Entomol. exp. appl. 57: 143-150, 1990. �9 1990 Kluwer Academic Publishers. Printed in Belgium. 143

Are the oviposition traits of the South India strain of Callosobruchus maculatus maintained by natural selection ?

Rodger Mitchell & Clement Thanthianga l Department of Zoology, Ohio State University, Columbus, OH 43210, USA ; ~ Present address: Pachunga College. Aizawl, Mizoram, India

Accepted: June 7, 1990

Key words: Bruchidae, Callosobruchus maculatus, competition, development, evolution, fecundity, growth rates, host preferences, life tables, mortality, natural selection, net reproductive rate, oviposition traits

Abstract

The deposition of eggs by this strain of Callosobruchus maculatus (Fab.) (Bruchidae: Coleoptera) departs from randomness in three ways; eggs are uniformly dispersed, oviposition rates drop when beans begin to carry 2 or more eggs, and there are sharp host preferences. Using random egg placement for the unspecialized condition, these traits are evaluated for their effect on a female's contributions of offspring to the next generation (R o, the net reproductive rate). The major increases in R o result from females dispersing eggs so uniformly that larval competition is either reduced or eliminated. Females reduce their oviposition rate when the larva from an egg added to a bean is almost certain to die in competitive encounters. Host preferences and larval survival in a host are positively associated with the abundance of the host in South India. The three oviposition traits act together to give and R o that is 25-50~o than that of eggs placed at random. These traits are known to be variable and heritable, hence, the conditions necessary for natural selection are satisfied.

Introduction

Darwin's (1859) succinct definition of natural selection - 'But if variations to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterized' - can be recast as a syllogism. Natural selection will act if(1) traits are variable, (2)variants differ in their fitness, and (3) the trait-fitness association is heritable (Endler, 1986). We use that syllogism to evaluate the oviposition traits of the South India strain of Callosobruchus maculatus.

The oviposition behavior of this strain deviates from randomness in three ways (Thanthianga & Mitchell, 1990): (1)females disperse their eggs uniformly (Mitchell, 1983; Messina, 1989; Messina & Mitchell, 1989); (2)the host prefer- ences are among the most pronounced in the species (Wasserman, 1986); and (3)females retain eggs as the density of eggs in an environ- ment approaches 2 eggs/bean (Credland, 1986). Interfertile local populations differ in these three traits and breed true (Credland et al., 1986; Dick & Credland, 1984; Messina, 1989; Messina & Mitchell, 1989; Wasserman, 1986) which estab- lishes two of the conditions necessary for natural selection. The proof for the maintenance of these

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traits by natural selection will be complete if these traits result in females contributing more daughters to the next generation than they would with random oviposition.

The first paper in this set (Thanthianga & Mitchell, 1990) characterized oviposition be- havior and density dependent inhibition of fecundity (Thanthianga & Mitchell, 1990). Here we describe the growth rates, burrowing behavior, survival, and fecundity for individuals developing at densities of 1 to 3 eggs/bean in the four major hosts. The details of larval growth and behavior are given because deviant feeding re- sponses indicate times of stress and may help identify the causes for the differences in host specific mortality. Survivorship and fecundity are used to calculate a partial net reproductive rate (Ro*) for eggs at three densities in each of the hosts. Ro* is the product of survival of daughters times their fecundity. It excludes post-emergence factors that are independent of oviposition deci- sions, adult survival, the probability of host dis- covery by females, and losses to parasites. The rate of population growth (rm) cannot be cal- culated because fecundity is altered in response to the number of egg/bean and the post-emergence adult demography is not known.

When Ro* is known for each oviposition site used by a female, her Ro* can be calculated. If the Ro* of an observed set of eggs is greater than the same number of eggs placed at random, it com- pletes the proof for a contemporary selective advantage for the oviposition behavior. Such an advantage does not necessarily explain how traits evolved because the traits may have evolved under quite different conditions. These findings apply only to the evolution of the traits of the sedentary form under storage conditions.

Materials and methods

The history of the South India strain, basic cul- turing, and experimental procedures are in Than- thianga & Mitchell, 1990). Representatives of hosts common in South India were used: mung bean (the Berkin variety of Vigna radiata L.),

cowpeas (California # 5 black-eyed pea, V. unguiculata (L.) Walp.), chickpea (Cicer arie- tinum L.), and pigeonpea (Cajanus cajan (L.) Millsp.). The first two, from Johnson Seed Co., Enid, OK., are used because they are genetically stable varieties. The other pulses were single lots purchased in Columbus, Ohio.

Data on the growth and life tables were taken from even aged cohorts of eggs opened daily to record the instar, location, burrowing activity, and weight (wet and dry). Competing larvae were identified by putting a mark by an egg of known age and allowing a second egg to be laid 4, 8, or 12 days later. The burrows from the marked and unmarked eggs were traced. Beans with no exit after 60 days were opened to determine when the larva died.

Results

Development of single larvae in mung (Table I). Eggs developed to the adult in 29-32 days at 22-24 ~ Milky colored eggs (0.0235 mg fresh wt) developed into a segmented embryo by day 4. The head, apparent by day 5, acquired a black pigment by day7. Eggs hatched on days 7-8. Instar I larvae carry numerous long spine-like setae and the body tapers posteriorly. The larva took 2 days to chew perpendicularly through the bean test and begin its feeding on the cotyledons.

When inside the bean, the larva turned almost 90 degrees to tunnel just below the bean test as Raina (1970) reported. Larval weight increased 8-fold during the 5 days of instar I. The body of instar II distended dorsally and the body took on a convex shape. After 2 days of burrowing parallel to the surface, instar II turned toward the center of the bean. Instar II fed for 3 days and its weight increased 4-fold.

After 3 days instar III had increased in weight 5 times and molted. Instar IV fed vigorously, opened out a large central chamber. It assumed a pupa-like shape after 7-8 days and is called the prepupa. The prepupa chewed a passage to the bean test, cleared a window beneath the test,

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Table 1. Duration of development in days and average _+ s.e. fresh weight (mg) of larvae at the end of each instar in four hosts. Ten larvae were weighed at the end of each molt and 10 pupae were weighed at the start of pupation. Mung was the control in t-tests for differences in weight

Instar Hosts

Mung Cowpea Chickpea Pigeonpea

I duration 5 5 5 7 weight 0.11 _ 0.01 0.10 + 0.01 0.10 _+ 0.01 0.07 + 0.004**

II duration 3 3 3 4 weight 0.21 _+ 0.02 0.17 + 0.02 0.16 + 0.02* 0.08 _+ 0.01"**

III duration 3 3 3 4-5 weight 1.19 _+ 0.06 0.99 + 0.13 0.91 + 0 .11 ' 0.27 + 0.02***

IV duration 5 5 5 -6 6-7 weight 8.58 + 0.91 7.63 + 0.37 7.86 + 0.41 8.24 + 0.32

Pupa duration 7-8 8-9 9-10 8-10 weight 8.27 _+ 0.55 7.37 + 0.53 7.46 + 0.48 7.51 + 0.37

* P < 005; *** P < 0001; t-tests against mung.

retreated to the central chamber, lined it with frass as described for Ctenocolum janzeni (Johnson, 1977), and pupated. The weight at pupation indicates a 4-fold increase in weight during instar IV. After 4 days of pupation the mandibles and compound eyes became pigmented. The adult body was apparent on day 7 and the adults first emerged 29 days after the eggs were laid. Survival was high, 80~o of the eggs matured to the adult (Table 2).

Single larvae in other hosts. The burrows in cow- peas followed the sinuous patch seen in mung, but

the burrowing was inconsistent in chickpeas and pigeonpeas. About half the larvae burrowed as in mung, others chewed a path directly toward the center, and some paths were intermediate. Some prepupae in chickpea cut incomplete exits. A few larvae in pigeonpea cut normal exits and pupated backwards. These pupae eclosed normally but they could not exit and died in the pupal chamber.

Females dissected from faulty pupal chambers were paired with normal males and given 50 beans for oviposition. Five of the 10 females from chick- peas deposited a few infertile or oddly shaped eggs during a normal life span. The other 5 females

Table2. Survival (10 and fecundity for beetles developing singly in the four hosts at 22-24 ~ Devlations from mung were tested with confidence limits for percentages for survivorship and t-tests for differences in fecundities

Control Experimentals mung

Cowpea Chickpea Pigeonpea

n 232 legg 1.000 1, 0.927 ln-lv 0.867 ]pupa 0.833

l,dult 0.800 Development (days) 35.7 Fecundity 74.8 _+ 4.6 Ro* 29.9

209 189 106 1.000 1.000 1.000 0.947 0.921 0.920 0.923 0.921 0.613 ** 0.923 0.783 0.532'* 0.828 0.619'* 0.336**

36.6"* 38.0"* 42.1"* 68.1 _+ 5.1 69.0 + 2.2 72.4 + 4.7 28.2 21 7 12.5

** P < 0.01.

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were fertile, but laid fewer eggs, 53.2 + 3.8, than normal females. Ten females pupating backwards in pigeonpeas had normal fecundity (68.5 + 7.0). Oviposition behavior was normal among females that pupated abnormally.

Larvae feeding in cowpeas always weighed insignificantly less than those in mung (Table 1) but their survival was identical (Table 2). Instars I I - I I I always weighed less in chickpea and pigeonpea, but instar IV grew very rapidly. Instead of the 7.2 increase in weight seen in mung, instar IV weight increased 8.6 times in chickpea and 30 times in pigeonpea (Table 1). If some larvae could not digest the food, while others grew at a con- sistently high rate, the weights should be less variable as maladapted larvae die. The weights of instar III (coefficients of variation = 0.01-0.03) were less variable than after instar IV (pupal c.v. = 0.3-0.6). It appeared that instar IV de-

veloped a physiological tolerance for chickpea and pigeonpea.

Competition. One larva survives competition (Thanthianga & Mitchell, 1987) and if the winner had the same survival as that of a larva alone, then survival in beans with n-eggs would be the single larva survival divided by n. Survival fits this model in all the hosts (Table 3). Larvae respond to each other in mung and cowpea. One larva fed as it would when alone, while the other was inhibited and fed slowly in a superficial burrow. Of 60 pairs, 59 of the larvae shallow burrows weighed less than the smallest larva in a central burrow (Table 4). Larvae in shallow burrows died if their burrow was intersected by the dominant larva (Thanthianga & Mitchell, 1987).

Competitors are normally excluded in chick- peas and pigeonpeas. Larvae showed no bur-

Table 3. Survival of adults when similarly aged larvae compete tested against the hypothesis that survival = (survival of single larvae)/no, of eggs). The observed survival fit the null hypothesis (confidence limits for percentages)

Host (alpha) Eggs/bean

1 Egg 2 Eggs 3 Eggs 4 Eggs

Mung (0.684) beans 1295 1551 363 24 ladut t 0.744 0.3 0.247 0.177 (expected) (0.372) (0.248) (0.186) Ro* 27.8 14.6 9.2 6.6

Cowpea (0.876) beans 693 ladul t 0.758 (expected) Ro* 25.8

263 33 0.385 0.242

(0.379) (0.253) 13.1 8.2

Pigeonpea (0.126) beans 42 36 45 1.dul t 0.394 0.230 0.150 (expected) (0.197) (0.131) Ro* 14.2 8.3 5.4

Chickpea (0.347) beans 53 21 5 ladui t 0.591 0.373 0.184 (expected) (0.296) (0.197) Ro* 20.4 12.6 6.4

Table 4. Average + s.e. weight (rag) of controls (20 larvae alone) and 10 pairs of competing larvae. Competitors in mung and cowpea were segregated by the location of their burrows. In chickpea and pigeonpea the burrows were identical and larvae were segregated by weight to determine whether the weight differences were as great as in the other hosts. T-tests were used to test for deviations from the controls

Age in 1 Larva/bean 2 Larvae/bean (days) (control)

Central Superficial

Mung 8 0.44 + 0.004 0.39 + 0.038 0.19 + 0.022***

14 5.98 + 0.009 6.02 + 0.018 2.46 + 0.013"**

Cowpea 8 0.38_+ 0.004 0.41 + 0.007* 0.31 + 0.020***

14 4.08 _+ 0.416 3.35 + 0.340 1.32 + 0.132'**

Heaviest Lightest

Chickpea 8 0.39 + 0.001 0.39 + 0.007 0.34 _+ 0.011"*

14 3.46 + 0.340 3.94 + 0.340 2.02 + 0.147"

8 0.15 + 0.004 0.20 + 0.002** 0.15 + 0.009 14 0.90 + 0.054 1.06 + 0.076 0.60 + 0.054*

* P < 0.05; *** P < 0.001.

rowing response to each other. To see if these larvae differed in their growth, competing pairs of larvae were segregated by weight. There seemed to be no growth response because differences in weight were less than those of larvae segregated by behavior in mung and cowpea (Table 4). The first larva to enter has an advantage over larvae that enter up to 3 days later (Thanthianga & Mitchell, 1987). The second egg is not added until after most beans carry an egg (Mitchell, 1975; Thanthianga & Mitchell, 1990) and survival varied among larvae from eggs laid 4-12 days apart. Older larvae survived if the younger larva remained inhibited during pupation. If the younger larva started feeding when the dominant larva pupated and breached the pupal chamber, the older individual usually died. When eggs differed in age by 4 or more days, the youngest larva had about a 50 per cent chance (n = 97) of surviving. Multiple exits do not occur when eggs are similar in age but if the difference was 4 or more days, two

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Table 5. Alpha, the relative preference (Thanthianga and Mitchell, 1990) and site specific R0* from Table 2

Alpha Ro* On beans with X eggs

1 Egg 2 Eggs 3 Eggs

Mung 0.684 27.8 14.6 9.2 Cowpea 0.876 25.8 13.1 8.2 Pigeonpea 0.347 14.2 8.3 5.4 Chickpea 0.126 20.4 12.6 6.4

exits are common (29%) from cowpeas, but rare in mung (1 ~o ).

Discussion

Grain stores in South India are dry with tempera- tures of 22-30 ~ and carbohydrates (e.g. nectar) are not available. That environment resembles most ambient laboratory conditions, hence, the demography in the laboratory may be representa- tive of feral populations.

A partial net reproductive rate (Ro*), was cal- culated from fecundity and survival to the adult. This measures the contributions of adults to the next generation for laboratory populations in which parasitism is excluded and the losses asso- ciated with dispersal and host discovery by the adult are excluded. Host species affects Ro* but mortality from larval competition is the major determinant of Ro* (Tables 3 and 5). Ro* is re- duced in proportion to the eggs/bean because all but one larva is usually eliminated in competition (Thanthianga & Mitchell, 1987). The oviposition responses to eggs/bean, which reduce competi- tion, take precedence over the cues identifying the kind and size of hosts in releasing oviposition (Thanthianga & Mitchell, 1990).

When presented with two species of host, mung or cowpea are clearly preferred over either chick- pea or pigeonpea (Table 5). Survival to the adult is over 0.75 in mung and cowpea giving an Ro* of 26-28 and preferences are weak when choosing between mung and cowpea. Survival, which is 0.60 or less in pigeonpea and chickpea, reduces

148

the Ro* to 14-20. The two hosts supporting the highest Ro* are more than twice as attractive than the other hosts (Table 5).

The effect of host selection and hyperdispersion is obvious in a comparison of the observed Ro* with Ro* calculated for two hypothetical distribu- tions: the Poisson representing the unspecialized trait (random egg placement) and the uniform dispersion. The records of females given 10 or 50 mung were sorted by eggs/bean and the Ro* for the three distributions calculated for each female (Table 6). The observed Ro* was usually within 4 ~o of the uniform distribution, hence, very little would be gained if females achieved perfect uni- formity. Deviations toward randomness at den- sities below 2 eggs/bean could reduce a female's

Table 6. The effect of hyperdispersion on average Ro* for eggs a female laid on mung. Ro* is given for egg distributions from Thanthianga and Mitchell (1990) within 5 eggs of the designated value

Eggs/bean (n) Ro* and % increase over the Poisson

Poisson Observed Uniform

On 50 beans

0.50 (56) 22.0** 26.9 i 0.25 27.8** 22% 26%

1.00 (118) 17.8"* 26.6 + 0.10 27.0** 49% 52%

1.50 (45) 14.5"* 18.8 + 0.08 18.9 30% 30~

2.00 (52) 12.2"* 14.3 + 0.07 14.6"* 17% 20%

On 10 beans

0.50 (3) 20.9** 27.8 _+ 0.0 27.8 33% 33%

1.00 (26) 17.0'* 24.6 + 0.6 24.9 45~o 46%

2.00 (21) 12.0"* 14.3 + 0.8 14.4 19% 20%

3.00 (18) 8.9 9.4 __ 0.3 9.4 6% 6%

4.00 (15) 6.8 6.9 _+ 0.2 6.8 2% 0%

5.00 (11) 5.4 5.00 _+ 0.1 5.8 -7% 2%

** > 99% confidence limits for the observed Ro*.

contribution to the next generation by as much as 25~o. At high densities, 5 eggs/bean, little is gained from hyperdispersion because the yield of adults from a Poisson distribution converges to that of the uniform (Table 6). The failure to discriminate eggs/bean at high densities could be adaptive in the sense of females ceasing to show discrimination when there is nothing to gain. Of course it may be an artifact shown by females confined to densities they always escape in nature.

The observed placement of eggs in preference experiments (Thanthianga & Mitchell, 1990) and the site specific Ro* (Table 7) were used to calculate R0* for a female's eggs in: (1) observed dispersion, (2) the Poisson, (3) the observed dis- tribution with no choice of host (both hosts used equally), and (4)the dispersion giving the maxi- mum Ro*. The beetles always attained an Ro* greater than the Poisson and, except for pigeon- pea/chickpea, their Ro* was greater than the no choice model. The difference between the 'no choice' and 'observed' columns are the gains attributable to host preferences. When prefer- ences are strongest (alpha 0.91-0.94, Table 7) Ro* is much greater than that of the Poisson and no choice distribution, although it was still below the maximum. Except for pigeonpea/chickpea combinations, females significantly increased Ro*, even when preferences were weak (alpha = 0.70-0.78).

The difference between the 'no choice' model and a random egg placement (Poisson) is the gain attributable to the avoidance of beans with above average egg numbers when the host species is not discriminated. Hyperdispersion increases Ro* much more than host discrimination except for the combinations ofcowpea or mung with pigeon- pea. Females begin to withhold eggs at densities of 1.1 eggs/bean (Thanthianga & Mitchell, 1990). Survival of eggs added to occupied beans within 4 days of the first egg is so low that nothing is lost by withholding eggs under crowded condi- tions. The eggs held back would enjoy a much higher survivorship if a female finds a new supply of beans.

Females show the strongest host preferences when presented with representatives of the hosts

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Table 7. The effect of selective oviposition on Ro*. Site specific Ro* and data on preferences in 1 : 1 combinations of two hosts (Thanthianga and Mitchell, 1990) are used to obtain the Ro* for four distributions: the Poisson, the observed, the observed with no host discrimination, and the maximum. The percent deviation from the Ro* of the Poisson is given. The host preferred in the combination xs listed first and its alpha is given

Host Alpha i Ro,

Poisson Observed dispersion Maximum

No choice Observed

Cowpea 0.9% 19.7 ** 23.7 25.0 • 0.7 28.0 ** chickpea 20 ~ 27 % 47 %

Mung 0.94 d 20.7"* 25.1 * 27.8 + 0.9 29.6 chickpea 21% 34 % 43 %

Cowpea 0.91 d 16.1 ** 19.3 ** 24.6 + 0.6 27.5 ** pigeonpea 20 % 53 % 71 o

Mung 0.78 b 16.9"* 21.1 ** 26.0 _+ 0.9 28.9** pigeonpea 25 % 56 % 71 ~

Pigeonpea 0.70 b 14.1 16.8 * 14.6 + 0.7 21.3 ** chickpea 197o 4% 51 ~ o

Cowpea 0.7% 22 2"* 28.2 27.8 + 0.2 29.7'* mung 27 ~ 25 % 34 %0

* > 95%; ** > 99% confidence limits for the observed Ro*. ' Subscripts indicate values that are not significantly different from each other.

grown together in intercrops, mung, cowpea, and pigeonpea (Table 7). The abundances of the host in the Tirunelveli District (Department of Statis- tics 1985) are positively associated with prefer- ences. Mung beans and the closely related black- gram occupy nearly twice the acreage (27,000 ha) as the next most common host, cowpea (2,000 ha). Pigeonpeas are less common (under 500 ha) and less than 200 ha of chickpea are usually planted in the district.

Females leave no more offspring than they would leave from random egg placement when presented with the hosts they are least likely to encounter, pigeonpea/chickpea (Table 7). Clear preferences were shown in combinations, but the hosts were accepted if presented alone. This tolerance allows females to persist on poor hosts.

The three conditions required for natural selec- tion to maintain the oviposition traits of this strain are established: (1) oviposition traits vary, (2) Ro is positively associated with the deviations of the traits from randomness, and (3) there is heritable variation in host preferences, hyperdispersion of eggs, and responses to crowding (Credland et al.,

1986; Messina, 1989; Wasserman, 1986). Pheno- typic differences in the net reproductive rates are the basis for natural selection maintaining the behaviors producing hyperdispersion of eggs, host preferences and density dependent control of oviposition rates.

Thus, the traits of this strain seem to be well suited to the local resources. The four major cultivars grown in South India are tolerated. Species of Vigna, which accounts for 98 ~o of the pulses planted in the area, support the highest net reproductive rate and are preferred over the others. The density dependent larval behavior (Thanthianga & Mitchell, 1987) may have evolved in response to the small seeded cultivars (com- monly under 50 mg, Wood, 1920; Dept. of Agri- culture, 1970) that dominate South Indian agri- culture.

Acknowledgements

We benefited from advice given by Kenneth Cooper, Dana Wrench, and Frank Messina. This

150

research is an extension of the studies of C. Thanthianga which were supported in part by a Government of India National Overseas Scholar- ship.

R~sum~

Les particularit(s de la ponte d'une souche de Callosobruchus maculatus du Sud de rlnde sont- elles rnaintenues par sdlection naturelle ?

Le taux partiel de reproduction nette (Ro*) d6pend de l'esp6ce de la plante sur laquelle les oeufs sont pondus et du nombre de larves entrant dans la graine. La survie larvaire est r6duite par 1/(le nombre de larves par graine) parce qu'une seule larve se d6veloppe dans une graine. La f6condit6 n'est pas modifi6e par la comp6tition subie par les larves, la mortalit6 larvaire a l'effet le plus impor- tant sur Ro*. Les femelles 61iminent ou r6duisent la comp6tition larvaire en dispersant leurs ~eufs uniform6ment et font si peu d'erreurs avec une hyperdispersion que l'6volution d'un comporte- ment plus pr6cis n'accroitrait Ro* que de 4~o au maximum. Des femelles retournant ~t une distri- bution des ~eufs au hasard provoqueraient une r6duction de Ro* de 25 ~o au moins.

Les 16gumineuses g6n6ralement cultiv6es dans l'Inde du Sud sont des h6tes acceptables quand elles sont pr6sent6es seules. Le choix des femelles entre 2 h6tes 616ve Ro* de 30 ~o ou plus par rap- port ~ une distribution au hasard. Les pr6f6rences les plus nettes concernent des combinaisons pr6- sentant la plus grande diff6rence de Ro*. Les femelles qui hyperdispersent leurs ceufs, choisis- sent leurs h6tes et ~vitent les pertes par comp6ti- tion en emp~chant que les ceufs ne donnent plus de descendants que ne le ferait une ponte au hasard.

Les particularit6s de la ponte sont variables et h6ritables. Les lign6es se s61ectionnent bien, en fonction de la dispersion de leurs oeufs sur les gaines, de la discrimination des plantes h6tes, et de la modulation de leur taux de ponte. La s61ec- tion naturelle maintient ces particularit6s du com- portement d'une far s6dentaire.

References

Credland, P. F., 1986. Effect of host availability on reproduc- tive performance in Callosobruchus maculatus (F.) (Coleoptera: Bruchidae). J. stored Prod. Res. 22: 49-54.

Credland, P. F., K. M. Dick & A. M. Wright, 1986. Bionomic variation among three populations of the Southern cowpea weevil Callosobruchus maculatus. Ecological Entomol. 11: 41-50.

Darwin, C., 1859. On the Origin of Species. John Murray, London. [Facsimile, 1964. Harvard University Press, Cambridge, Mass., U.S.A., 502 pp.]

Dick, K.M. & P. F. Credland, 1984. Egg production and development of three strains of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae). J. stored. Prod. Res. 19: 189-198.

Dept. of Agriculture, Tamil Nadu, 1970. A Note Book of Agricultural Facts and Figures. Seventh Edition. Madras, India, 440 + xxi pp.

Dept. of Statistics, 1985. Statistical Handbook of Tamil Nadu. Government Central Press, Madras, India.

Endler, J. A., 1986. Natural Selection in the Wild. Princeton University Press. Princeton, New Jersey, U.S.A., 336 pp.

Johnson, C.D., 1977. Life history of Ctenoculum janzeni (Coleoptera: Bruchidae) in seeds of Piscidia mollis (Leguminosae). Coleopterists Bull. 31: 313-318.

Messina, F.J., 1989. Genetic basis of variable oviposition behavior in Callosobruchus maculatus (Coleoptera: Bru- chidae). Annals Entomol. Soc. America 82: 792-796.

Messina, F. J. & R. Mitchell, 1989. Intraspecific variation in the egg-spacing behavior of Callosobruchus maculatus. J. Insect Behav. 2: 727-742.

Mitchell, R., 1975. The evolution of oviposition tactics in the bean weevil, Callosobruchus maculatus (F.). Ecology 56: 696-702.

Mitchell, R., 1983. Effects of host-plant variability on the fitness of sedentary herbivorous insects. In: R. F. Denno & M. S. McClure (eds.). Variable Plants and Herbivores in Natural and Managed Systems, pp. 343-369. Academic Press, Inc., New York, U.S.A., 717 pp.

Raina, A.K., 1970. Callosobruchus spp. infesting stored pulses (grain legumes) in India and a comparative study of their biology. Indian J. Entomol. 32: 303-310.

Thanthianga, C. & R. Mitchell, 1987. Vibrations mediate prudent resource exploitation by competing larvae of the bruchid bean weevil Callosobruchus maculatus. Entomol. exp. appl. 44: 15-21.

Thanthianga, C. & R. Mitchell, 1990. The fecundity and oviposition behavior of a South Indian strain of Calloso- bruchus maculatus. Entomol. exp. appl. 57: 133-142.

Wasserman, S. S., 1986. Genetic variation in adaptation to foodplants of the southern cowpea weevil, Callosobruchus maculatus: Evolution of oviposition preference. Entomol. exp. appl. 42: 201-212.

Wood, R. C., 1920. A Note-Book of Agricultural Facts and Figures. Third Edition. Government Press, Madras, India, 194 pp.