences in earlymortality. Larvae fed on turnip had considerably reduced survivalrates compared with the other host plants tested, in addition, lowlevel of prepupal mortality observed. Therefore, we suggest that thelow survival on turnip was due to the heavy early mortalityobserved which could be attributed to the unsuitability of the hostplants. The early stage mortality of an insect population is a keyfactor for establishing adult populations. Similarly, the larval hostplants can have a significant effect on both adult longevity andreproduction (fecundity).

Host plant chemical cues could have a great affect on ditrophicinteractions (Vet and Dicke, 1992). The differences in egg laying ondifferent host plants demonstrated that chemical cues mediatedhost plant selection in P. xylostella. Females emerged from larvaereared on canola laid significantly higher number of eggs comparedwith females from larvae emerged from other host plants. Thevariation in P. xylostella has previously been shown to be associatedwith the host plants and the conditions under which the plantswere grown (Lu et al., 2004; Badenes-Perez et al., 2005). Forexample, Begum et al. (1996) reported that females P. xylostellareared on wild canola were less fecund compared with those rearedon canola grown in the laboratory. Additionally, the shorterdevelopmental duration of immature stages (egg, larva, prepupa,and pupa) on canola (Table 1) suggest that canola is relativelynutritious compared with cabbage, cauliflower, mustard, turnipand radish. Our results are similar to Pivnick et al. (1994) who foundthat female P. xylostella preferred to oviposit on cabbage, cauli-flower, broccoli, and kohlrabi. This host preference may be due inpart to different chemical cues possibly involved in the acceptanceor rejection of potential hosts.

The intrinsic rate of population increase is an important indi-cator of population dynamics (Varley and Gradwell, 1970).Comparisons of intrinsic growth rates often provide considerableinsight beyond that which is apparent from individual life-historyparameters. Although the component variables of populations areaffected by a large number of factors, we found a close and positivecorrelation between individual measures of the host plant suit-ability and the intrinsic rate of population increase. It could reflectthe potential of host plants to contribute to P. xylostella populations.However, the net reproductive rate is not the only indication ofpopulation growth potential since the rate of population increase isdependent on the number of eggs laid, eggs hatched, growth andadult eclosion (Sayyed et al., 2008a). A variation in any of the abovetraits could therefore affect the rate of population increase. Incontrast an extremely close relationship between mean relativegrowth rate and intrinsic rate of increase has been shown in fourlepidopteran species: Spodoptera frugiperda, Panolis ammea, Anaitisefformata, and Lymantria dispar (Leather and Dixon, 1984; Leather,1994). Also as previously noted, a shorter developmental time allowsmore generations on a host. Therefore, considering the develop-mental time, the six host plants can be arranged in descending order

of host suitability: canola>mustard> cauliflower> cabbage>radish> turnip. These results are consistent with field observations.In Pakistan, the canola and mustard are sown in the field first andwhen in February cabbage and cauliflowers are planted, theP. xylostella move to vegetables and cause large economic losses tothe vegetables every year. It is a key pest of these crops in Pakistan(Khaliq et al., 2007).

There are many factors that can affect host suitability, includingnutrient content. The exact cause of the differences found amonghost plants in larval growth rates, mortality, and adult fecundityand survival remains however unknown and therefore requiresfurther work to establish biochemically the reasons for thedifferences. The work in present studies suggests that crops likemustard and canola could be used as trap crops when cabbage andcauliflowers are the major vegetable cash crops (Shelton andNault, 2004). This study presents information on fitness of P.xylostella on various host plants. We not only compared lifeparameters but also focused on the issue of intrinsic rate of pop-ulation increase on various host plants under the same environ-mental conditions.

Acknowledgments

We are thankful to Dr Ben Raymond (University of Oxford) andDr Paul Johnston (University of Manchester) for reading ourmanuscript and commenting on it. We are also grateful to Dr JerryCross for his useful comments during the review process. ForeignFaculty hiring programme of Higher Education Commission ofPakistan supported AHS and he appreciates their support.

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