effects of within-plant variation in leaf quality on the ecology of g. viridula

Introduction

Leaf age is the most important attribute affecting theuse of leaves by herbivores and also in determiningthe impact of leaf-removal on plant growth (Crawley1983). It has a profound effect on its suitability as foodfor both chewing and sucking herbivores. Many leaf-eating insects often have a restriction in their feedingto a certain age-class of leaves (Raupp and Denno,1983). As they age, leaves change physically (surfacearea, colour, surface toughness, etc.) and chemically(concentrations of soluble N, tannins, amino acids,etc.). Also, leaves of the same age produced in differentplaces on the plant, or from the shoots with differenthistories of defoliation, will differ in their quality asfood (Bryant and Kuropat, 1980; Haukioja, 1980). Theexperiments were set up to see the effects of within-plant variation in leaf quality on the ecology of G.viridula. Two main objectives of the experiments areas follows;

1) Is there any effect on the ecology of G. viriduladue to the leaf age ? (EXP1)

2) If so, is this effect correlated to the changes in N

concentration of the leaves ? (EXP2)The findings of these experiments would hopefully

help in understanding the population dynamics of G.viridula on R. obtusifolius. It would also help unders-tanding the effect of the variations in leaf N concen-tration on herbivores associated with R. obtusifolius.

Materials and Methods

1. The effect of the leaf age on rates of growthof instars of G. viridula (EXP1)

A total of 240 R. obtusifoliusseeds (Herbiseed,Billingbear Park, Wokingham U.K.) was sown intoJohn Innes Compost No 2 on 20 April 1995. Then theseedlings were transplanted into 9 cm-diameter potson 11 May. The compost mixture used in 9 cm-diame-ter pots and thereafter consisted of soil, peat, and sand(1 : 4 : 2, respectively). The pots were divided into 4categories (O, L, M, H) of 60 plants each. Initial appli-cation of Growmore pellets (7-7-7 N : P : K) (O : 0 g, L: 0.5 g, M : 1.0 g, H : 1.5 g) was done at the time of thetransplantation of the pots on 11 May. Afterwards,biweekly application of the powdered Growmore pel-lets into plants was done on 24 May, 9 June, 24 June.On 20 May nicotine fumigation was carried out to kill

ENTOMOLOGICAL RESEARCHVol. 35, No. 4, pp. 243-252, December 2005

Effects of within-plant Variation in Leaf Quality on the Ecology of G. viridula

KWON, Ohseok

Division of Weed Management, Department of Agricultural Biology, National Institute of Agricultural Science and Technology, Suwon 441-100, Korea

ABSTRACT In order to find out the relationship between the leaf quality of R. obtusifoliusandthe ecology of G. viridula, a series of experiments was carried out. The effects of leaf age on ratesof growth of instars of G. viridula showed that the leaf age was closely related to the developmentof the larvae of G. viridula. Also, the results showed quite clearly that the N concentration of R.obtusifoliusleaves have significant effects on the growth rates of G. viridula larvae. It is suggestedthat C : N ratio of food plant plays an important role in the population dynamics of phytophagousinsects. A small increase in leaf N concentration in a nutrient poor habitat will substantiallyincrease the G. viridulapopulation.

Key words : Leaf quality, Gastrophysa viridula, Rumex, N concentration

*Corresponding authorE-mail: [email protected], Tel: 81-11-704-4374

Research Paper

aphids. On 9 July, a total of 80 plants (20 plants fromeach category) was randomly selected. They weretransplanted into 2 litre pots with the initial applicationof the powdered Growmore pellets (O : 0 g, L : 1 g, M :2 g, H : 3 g). Afterwards, biweekly application of thepowdered Growmore pellets into plants was done on24 July, 9 August, 24 August.

1-a) Preliminary experiment on leaf N concentration

A preliminary investigation of leaf N concentrationon R. obtusifoliuswas done on 23 July and on 28 July.The aim was to check if there are significant diffe-rences in leaf N concentrations a) among the O, L, M,H plants, and b) among the leaves of different ages ina plant. One plant from each category was randomlyselected. Four basal leaves from each plant werecollected on 23 July (2 weeks from N application on 9July) and on 28 July (4 days from N application on 24July). They were analysed for N concentration intissue.

1-b) Experiment 1-instar IOn 10 October, a total of thirty basal leaves of similar

size and shape from 10 randomly chosen plants (threeleaves from each plant) in 2 litre pots (H category,randomly chosen) were collected for the experiment.They were labelled as B1, B2, B3 in the order of age(oldest to youngest). A circular disc of 5cm diameterwas cut from each of them. Then, each of these discswas placed into a plastic Petri dish in which a filterpaper (Whatman No. 1, 9 cm diameter) was placed. Afew drops of water were applied on a filter paper tokeep the humidity high.

An egg batch (45 eggs in total) of G. viridula wasselected randomly from the insect culture maintainedin the laboratory on 5 October. By 10 October, all ofthe larvae hatching from these eggs were first instars.Thirty of them were randomly selected, and thenweighed on a Cahn C-31 Microbalance (Scientific &Medical Products Ltd., Shirley Institute, Manchester,U.K.). Each of them was then placed into a Petri dishprepared containing one leaf disc (as above). The Petridishes were then randomised on the bench in thelaboratory room. The G. viridula larvae were leftundisturbed for two days, and then weighed again.

1-c) Experiment 2-instar IIOn 11 September, a total of thirty basal leaves of

similar size and shape from 10 randomly chosenplants (three leaves from each plant) in 2 litre pots (Mcategory, randomly chosen) were collected for another

experiment. They were prepared, and executed in thesame manner as the above experiment except that ins-tead of first instars of G. viridula, second instars wereused in the experiment. It was based on the assumptionthat the feeding of elder individuals would be affectedby the leaf age less than that of the younger ones.

An egg batch (48 eggs in total) of G. viridula wasselected randomly from the insect culture maintainedin the laboratory room on 1 September. By 11 Sep-tember, all of the larvae hatching from these eggs weresecond instars. Thirty of them were randomly selec-ted, and then weighed on a Cahn C-31 Microbalance.Each of them was then placed into a Petri dish pre-pared as above. The Petri dishes were then rando-mised on the bench in the laboratory room. The G.viridula larvae were left undisturbed for two days, andthen weighed again.

2. The effect of different levels of nitrogen in R. obtusifolius leaves on the growth anddevelopment of G. viridula over onegeneration (EXP2)

2-a) Experiment 1-Analysis of leaf N concentration

A total of 20 leaves from each category of N con-centration (one leaf per plant×5 plant×4 category)was collected at the start and at the end of the Experi-ment 2 - Effect of leaf N concentration on G. viridula(2-b)). Collected leaves were individually marked foridentification, and freeze-dried for 48 hours. Then, thesample was prepared by grinding the leaves in amortar to a fine powder. The chemical analysis ofnitrogen was then done using the Carlo Erba 1108Elemental Analyser (Fisons Instruments, Crawley,U.K.). While use of total leaf N as a nutritional indexmay overlook subtleties in microdistribution of solubleand insoluble nitrogen within tissues of leaves, it hasshown to be a reliable indicator for phytophagousinsects (Faeth et al., 1981).

2-b) Experiment 2-Effect of leaf N concentrationon G. viridula

A total of 240 R. obtusifoliusseeds (Herbiseed,Billingbear Park, Wokingham U.K.) was sown intoJohn Innes Compost No 2 on 20 April 1995. Then theseedlings were prepared for the experiment. The potswere divided into 4 categories (O, L, M, H) of 60 plantseach.

On 14 June, a total of forty basal leaves of similarsize and age from the plants of each category (1 leafper plant×10 plant) was collected for the experiment.

Entomol. Res., 35(4), December 2005244

They were labelled as O, L, M, H in the order of leafN concentration level (lowest to highest, respectively).A circular disc of 5 cm diameter was cut from each ofthem. Then, each of these discs was placed into aplastic Petri dish in which a filter paper (Whatman No.1, 9 cm diameter) was placed. A few drops of waterwere applied on a filter paper to keep the moisturelevel high. During the experimental period, a new leaf(a circular disc of 5 cm diameter) was put into the Petridish every 2 days after the measurement of the indivi-dual beetle was made, and old leaves were removed.

Two egg batches (78 eggs in total) of G. viridula

were selected randomly from the insect culture main-tained in the laboratory room on 5 June. By 9 June, allof the eggs were hatched out. They were too young tobe handled for the measurement of the body weight.Forty of them were randomly selected, and then pla-ced into 4 Petri dishes (10 individuals per dish). EachPetri dish had a leaf from O, L, M, or H plants. Thelarvae were left undisturbed until 14 June, and thenweighed on Cahn C-31 Microbalance (Scientific &Medical Products Ltd., Shirley Institute, Manchester,U.K.). Each of them was then placed into the preparedPetri dish. The Petri dishes were then placed on thebench stand in the laboratory room. The G. viridulalarvae were left undisturbed for two days, and thenweighed again every two days until all individuals hadcompleted their life cycle to the adult stage. The de-velopment rate of the individuals was also recorded.

Results

1. The effect of the leaf age on rates of growthof instars of G. viridula (EXP1)

1-a) Preliminary experiment on leaf N concentration

First, the preliminary analysis of leaf N concentration

Kwon : Effects of within--plant Variation on G. viridula 245

Table 1. a) Preliminary experiment on leaf N concentration:Nitrogen and Carbon concentrations of leaves in R. obtusi-folius plants (Basal leaf order : 1 being the oldest, and 4 beingthe youngest basal leaf) were not significant (F = 1.815, p =0.167). Also, the variation of leaf N concentrations among O,L, M, H plants was not significant (F = 1.233, p = 0.316)

Date Plant CategoryBasal leaf % N % C C : Norder

1 1.413 44.500 31.493

1 O2 1.541 41.202 26.7373 1.575 40.481 25.7024 1.621 42.420 26.169

1 1.780 43.478 24.426

2 L2 2.073 43.161 20.8213 2.201 46.115 20.952

23-Jul 4 2.386 47.176 19.772

1 2.163 42.507 19.652

3 M2 2.392 47.238 19.7483 2.564 45.173 17.6184 3.465 48.355 13.955

1 2.353 44.088 18.736

4 H2 2.426 44.417 18.3093 2.961 46.085 15.5644 3.912 46.173 11.803

1 3.566 42.536 11.928

5 O2 3.617 44.691 12.3563 4.492 47.134 10.4934 4.914 44.773 9.111

1 2.067 45.869 22.191

6 L2 2.121 44.736 21.0923 2.656 46.371 17.459

28-Jul 4 3.033 45.893 15.131

1 3.091 44.467 14.386

7 M2 3.367 45.678 13.5663 3.814 45.637 11.9664 3.950 46.218 11.701

1 1.876 44.326 23.628

8 H2 2.393 45.953 19.2033 2.422 44.872 18.6274 2.473 46.096 18.640

0

1

2

3

4

1 2 3 4

% N

con

c. (

dry

wei

ght)

(O)(L)

(M)(H)

(O)(L)

(M)(H)

0

1

2

3

4

5

1 2 3 4

Basal leaf order

% N

con

c. (

dry

wei

ght)

i) 23 July

ii) 28 July

Fig. 1. a) Preliminary experiment on leaf N concentration:Nitrogen concentration of leaves in R. obtusifoliuson i) 23July and on ii) 28 July, 1995.

on the basal leaves of R. obtusifoliuswas done (seeTable 1 and Fig. 1). Although there was a trend of theleaf N concentration being inversely related to the ageof the leaves, the differences were not significant (F =1.815, p = 0.167). Also, the variation of leaf N concen-trations among O, L, M, H plants was not significant(F = 1.233, p = 0.316).

1-b) Experiment 1-instar IFirst instar larvae did not show significant diffe-

rences in weights between before and after the feedingon the leaves of different ages (F = 1.108, p = 0.340).A total of 6 individuals died on 12 October (Table 2).It may be due to the stress caused by measuring theweights. A further study with a larger sample size maymake the picture clearer. Fig. 2 shows the differencesin larval weights of G. viridula before and after thefeeding on the leaves of different ages.

1-c) Experiment 2-instar IITable 3 shows the result of the experiment. The


Table 2. 1-b) Experiment 1-instar I: The weights of G.viridula individuals before and after the feeding on the leavesof different ages (B1, B2, B3) (Unit = mg)

10-Oct 12-OctLeaf

B1 B2 B3 B1 B2 B3

1 0.768 0.587 0.606 1.666 N/A 1.0222 0.514 0.748 0.759 N/A 2.431 2.2403 0.725 0.871 0.758 1.618 2.738 1.7684 0.727 0.763 0.771 2.004 2.185 2.2315 0.835 0.869 0.901 2.598 2.710 2.4476 0.675 0.698 0.717 0.937 2.345 2.2167 0.689 0.572 0.539 2.012 N/A 0.9858 0.803 0.661 0.543 2.010 2.041 1.7979 0.426 0.393 0.381 0.595 0.747 N/A

10 0.312 0.408 0.599 N/A N/A 1.975

Mean 0.647 0.657 0.657 1.68 2.171 1.853

S.E. 0.055 0.053 0.048 0.228 0.256 0.177

0

0.5

1

1.5

2

B1 B2 B3

Leaf age

Mea

n di

ffere

nce

of

(mg)

wei

ghts

Fig. 2. 1-b) Experiment 1-instar I: The mean difference ofweights of first instar larvae before and after the feeding onthe leaves of different ages (B1, B2, B3, error bars are 1S.E.M.).

Table 3. 1-c) Experiment 2-instar II: The weights of G.viridula individuals before and after the feeding on the leavesof different ages (B1, B2, B3) (Unit = mg)

11-Sep 13-SepLeaf

B1 B2 B3 B1 B2 B3

1 1.305 1.208 2.028 3.087 3.652 4.9152 1.466 1.250 1.368 3.313 4.062 3.5923 1.402 1.583 1.206 4.174 4.495 4.3774 1.423 1.170 1.139 2.872 3.889 3.0305 1.416 1.340 1.650 2.980 4.113 4.8006 1.384 1.393 1.258 3.714 4.263 4.6757 1.260 1.237 1.350 3.171 4.002 4.0718 1.403 1.265 1.184 4.128 3.527 3.9909 1.053 1.006 1.165 3.663 3.440 4.260

10 1.056 0.871 0.797 3.265 2.851 3.060

Mean 1.317 1.232 1.315 3.437 3.829 4.077

S.E. 0.048 0.062 0.104 0.146 0.15 0.213

0

1

2

3

B1 B2 B3

Leaf age

Mea

n di

ffere

nce

of

(mg)

wei

ghts

Fig. 3. 1-c) Experiment 2-instar II: The mean difference ofweights of second instar larvae before and after the feedingon the leaves of different ages (B1, B2, B3, error bars are 1S.E.M.).

0

1

2

3

4

5

14-Jun 02-Jul

% N

con

c. (

Dry

wei

ght)

OLMH

Fig. 4. 2-a) Experiment 1-Analysis of leaf N concentration:Nitrogen concentration of leaves in each category at the startand at the end of the Experiment 2 (Error bars are 1 S.E.M.).


Table 4. 2-a) Experiment 1-Analysis of leaf N concentration: Nitrogen and Carbon concentrations of the leaves in each category(O, L, M, H) at the start and at the end of the Experiment 2

Date Category Leaf % N % C C : N

1 1.607 35.115 21.8512 1.724 38.666 22.428

O 3 1.775 35.770 20.1524 2.190 32.057 14.6385 1.687 38.116 22.594

1 3.534 36.728 10.3932 3.798 36.471 9.603

L 3 3.844 42.955 11.1754 4.350 37.527 8.627

14 June 5 3.237 39.267 12.131

1 3.343 37.728 11.2862 4.254 38.285 9.000

M 3 4.260 40.781 9.5734 3.276 36.527 11.155 4.447 38.433 8.642

1 4.373 38.869 8.8882 4.664 38.073 8.163

H 3 4.225 36.692 8.6844 4.070 37.757 9.2775 4.123 37.865 9.184

Table 5. 2-b) Experiment 2-Effect of leaf N concentration on G. viridula: The weights of G. viridula individuals feeding on theleaves of R. obtusifoliusplants from each category (O, L, M, H) (Unit = Mean Wt. (mg) per individual)

Date Life Categorystage O S.E. L S.E. M S.E. H S.E.

14-Jun Larvae I 0.690 0.067 0.579 0.066Larvae II 0.846 0.091 1.169 0.117 1.124 0.117 1.314 0.141

Larvae I 1.245 0.07516-Jun Larvae II 1.793 0.311 2.342 0.309 2.918 0.635 2.681 0.287

Larvae III 4.005 0.427 4.925 0.336 4.102 0.272

18-Jun Larvae II 2.797 0.386 3.578 0.548 7.424 N/A 5.228 N/ALarvae III 6.136 N/A 7.048 0.796 9.220 1.100 7.613 0.982

20-Jun Larvae II 4.080 0.575 7.040 2.360 6.536 N/ALarvae III 9.810 1.160 13.364 0.322 13.632 0.791 12.450 1.140

Larvae II 6.431 0.26322-Jun Larvae III 12.090 1.110 11.67 0.315 11.680 1.160 11.743 0.992

Pupae 14.233 N/A 12.372 0.942 12.703 N/A

24-Jun Larvae III 10.300 0.561 10.640 1.100 10.359 N/APupae 8.967 N/A 10.671 0.336 11.345 0.635 11.495 0.360

Larvae III 8.082 N/A26-Jun Pupae 8.957 0.559 9.682 0.242 10.180 1.210 10.175 0.381

Adult 12.005 N/A 11.052 0.519 11.165 0.648

28-Jun Adult 8.610 N/A 9.057 0.127 10.417 N/A 9.724 0.384

30-Jun Adult 8.360 0.516 7.570 1.570 7.706 N/A

02-Jul Adult 5.550 1.050

Date Category Leaf % N % C C : N

1 1.537 37.185 24.1932 1.692 35.671 21.082

O 3 1.621 35.640 21.9864 1.550 37.511 24.2015 1.653 40.183 24.309

1 2.785 36.981 13.2792 2.754 37.083 13.465

L 3 2.752 37.222 13.5254 2.674 39.330 14.708

2 July 5 1.975 37.800 19.139

1 4.202 42.283 10.0632 2.866 36.002 12.562

M 3 3.608 40.303 11.1704 4.422 40.464 9.1515 4.870 41.325 8.486

1 3.544 41.225 11.6322 3.850 37.976 9.864

H 3 3.969 40.400 10.1794 5.114 41.498 8.1155 3.682 38.730 10.519

differences in larval weights of G. viridula before andafter the feeding on the leaves of different ages weresignificant in second instar larvae (F = 3.146, p = 0.05).Unlike the Experiment 1, there was no mortality of theindividuals on 13 September. Comparing to the Expe-riment 1-instar I, the significant relationship betweenthe leaf N concentration and the weight gained by 2nd

instar larvae indicate that growth rates of 1st instarlarvae may have a significant relationship with leaf N

concentration as well. The result shown above may bedue to the energy consumed to settle down on newfeeding sites. While this may offset the energy gainedby higher leaf N concentration for 1st instar larvae,larger and stronger 2nd instar larvae may not suffer thesame degree of offset. Fig. 3 shows the differences inlarval weights of G. viridula before and after the


Table 6. 2-b) Experiment 2-Effect of leaf N concentration on G. viridula: The maximum weights of G. viridula individualsfeeding on the leaves of R. obtusifoliusplants from each category (O, L, M, H) (Unit = mg)

Life Category Life Category

Stage O L M H Stage O L M H

Larvae II 4.997 1.182 2.012 1.718 Pupae 8.901 10.138 11.431 N/A3.106 0.965 0.995 2.817 8.338 10.921 12.472 12.7035.928 2.622 1.617 1.824 5.978 9.942 8.239 N/A4.161 9.405 6.536 1.057 11.237 11.007 N/A 10.8246.817 1.655 1.447 1.852 N/A 14.233 13.314 11.7814.231 4.35 1.105 1.341 10.639 10.065 8.967 13.0956.547 2.449 0.989 5.288 8.585 10.849 13.041 10.2036.421 3.235 1.787 3.047 9.144 10.398 N/A 10.9051.347 1.52 2.985 1.002 N/A 10.201 12.157 12.0514.099 4.68 1.035 2.939 8.834 8.413 12.857 9.591

Mean 4.765 3.206 2.051 2.289 Mean 8.957 10.617 11.56 11.394

S.E. 0.548 0.797 0.535 0.409 S.E 0.559 0.465 0.68 0.431

Larvae III 10.206 11.431 14.317 5.538 Adult N/A 8.998 9.181 N/A10.053 13.338 14.481 13.760 8.61 9.551 11.124 10.0737.471 12.428 9.234 N/A 4.501 8.761 N/A N/A

15.709 13.617 6.384 14.547 9.713 9.147 N/A 9.86310.448 15.113 14.497 14.202 N/A 12.005 10.931 11.10614.946 12.385 12.294 15.651 8.615 9.002 N/A 12.3159.487 11.838 15.717 11.544 7.52 9.293 12.131 9.017

10.027 13.197 N/A 13.100 7.592 8.552 N/A 9.271N/A 12.744 14.858 14.233 N/A 9.241 10.417 10.745

10.217 10.167 15.760 12.633 6.595 6.003 11.891 7.706

Mean 10.952 12.626 13.06 12.801 Mean 7.952 9.055 10.946 10.012

S.E. 0.881 0.424 1.074 0.99 S.E. 0.639 0.456 0.437 0.499

y = 42.6×-1.016

R2 = 0.9634

0

10

20

30

40

0 1 2 3 4 5 6 7

% N conc. (dry weight)

C : N

rat

io

C : NPower (C : N)

Fig. 5. The relationship between N concentration and C:Nratio of R. obtusifoliusbasal leaves.

0

4

8

12

16

Larvae II Larvae III Pupae Adult

Life stage

Mea

n m

axim

um w

eigh

t per

in

divi

dual

(m

g)

OLMH

Fig. 6. 2-b) Experiment 2-Effect of leaf N concentration onG. viridula: The mean maximum weights of G. viridulaindividuals at each life stage among each category (O, L, M,H, error bars are 1 S.E.M.).

feeding on the leaves of different ages.

2. The effect of different levels of nitrogen in R. obtusifolius leaves on the growth anddevelopment of G. viridula over onegeneration (EXP2)

2-a) Experiment 1-Analysis of leaf Nconcentration

Table 4 shows the result of the chemical analysis ofleaf N concentrations from each category of leaves at

the start and at the end of the Experiment 2-Effect ofleaf N concentration on G. viridula. There were sig-nificant differences in leaf N concentration betweenthe categories, and between the dates (F = 65.707, p⁄

0.001 and F = 6.140, p⁄0.05, respectively). However,the differences among the categories over time werenot significant (F = 3.059, p = 0.058). Fig. 4 shows thechanges in leaf N concentration among the plants ineach category at the start and at the end of the Experi-ment 2 (2-b)).

The data collected from the chemical analysis of the


Table 7. 2-b) Experiment 2-Effect of leaf N concentration on G. viridula: The numbers of G. viridula individuals survived, andthe index of development on each observation date

Category Day Lar I Lar II Lar III Pupae Adult Index of development

0 7 3 1.302 4 6 1.604 8 1 2.116 6 3 2.33

O 8 3 6 2.6710 8 1 3.1112 1 8 3.8914 7 1 4.1316 3 4 4.5718 7 5.00

0 3 7 1.702 6 4 2.404 3 7 2.706 2 8 2.80

L 8 9 1 3.1010 2 8 3.8012 9 1 3.9014 2 7 4.7816 9 5.0018

0 10 2.002 3 7 2.704 1 8 2.896 1 8 2.89

M 8 7 2 3.2210 8 4.0012 2 5 4.7114 7 5.001618

0 10 2.002 4 6 2.604 1 8 2.896 9 3.00

H 8 8 1 3.1110 1 7 3.8812 5 3 4.3814 4 4 4.5016 8 5.0018

leaves were analysed for the relationship between Nconcentration and C : N ratio of R. obtusifoliusbasalleaves. Fig. 5 shows the significant relationship be-tween leaf N concentration and C : N ratio of R.obtusifolius(DF = 190, R2 = 0.9634). The data used forthe analysis were from the sampled leaves of the FieldStation Plot in 1992 (n = 120), from Table 1 (n = 32),and from Table 4 (n = 40).

2-b) Experiment 2-Effect of leaf N concentration on G. viridula

Table 5 shows the life stage of individuals at eachcategory and their mean weight on each observationdate. The maximum weights of individuals at each lifestage were compared to each other among the catego-ries of leaf N concentration (Table 6). There is a signi-ficant difference in the maximum weights of indivi-duals among the categories across life stages (F =3.651, p⁄0.001). Fig. 6 shows the differences in ma-ximum weights of G. viridula individuals at each lifestage among each category (O, L, M, H).

Table 7 shows the number of individuals survivedon each observation date, their life stages, and the

index of development. Fig. 7 shows the differences indevelopment patterns of G. viridula individuals amongthe categories of leaf N concentration (O, L, M, H).There are significant differences in the index of devel-opment of G. viridula individuals among the cate-gories;

There is also a significant difference in the timetaken for the completion of each life stage amongcategories (F = 6.612, p⁄0.001). Fig. 8 shows the

O H

L

ns

ns

M

p⁄0.001

p⁄0.001

p = 0.01

p⁄0.001


0

1

2

3

4

5

0 3 6 9 12 15 18

Time (days)

Inde

x of

dev

elop

men

t

OLMH

5Σni xii = 1Index of Development for each date = mmmmm

N

where ni = number of individuals at i stage of life cycle;xi = instar code at i stage of life cycle (Egg = 0, instar I = 1, instar II =

2, instar III = 3, pupae = 4, adult = 5);N= total number of individuals present

Fig. 7. 2-b) Experiment 2-Effect of leaf N concentration onG. viridula: Index of development for G. viridula in eachcategory (O, L, M, H).

0

5

10

0 2 4 6 8 10 12 14 16 18

0

5

10

0 2 4 6 8 10 12 14 16 18

0

5

10

0 2 4 6 8 10 12 14 16 18

0

5

10

0 2 4 6 8 10 12 14 16 18

Time (days)

No.

of i

ndiv

idua

ls

Lar I Lar II Lar III Pupae Adult

a) O

b) L

c) M

d) H

Fig. 8. 2-b) Experiment 2-Effect of leaf N concentration onG. viridula: The survivorships of G. viridula individuals ateach life stages in each category (O, L, M, H).

number of individuals at each life stage in each cate-gory (O, L, M, H).

Discussion

The results from the experiments in this chaptershow quite clearly that the age and the N concentrationof R. obtusifoliusleaves have significant effects on thegrowth rates of G. viridula larvae. First of all, the Nconcentration of R. obtusifoliusleaves is inverselyrelated to the leaf age, although the differences amongthe age categories (B1, B2, B3) were not significant.The decline of quality with leaf age was shown inseveral studies (Raymond, 1969; van Emden et al.,1969; Feeny, 1970; Rockwood, 1974; Larsson andOhmart, 1988). Larsson and Ohmart suggested thatthe leaf toughness may be the cause of the differencesin larval growth when there is no significant differ-ences in leaf N concentration related to leaf ages. Thismay not be the case for G. viridula larvae feeding onR. obtusifolius leaves. If the assumption suggested byLarsson and Ohmart applies to G. viridula, the resultfrom the experiment with 1st instar larvae should bringsignificant differences in larval growth relating to theage of leaves fed to the larvae . But this was not thecase.

However, as Scriber and Feeny (1979) suggested,there may be several factors in plants responsible forthe larval growth: leaf water content, nitrogen content,toughness, and fibre content. Often, these factors arerelated to each others, making it more difficult tounderstand the effect of any one factor on the larvalgrowth. The results from EXP1 may suggest that theeffect on the larval growth of G. viridula may be rela-ted to the combination of leaf toughness and N con-centration of R. obtusifoliusleaves.

On the other hand, there is a clear relationship be-tween leaf N concentration of R. obtusifoliusand theecology of G. viridula. As shown in EXP2, both themaximum weights of individuals at each life stage andthe time taken for the completion of each life stagewere significantly related to the leaf N concentration.

Watt et al. (1995) investigated the relationship be-tween atmospheric CO2 and its effect on insect her-bivores. They suggested that the elevated CO2 inducesthe increased C : N ratio of the plants. Port et. al. (1995)also investigated the effects of gaseous N pollutants oninsect herbivores. They found that the species of nitro-gen pollution (NOx or NHy) may have a substantialeffect on the response of some herbivorous insects. The

understanding of the effects on herbivore populationdue to the increased CO2 and N concentration in habi-tat is becoming more and more important as the pollu-tion level and fertilizer application increase.

The above findings and the findings from this studyall come down to the fact that C : N ratio of food plantplays an important role in the population dynamics ofphytophagous insects. It may therefore be importantto note that the relationship between leaf N concen-tration of R. obtusifoliusand its C : N ratio is not linear,but power. A small increase in leaf N concentration ina nutrient poor habitat will substantially decrease C :N ratio. Therefore, further studies on the combinedeffects of CO2, gaseous N compounds, and nitrogenousfertilizer on the ecology of the insects associated withR. obtusifoliuswould be most desirable.

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(Received 9 December 2005; Accepted 21 December 2005)


effects of within-plant variation in leaf quality on the ecology of g. viridula

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