Res. Popul. Ecol. (1985) 27, 145-157. �9 by the Society of Population Ecology

R E S O U R C E U T I L I Z A T I O N P A T T E R N O F T W O S P E C I E S O F

T E T R A N Y C H I D M I T E S ( A C A R I N A : T E T R A N Y C H I D A E )

Akira KONDO .1 and Akio TAKAFUJI *.2

* Laboratory of Applied Entomology, College of Agriculture, Okayama University, Okayama 700, Japan

** Entomological Laboratory, College of Agriculture, Kyoto University, Kyoto 606, Japan

INTRODUCTION

The two spotted spider mite, Tetranychus urticae K o ch and the kanzawa spider mite,

Tetranychus kanzawai KISHIDA are both important agricultural pests on a variety of crops

in Japan. Both of these closely related species are highly polyphagous, have many kinds

of host plants in c o m m o n (OSAKABE, 1967) and often occur in the same pear and peach

orchards (e.g. TAKAFUJI et al., 1982; UCHIDA, 1980). Most of the tetranychid mites of

the genus Tetranychus including these two species have very high capacity for population

increase and are typical colonizing species (e.g. TAKAFUJI and KAMIBA'ZASHI, 1984).

Although the two species have many similar ecological characteristics, observations

show that there is a marked difference between the patterns of injury caused by them on

their host plants. Injuring scars on leaves by T. urticae exhibit white-green spots and they

spread all over the leaves as the injury advances. With T. kanzawai, the injured leaves

often become brown and they tend to wither faster than those injured by T. urticae.

This difference seems apparent when we rear these mites on potted plants in the labora-

tory: it is much easier to maintain a population of T. urticae at high levels, than that of

T. kanzawai whose population often crashes because of severe deterioration of host plants.

This suggests that the pattern of resource utilization will differ considerably between the

two species.

There are a number of studies on the life history traits of T. urticae (e.g. LAING, 1969;

SAITO, 1979; HERBERT, 1981), whereas only a few are published on T. kanzawai (e.g.

OSAKABE, 1967). However, most of the data obtained are under different physical

conditions and on different host plants, so that comparison of their life history traits is

difficult.

The purpose of this study is to compare the resource utilization patterns of the two

species, based on their life history parameters and the interactions between the mites

and their host plant under the same experimental conditions.

1 Present address: Department of Pest Control, Okayama Prefectural Agricultural Experiment Station, Sanyo-cho, Okayama 709-08, Japan. Author to whom reprint request should be sent.

146

MATERIALS AND METHODS

The mites

The T. urticae population used in this study was collected from Lamium purpueum

growing in a pear orchard in the south of Okayama city in western Honshfl and the

T. kanzawai population from Clerodendron trichotomum around a pear orchard close to the

orchard where T. urticae was collected. The populations were maintained on the seedlings

of the mottled kidney bean, Phaseolus vulgaris L. under constant fluorescent illumination

at 25•176 70-80% R.H. The T. urticae population is a non-diapausing type, whereas

the population of T. kanzawai is a diapausing one (see TAKAFUJI and KAMIBAYASHI, 1984).

For ovipositing adult females from 4.75 to 5.25 days' postmaturation, the length and

width of idiosoma are both significantly longer for T. urticae than for T. kanzawai

(p<0.001).

Experiments were carried out under the same condition as where the populations

were maintained, except for the experiment in a glasshouse, and all replicates in each

experiment were run simultaneously. The host plant used in the experiments was also

the mottled kidney bean.

Life history parameters

One adult female of each species was introduced onto a 1.3-sq. cm leaflet placed on

water-soaked cotton in a plastic petri dish (9 cm in diameter and 2 cm in depth) and was

allowed to oviposit for 4 h. The eggs laid were removed leaving only one egg per leaflet.

The individual cultures were observed at intervals of 6 h until the first egg was laid by

the female of the next generation to determine the developmental times of each stage and

pre-oviposition period. Thereafter, observation was conducted at intervals of 24 h to

record ovisposition and mortality. The leaflets were replaced with new ones on the

10th day after the adult females were introduced and thereafter, every five days. One

adult male was introduced for mating onto each leaflet when females reached the adult

stage. The sex ratio and hatchability were determined from the eggs which were laid

during 24 h by 200 females of various ages.

Tolerance of starvation

This experiment was designed to determine the capacity of the mites to tolerate

deprivation of food and water and thereby mimic conditions during dispersal. Spider

mites which have low dispersal ability are thought to face severe mortality in the process

of their dispersal.

One adult female from 4.75 to 5.25 days' postmaturation was introduced into a

small plastic cage (1.1 cm in diameter and 0.5 cm in depth) covered with a cover glass.

Mortality was checked at intervals of 12 h.

147

Oviposition pattern within a single leaf

In order to analyze the spatial distribution pattern of eggs oviposited by the mites,

one adult female from 4 to 5 days' postmaturation of each species was introduced onto

a 4-sq. cm leaf disc placed on water-soaked cotton in a plastic petri dish as described

above. A plastic cover with 2-sq. m m quadrats was placed on it so that when viewed

through the cover the leaf disc could be visualized as divided into quadrats. Eight holes

(about 8 m m in diameter) were made in the cover to allow for air circulation. The

number of eggs in each quadrat was recorded at intervals of 12 h for 84 h until the eggs

hatched.

The distribution pattern of the eggs on the leaf disc was analyzed by using the relative

index of aggregation ~/m (IwAo, 1968), where m is mean density and/~ is mean crowding.

In the present study, three spatial units were used; 2, 4 and 8-sq. mm. Although there

were eight replicates for each species, the number of eggs laid on leaf disc varied between

the two species. Therefore, in order to compare the values of ~n/m between the two

species, four replicates in which cumulative number of eggs laid for 84 h was nearly equal

were selected for the analysis.

The effect of leaf quality on dispersal

Three kinds of leaf discs (2.6-sq. cm in diameter) with different degrees of mite injury

were prepared for the experiment, i.e., one without any mite injury (G-leaf), one that

had been injured for 4 days by 10 adult females of each of T. urticae and T. kanzawai (Pl-leaf) and one that had been injured by the same number of each species but for 8 days

(P2-1eaf).

Each of the three leaf discs and one G-leaf were placed 1.5 cm apar t on water-soaked

cotton in a petri dish. Sixteen adult females from 4 to 5 days' postmaturation were

introduced onto the former leaf disc. Each pair of leaf discs was bridged with filter paper

(3 cm in length and 1.3 cm in width) which did not touch water in order to allow the

mites to disperse from the leaf disc to the G-leaf. Thereafter, the number of individuals

which dispersed to a G-leaf or drowned on the water-soaked cotton were counted at

intervals of 6 to 12 h for 120 h, to describe the dispersal tendency of the mites. The

individuals which dispersed to a G-leaf were removed at each census.

The patterns of population increase and dispersal on potted plants

This experiment was carried out from early March to mid-May, 1983 in a glasshouse

at the Faculty of Agriculture, Okayama University. The temperature in the glasshouse

was controlled between 15~ and 35~ by using an oil heater and a fan.

Mottled kidney plants cultivated individually in flower pots (24 cm in diameter and

19 cm in depth) were used for the experiment. A white foam plastic disc (34 cm in

diameter and 2 cm in thickness) with a small central hole to accomodate the plant stem

was placed on the surface of the flower pot. The gap between the stem and the hole

148

was filled with foam plastic and the leaves were prevented from touching the disc. The

flower pots were widely spaced in a large water-filled basin to prevent the mites f rom

migrat ing between the pots.

The bean plants used in the experiments were all about 15 cm tall with 11 leaves.

Sixteen adult females from 4 to 5 days ' pos tmatura t ion of each species were in t roduced

onto a single leaf of each plant. T h e y were confined on the leaf for 6 h to allow them

to settle by covering the stem with water-soaked cotton. To record the number of

dispersing adult females, a drawing paper (6 cm in width) coated with t ransparent adhe-

sive (Kinryfl| was fixed a round the white foam plastic disc and the paper was replaced

with new one at each census.

A populat ion census was made at intervals of 2 to 4 days, count ing the number o f

adult females on each leaf and the number of dispersing adult females which were t rapped

on the paper. T h e number of leaves, the number of defoliated leaves and the relative

injury level were also measured. The grade tbr visual assesment of foliage injury was as

follows: 0, no mite injury; 1, up to 25% surface area spotted; 2, 2 5 - 5 0 % ; 3, 5 0 - 7 5 % ;

4, 75 -100%; 5, dead or defoliated leaf. For each species, the experimet was replicated

five times.

RESULTS

Life history parameters

Table 1 shows the developmental times of the immature female stages of the two

species. For most of the stages, the developmental times were shorter for T. urticae than

for T. kanzawa i and the total developmental time of the former was slightly, but signifi-

cantly (p<0 .05) , shorter than that of the latter. Al though the or• period and

the total adult longevity were not significantly different between the two species (p > 0.05),

the pre-oviposition period was significantly shorter for T. urticae (p<0.001) than for T.

kanzawai (Table 2).

Table 1. Developmental times of the immature female stages of the two species of tetranychid mites in days.

Stage T. urticae a T. kanzawai b

Egg 4. 35• 03 4. 39• 02 Larva 0. 91 • 02 0. 94• 02 Protochrysalis 0. 83 • 0. 02 0. 82 • 0. 02 Protonymph 0. 65 • 02 0. 68 • 0. 02 Deutochrysalis 0. 85• 02 0. 84• 02 Deutonymph 0. 84+0. 02 0. 80• 02 Telochrysalis 1.11 • 02 1.15• 02

Total 9. 53• 02 9. 62• 03

a Average of 52 replicates~S.E. b Average of 37 replicates+S.E.

Table 2. The length of various periods in the adult stages of the two species of tetranychid mites in days.

149

T. urticae T. kanzawai

Pre-oviposition period 0. 94+0. 02 (52) 1.30• 03 (37) Oviposition period 19. 65 • 1.41 (31) 16. 36• 98 (27) Post-oviposition period 4. 29• 84 (31) 4. 77• 69 (27)

Total adult longevity 25. 48+ 1.59 (31) 22. 42• 1.08 (27)

Numerals in parentheses are the number of individuals tested. MeanzFS.E.

F igure t shows the age-specific survival and oviposi t ion rates of the two species.

T h e oviposi t ion rates o f the two species reached m a x i m a on the 14th d a y and then

decreased at a n e a r l y equa l rate. T h e da i ly egg p roduc t ion of T. urticae was grea te r t han

tha t of T. kanzawai t h roughou t the per iod. T h e ha t chab i l i t y of eggs was not s ignif icant ly

different ( p > 0 . 0 5 ) be tween the two species (T . urticae: 95.6%, n-=1866; T. kanzawai:

94 .5%, n-~1732). T h e survival ra te of the i m m a t u r e stages was 9 8 % for T. urticae a n d

1,0 , . , ~L -

O.S _J

lo * ; T. URTICAE W

8 ,

o r 10 2 0 30 ,40 5 0

AGE IN DAYS

Fig. 1. Age-specific survival and oviposition rates of two species of tetranychid mites. The numbers of individuals tested were 31 for T. urticae and 27 for T. kanzawai. Survival rate (upper figure) indicates the ratio of the number of individuals which were surviving at each age. Oviposition rate (lower figure) indicates the mean number of eggs oviposited per surviving female.

150

100 ...... .~_ . . . . .

5 0

<> -- URTICAE (3.11 �9 o.lo) 'b,,,, ~ _ C~

co 0 o---.o T. KANZAWAI~ (2.41 + 0.09)1 I " ' ' "O, . "O. n ~ ~

1 2 3 4 5 DAYS

Fig. 2. Survival rates of adult females of two species of tetranychid mites deprived of food and water. The numbers of individuals tested were 62 for T. urticae and 58 for T. kanzawai. Numerals in parentheses are the mean life span (average--4-S.E.) in days.

91% for T. kanzawai. T h e sex ra t io ( ~ / ( ~ + ~ ) ) was s ignif icant ly h igher (p<0 .001)

for T. urticae (T. urticae: 74.3%, n = 1 7 6 1 ; T. kanzawai: 65.9%, n = 1 6 2 5 ) . T h e decl ine

in survival ra te for a d u l t females of T. kanzawai occur red ear l ier t han tha t of T. urticae.

T a b l e 3 shows the pa rame te r s re la ted to the popu la t ion increase of the two species.

A l though the mean genera t ion t ime (T) was sl ightly shor ter for T. kanzawai t han for

T. urticae, the first oviposi t ion of the la t te r occur red signif icant ly ear l ier (p<0 .001) and

the total egg p roduc t ion of the l a t t e r was also s ignif icant ly grea te r ( p<0 .01 ) . Conse-

quent ly , the values of the intr insic ra te of na tu ra l increase (r~,) and the net r eproduc t ive

ra te (Ro) were both g rea te r for T. urticae t han for T. kanzawai.

Tole rance of s ta rva t ion

F igure 2 shows the survival rates of the adu l t females of the two species when depr ived

of food and water . T h e survival ra te of T. urticae was h igher than tha t of T. kanzawai

t h roughou t the per iod, thei r decreas ing pa t te rns be ing s imi lar to those observed in the

age-specific survival rates, and the m e a n length of survival per iod was s ignif icant ly longer

for T. urticae than for T. kanzawai (p<0 .001) .

Table 3. Population parameters of the two species of tetranychid mites.

T. urticae 7". kanzawai

Age at first reproduction (days) Mean generation time, T Total number of eggs/ Net reproductive rate, Ro Intrinsic rate of natural increase, rm per day

10.52_+ 0.05 (52) 10.92:k0.04 (37) 20. 60 19. 00

178.58___10.20 (31) 142. 19:t:9.71 (27) 128. 86 82. 08

0. 292 0. 268

Numerals in parentheses are the number of individuals tested.

151

Ovisposition pattern within a single leaf

Figure 3 shows changes in cumulative number of eggs (m) and the degree of aggre-

gation in the spatial distribution of eggs (~) of the two species for detached leaf discs. The value of r~/m of T. kanzawai was higher than that of T. urticae in all the three spatial

units, showing that the former clusters its egg to a greater extent.

The effect of leaf quality on dispersal

Figure 4 shows Changes in the cumulative number of adult females which dispersed from each of the three kinds of leaf discs with different degrees of mite injury. In both

| BB T. URTICAE

~ [ ~ 20 ,x\\~, T. KANZAWA, ] ~ I

80

E 6O \ *E

v 40 Z o

20 (D laJ rr (D 0 (D

I_1_ 0

i , i LI_I

CD LI_I r'~

A

4ofB 20

0

2~ c 10

0 '

i .... i T. URTICAE

o- .... o T. KANZAWAI I I I I i I I

. . . . . . . . . + . . . . . . . . F . . . . . . . . . . . . . . . . . . . . . + . . . . . . . i I s i V T r T ~ ~ T

f I I I I I I

0 - . . . . . . . o - . . . . . . . . 0 - . . . . . . . O- . . . . . . . -o - . . . . . . . o - . . . . . . �9

12 24 ~6 4'8 gO ;2 84

HOURS Fig. 3. Spatial distribution patterns of the eggs oviposited by two species of tetranychid mites

on detached leaves. Three spatial units were used: A, 2 • B, 4 • C,

8 x 8 m m quadrats. - -O- - and - 0 - show the average of four replicaes ~z S.E.

152

/ ....... T /"

/ / f ,,,,, KANZAWAI

/ /

/ / ' l I

i l! '

/

/

418 712 9/ I 0 24 6 120

HOURS

Fig. 4. Effect of leaf quality on the dispersal tendency of adult females for two species of tetranychid mites. - -if]--, - i - (G-leaf ~ G-leaf) ; --A- -, -A- (Pl-leaf ~ G-leaf) ; --�9 -0-- (P~-leaf--+ G-leaf). G-leaf is a leaf without any mite injury. P1- and P2-1eaves are leaves that have been injured by 10 adult females of each of T. urticae and 7'. kanzawai for 4 and 8 days, respectively. Each point is the average of 8 replicates•

,_.J

n

n z

10

i , I

I.x.I O_ 09

i_L. o 5

Cr t.O r'n

z

~�9 s / , ~ I~ s

/

/ / f .J

= = T. U R T I C A E 0 - - - - 0 i~___.~ T. I O-- - - -Q

species, the poorer the leaf quality was, the sooner the dispersal occurred, and most

individuals finished dispersing by the 120th h, excepting the case of T. urticae in the

"G-leaf to G-leaf" system. T. kanzawai dispersed sooner than T. urticae. Especially in

the "G-leaf to G-leaf" system, the dispersal pattern differed greatly between them; the

majority of individuals of T. kanzawai dispersed by the 120th h, whereas the proportion

of dispersed individuals of T. urticae was only less than 50% by that time. This showed

that T. kanzawai responded more sensitively to the deterioration of leaf quality, dispersing

more quickly from leaves than T. urticae, and/or the former injured the leaves more severely

than the latter.

The patterns of population increase and dispersal on potted plants

Figures 5 and 6 show changes in the number of adult females on bean plants and the

number of adult females dispersing from plants for T. urticae and T. kanzawai, respectively.

Both populations reached their peaks in the 3rd generation, but the peaks occurred

earlier for T. kanzawai than for T. urticae (the 48th-52nd day for T. urticae and the 45th day

for T. kanzawai). The peak densities of T. urticae per plant, however, were 2 to 3 times

153

20000

Ld

C:; Ioooo Z

100

b~

t-', ~ L U 0

~C~ u J 5 0 LLI - J C l 0

6(33z/

. . . . . . . L J"

V / / ~ / / / H / / .

' 1 o " , ' o " go "

~L~s3~5(331) 31677

,, ..... ............. .Y

o ' 2'0 ' a'o ' 6'o '

DAYS

29874,! 33~~176176

1.0

Ct5

.................... o

20 40 60

Fig. 5. The pat tern of population increase and dispersal for T. urticae on living host plants. - 0 - , number of adult females per plant; ( ), number of adult females per leaf; -C~-, number of dispersing adult females; - - , number of living leaves; ~ , number of defoliated leaves; - - - , injury level. Three examples from 5 replicates are shown.

1 5 0 0 0

o ' )

:~ 10000

b 5 0 0 0

0 7

1 0 0

bm

_.J

123 0 i..u. IJ-I

I - - ( ~

t"-,

A~ I i ~

~233)

_/ . . . . . . I

.. ................... .......

i i 0 210 I 410 OlO I ' 2'o ' ,;o ' go ' o DAYS

Z . ............ 1.0 ~_

""' "" ' " " " t r-"

2'0 4'o 'go '

Fig. 6. The pattern of population increase and dispersal for 7". kanzawai on living host plants. -@-, number of adult females per plant; ( ), number of adult females per leaf; - O - , number of dispersing adult females; - - , number of leaves; [~, number of defoliated leaves; - - - , injury level. Three examples from 5 replicates are shown.

154

higher than those of T. kanzawai, and the peak densities per leaf were also higher for the

former, showing that the population size achieved on host plants was much greater for

T. urticae than for T. kanzawai. The peaks of the number of dispersing adults also

occurred earlier for T. kanzawai than for T. urticae (the 52nd-60th day for T. urticae and

the 49th-53rd day for T. kanzawai). This result corresponded well to the result from the

leaf disc experiment (Fig. 4) which showed that T. kanzawai dispersed sooner than T.

urticae.

Although the peak densities of T. urticae on plants were much higher than those of

T. kanzawai, conversely peaks in dispersing adult females were 2 to 4 times higher for

T. kanzawai than for T. urticae. In addition, the numbers of dispersing T. kanzawai

decreased abruptly after distinct peaks, whereas those of T. urticae populations showed

no distinct peaks but decreased gradually. This difference between species occurred

because most of the dispersing individuals of T. kanzawai dropped onto the white foam

plastic discs when heavily injured leaves defoliated, whereas some individuals of T. urticae

dispersed by spinning down from the tips of the leaves and drifting on air currents beyond

the edges of the discs where their dispersal was not recorded.

The growth of plants was much more heavily hindered by T. kanzawai than by

T. urticae. The maximum numbers of leaves on the plants with T. urticae were about

twice those with T. kanzawai. The numbers of leaves on the plants decreased with

increasing mite injury for each mite species. The leaves of the plants with T. kanzawai

were almost completely defoliated by about the 50th day, whereas those with T. urticae

began to decrease rapidly on about the 60th day but no complete defoliation occurred,

though most leaves deteriorated severely. The defoliation occurred more than 20 days

earlier on the plants with T. kanzawai than on those with T. urticae. Consequently, the

cumulative injury level of the plants was higher with T. kanzawai than with T. urticae

and its peaks occurred sooner with the former (around the 60th day for T. urticae and

around the 50th day for T. kanzawai). The leaves of the plants resurged after abrupt

decreases in mite density, but the new leaves were very small. During the course of this

experiment, there was no significant difference in mean leaf size between the plants with

the two species.

DISCUSSION

The feeding damage caused by the two closely related species were quite different

from each other. The growth of host plants was much more heavily hindered by T.

kanzawai than by T. urticae. It is known that cells punctured by T. urticae collapse and

then adjacent unpunctured cells show coagulated protoplasts and with further damage,

degenerative processes in the chloroplast structures (MoTHES et al., 1982). No such study

on the physiological mechanism in feeding damage by T. kanzawai has been reported.

However, it seems that there may be some qualitative differences between the feeding

mechanisms of the two species because the leaves injured by T. kanzawai become brown

155

and then wither, which seldom occurs in the leaves injured by T. urticae unless the injury

is extremely severe. This difference in feeding damage on host plants then resulted in

distinct differences between the patterns of population increase of the two. T. urticae

numbers increased to a maximum without severe defoliation, and then the mites dispersed.

This pattern in population increase for this species was quite different from that of T.

kanzawal which caused severe deterioration of host plants and dispersed before achieving

maximum population size. Thus, the peak density attained by T. urticae per plant was 2 to 3 times greater that of T. kanzawai and it is evident that the former utilizes a given

food resource much more efficiently, under the experimental conditions used in this study.

WANmUCm and SAITO (1983) studied the processes of population growth and resource

utilization pattern of two species of tetranychid mites, i.e., Olygonychus ununguis (JAcom)

and Panonychus citri (MeGREoOR) and showed that the boundary of the microhabitat

was a single leaf for O. ununguis but sapling or foliage for P. cirri. No such clear difference

is seen between the two species used in this study, except that T. kanzawai tends to cluster

its eggs more than T. urticae within a leaf. The difference in their resource utilization

pattern is therefore attributable to the difference in the interactive pattern with host

plants and the difference in their responses to crowding and/or the deterioration of host plants. The dispersal experiments on detached leaves with various degrees of injury

show that T. urticae is more tolerant than T. kanzawai of over-crowding and/or resource

deterioration. This is also supported by the fact that the former can survive longer when

deprived of food and water.

The intrinsic rate of natural increase, r,, is an index which shows the maximum rate

of potential increase for a species under given environmental conditions and is often used

as criteria when comparing the life history traits and habitat characteristics of species

(e.g. STEARNS, 1976; SOUTHWOOD, 1977). The value ofrm was higher for T. urticae than

for T. kanzawai but the difference was only slight. Nevertheless, the maximum popula-

tion size achieved on host plants was much greater in the former. This shows that for

comparing the capacity for population increase between species, it is essential to take

into consideration the interaction with their food resources.

In the present study, resource utilization patterns of the two species were compared

on a single plant and on the host plant common to them. However, both species are

highly polyphagous and in the field they utilize various host plants as they are available

and suitable as food in different seasons (see TAKAFUJI and KAMIBAYASHI, 1984). There-

fore, in order to understand the significance of the difference in resource utilization

pattern between the two species, it is necessary to compare their population dynamics

in a habitat where the availability of various host plants fluctuates both temporally and spatially.

SUMMARY

Life history and resource utilization pattern were compared between two closely

related mite species of the genus Tetranychus.

156

1. Tetranychus urticae deve loped m o r e qu ick ly a n d h a d a h ighe r ovipos i t ion ra te t h a n

T. kanzawai. Co n s eq u en t l y , the p o t en t i a l for p o p u l a t i o n increase was g rea te r for T.

urticae t h a n for T. kanzawai b u t the di f ference was sl ight.

2. T . urticae was m o r e t o l e r an t o f a de t e r io r a t i ng food resource a n d / o r ove r -c rowding .

T. kanzawai r e s p o n d e d m o r e sensi t ively to food de t e r io r a t i on a n d dispersed m o r e quickly .

3. T h e r e was a cons ide rab le d i f ference b e t w e e n i n j u r y pa t t e rn s by the two species.

T. kanzawai d a m a g e d host p l an t s m o r e severely, caus ing ear l ie r a n d m o r e extens ive

defo l ia t ion of the p lants .

4. T h e m a x i m u m p o p u l a t i o n size ach ieved b y T. urticae was 2 to 3 t imes g rea te r t h a n

t h a t o f T. kanzawai o n po t t ed host p lants . T h i s showed tha t the fo rmer could ut i l ize

a food resource m u c h m o r e efficiently.

ACKNOWLEDGEMENTS: We wish to thank Dr. R. LAMB, of Agriculture Canada at Winnipeg, for reading

the manuscript and verbal correction. We are indebted to Prof. T. YOSHIDA of Okayama University,

Prof. E. KtlNO and Dr. H. TAKEDA of Kyoto University for valuable advice to this study. Lastly, we thank

Mrs. C. TAm-KONDO, and Mr. H. KAMEZAKI of Okayama University for their technical assistance in this

study.

REFERENCES

HERBERT, H.J . (1981) Biology, life tables, and innate capacity for increase of the twospotted spider mite,

Tetranychus urticae (Acarina: Tetranychidae). Can. Entomol. 113: 371-378.

IWAO, S. (1968) A new regression method for analyzing the aggregation pattern of animal populations.

Res. Popul. Ecol. 10: 1-20.

LAING, J. E. (1969) Life history and life table of Tetranychus urticae KocH. Acarologia 11: 32~,2.

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157

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