1. When freshly eclosed females of the primitively eusocial wasp Ropalidia marginata (Lep.) are isolated, only about 50% of them build nests and lay eggs thereby suggesting a pre-imaginal biasing of caste.

2. Wasps that lay eggs take a very variable amount of time after eclosion to start doing so.

3. Females eclosing from nests where larvae are fed at a relatively higher rate are more likely to become egg-layers and are likely to take less time after eclosion to begin to lay eggs.

4. Thus, both forms of pre-imaginal biasing of caste, namely, differences in egg laying capacity and differences in the time taken to attain reproductive maturity, appear to be influenced by larval nutrition

Bhagavan, Seetha

Chandrashekara, K

Gadagkar, Raghavendra

Vinutha, C

1. When freshly eclosed females of the primitively eusocial wasp Ropalidia marginata (Lep.) are isolated, only about 50% of them build nests and lay eggs thereby suggesting a pre-imaginal biasing of caste.\ud
2. Wasps that lay eggs take a very variable amount of time after eclosion to start doing so.\ud
3. Females eclosing from nests where larvae are fed at a relatively higher rate are more likely to become egg-layers and are likely to take less time after eclosion to begin to lay eggs.\ud
4. Thus, both forms of pre-imaginal biasing of caste, namely, differences in egg laying capacity and differences in the time taken to attain reproductive maturity, appear to be influenced by larval nutrition

Name not available

The role of larval nutrition in pre-imaginal biasing of caste in the primitively eusocial wasp Ropalidia marginata (Hymenoptera: Vespidae)

1. When freshly eclosed females of the primitively eusocial wasp Ropalidia marginata (Lep.) are isolated, only about 50% of them build nests and lay eggs thereby suggesting a pre-imaginal biasing of caste. 2. Wasps that lay eggs take a very variable amount of time after eclosion to start doing so. 3. Females eclosing from nests where larvae are fed at a relatively higher rate are more likely to become egg-layers and are likely to take less time after eclosion to begin to lay eggs. 4. Thus, both forms of pre-imaginal biasing of caste, namely, differences in egg laying capacity and differences in the time taken to attain reproductive maturity, appear to be influenced by larval nutrition

Chandrashekara, K.

Vinutha, C.

Publications of the IAS Fellows

Ecological Entomology (1991) 16, 435-440
The role of larval nutrition in pre-imaginal biasing
of caste in the primitively eusocial wasp Ropalidia marginata
(Hymenoptera: V espidae)
RAGHAVENDRA GADAGKAR, SEETHA BHAGAVAN,*
K. CHANDRASHEKARA and C. VINUTHA Centre for Ecological Sciences,
Indian Institute of Science, Bangalore 560 012, India
\
Abstract. 1. When freshly eclosed females of the primitively eusocial
.' wasp Ropalidia marginata (Lep.) are isolated, only about 50% of
them build nests and lay eggs thereby suggesting a pre-imaginal
biasing of caste.
2. Wasps that lay eggs take a very variable amount of time after
eclosion to start doing so.
3. Females eclosing from nests where larvae are fed at a relatively
higher rate are more likely to become egg-layers and are likely to take
less time after eclosion to begin to lay eggs.
4. Thus, both forms of pre-imaginal biasing of caste, namely,
differences in egg laying capacity and differences in the time taken
to attain reproductive maturity, appear to be influenced by larval
nutrition.
Key words. Larval nutrition, pre-imaginal caste bias, primitively
eusocial wasp, Ropalidia marginata.
Introduction (see reviews in Engelmann, 1970; Wilson, 1971;
Spradbery, 1965, 1973; Brian, 1980; Wheeler,
A fundamental property of eusocial insects is 1986). In most primitively eusocial species,
the differentiation among colony members into however, there are few, if any, consistent mor-
I a fertile reproductive caste and a sterile worker phological differences between the reproductive
caste. In most highly eusocial insects there are and worker castes. This suggests that the caste
marked and consistent morphological differ- of an individual is not fixed at eclosion, thus
r ences between the reproductive and sterile leaving room for flexibility in the social roles it
castes. Differential larval nutrition has long may adopt in adult life. Nevertheless, there
been proposed as the underlying process that is accumulating evidence that there is some
gives rise to differential development of brood pre-imaginal biasing of caste at least in some
leading to such morphologically distinct castes primitively eusocial species (see reviews in
Gadagkar, 1990, 1991) and good reason to sus-
pect that larval nutrition may be responsible for
* Present address: Department of Entomology, this (Hunt, 1991).
Ohio State University, Q)lumbus, Ohio 43210, U.S.A. In one set of experiments, newly eclosed fe-
Q)rrespondence: Dr Raghavendra GadagkaT, males of the primitively eusocial wasp Ropalidia
Centre for Ecological Sciences, Indian Institute of marginata (Lep.) (Hymenoptera: Vespidae)
Science, Bangalore 560 012, India. were isolated into individual cages and tested
435
436 Raghavendra Gadagkar et al.
for any pre-imaginal effects on their ability to
initiate nests and lay eggs. About 50% (150 out
of 299) of the females so tested initiated nests ~ Q)]
and laid eggs while the remaining died with- ": ~ ~ 5
outdoing so (Gadagkar et al., 1988, 1990). In ~ ~ ~] 00 Ntd ..
th 150 .0 ~.. "" NO"" 0
a Ition, among e egg-layers, the tIme Z a ~ 8. 0 0 0 0 0 0
taken after eclosion to start laying eggs varied
considerably (mean :t SD = 65.9:t 36.9 days). 0/)
Here we test and confirm the prediction that ~ §
differential larval nutrition is a factor that is ~ ~ ~
responsible for both forms of pre-imaginal bias- ~ ~ ] ]0 >. ~ '"ing of caste -that leading to differentiation Z ~ ..2 BooN (;4 ~ S N ;
into egg-layers and non-egg-layers as well as
that leading to differentiation into early repro- 6 ~
ducers and late reproducers. ] ]
'0 ..2 "0 ..
.O/)BZo ,5 ~ N N ~ 10 00
"'... """" Materials and Methods
Six naturally occurring post-emergence colonies '0 ~
of Ropalidia marginata located in Bangalore Zo ~ 00 0\ 0 00 "" ",,""N""
(13:00:00 N, 77:32:00 E) and Mysore (12:25:00
N, 76:50:00 E), India, were studied between June 1986 and May 1987. Some characteris ics ~ ~
of these nests are described in Table 1. Behav- i 5 (0 S 0 0 0 ~
ioural data presented in this paper are based OR
observations of individually identified wasps 'a '0 '"
from the six colonies. These data were obtained ~ 0 ~ ~ 5 ~ ~ ~ ~
d . 1445 ' b . bl k I ' '" = (.) N""""""""
unng -mm 0 servatlon oc s per co ony ~
spread randomly between 08.00 and 18.00 hours ~ over a peri d of 4-6 days. Colo y N121 was :E ~ ~ '"
...05=attacked by the predatory hornet Vespa tropica = Z Q) 13 t-- ~ ~ "" ~
before the observations could be completed and ~
thus only seventy-two observation blocks were ~ ~
performed for this colony. This amounted to ~ ,~
6 h of observations on colony N121 and 12 h g ~'~ '"
of observations on each of the remaining five ~ i!.] 0 ~ N 0 0 0
colonies. Our methods used for making behav- .~
ioural observations are de;;cribed in more detail ~ '0 Q) :
elsewhere (Gadagkar & Joshi, 1983). Data were c.: 0 e.Z ~' ",,0\""000\10
pooled for all animals in a colony to compute ,~1:1. the number of times larvae were fed per hour. .s ,
Dividing this by the total number of larvae in '0 '0 ~
the colony, we obtained the number of times ~ Zo ~ 0 t-- N 00 00 t--
": -~ 10 ~ ~ ""
an average larva is fed per hour in a colony ,e
(Table 1). ~ At the end of observations, the nests were  ~ ~ ~ ~ 0\ N collected and maintained in the laboratory after 0 Z Q) t-- "" 10 ~
removing all eggs, larvae and adults. All females ,..:
that eclosed from such nests were immediately ~ ~ .g ~ (;4 ~ ~ ~ ~
isolated into individual 22 x 11 x 11 cm venti- ~ ~ 8 Z Z Z Z Z Z
lated plastic jars. These isolated females were
provided with a piece of soft wood as a source
Larval nutrition and caste in R.marginata 437
of building material and an ad libitum diet of Results
final instar Corcyra cephalonica (Stainton)
(Lepidoptera: Galleridae) larvae, honey and Eighty-seven freshly eclosed females of the
tap water from the same source. Records were primitively eusocial wasp Ropalidia marginata
maintained of the number of C.cephalonica were tested for their ability to initiate nests and
larvae consumed by each wasp throughout the lay eggs when isolated into laboratory cages. As
experiment. All wasps were observed for signs in previous experiments (Gadagkar et al., 1988,
of nest building and egg laying every day (Table 1990), about half of them (47 out of 87) built
1). These experimental procedures have been nests and laid eggs while ,the remaining forty
described in more detail elsewhere (Gadagkar died without doing so, in spite of living, on the
et al., 1988). average (128.6:t 94.0 days), longer than the
\ The model used to perform logistic regression time taken by the egg-layers to start laying eggs
analysis, to establish larval nutrition as a de- (73.4:t 64.1 days) (t test, P < 0.01). However,
terminant of the probability of egg laying by the probability that an individual develops into
eclosing wasps, is also described elsewhere an egg-layer or a non-egg-layer was significantlyJ (Gadagkar et al., 1988; Shanubhogue & Gore, influenced by the rate at which an average larva
1987). The independent variable, namely the was fed per hour in their nest of origin (Table 2,
number of times an average larva is fed per P < 0.03). The goodness of fit of the model used
hour in a nest, was modelled to influence the in the logistic regression analysis was confirmed
probability of egg laying by eclosing females in by the fact that the expected numbers of wasps
that nest such that in each probability class calculated from the
X model were indistinguishable from the corre-In [P/( l- p)] = A + A1 .PO P spondmg observed numbers (Table 3, P > 0.05).
The equation As in earlier experiments (Gadagkar et al.,
-Po+1l1X/1 Po+1I1X 1988, 1990), and, as mentioned above, therep -e +e was a large variation in the time taken by the
was solved using the maximum likelihood egg-layers to start laying eggs, ranging, in this
criterion: experiment, from 5 to 277 days after eclosion
m n (mean :t SD = 73.4:t 64.1 days). There is a
L = (.II p;) <. n [1 -pJ) weak but statistically significant negative cor-
,;1 ,;m+1 .
relation between the time taken by an ammal to
where p is the probability of becoming an egg- start laying eggs (age at reproductive maturity)
layer, 1 -P is the probability of becoming a non- and the rate at which larvae are fed in the nest
egg-layer, (30 is the intercept, (31 the regression from which they eclose (Fig. 1, P<O.Ol).
coefficient, X the independent variable, L the
likelihood, 1 to m the egg-layers and m + 1 to n
the non-egg-layers. The goodness of fit of the Discussion
model was tested as follows: p the probability of
" becoming an egg-layer was computed from the The results presented in this paper show that
above equations for each of the eighty-seven pre-imaginal caste bias takes two forms. First,
animals. Dividing the probability values from some animals lay eggs while others die without
tOto 1 into twenty equal probability classes, the doing so. It is reasonable to expect that those
.mean p values of all the animals falling in any animals that build nests and lay eggs under the
probability class multiplied by the total number conditions of these experiments will have a
of animals in the experiment (87) gives the relatively high probability of becoming egg-
expected number of egg-layers in that prob- layers under natural conditions compared to
ability class. Similarly, [1- (mean probability)] those animals that die without laying eggs.
x (total number of animals in the experiment) Second, among the egg-layers there is consider-
= expected number of non-egg-layers in that able variation in the time taken to lay eggs. It is
probability class. The observed number of egg- similarly reasonable to expect that such of the
layers and non-egg-layers among animals falling egg-layers in these experiments that started
in each probability class being known, a X2 test laying eggs soon after eclosion will have a higher
was performed. chance of capitalizing on egg-laying oppor-

Larval nutrition and caste in R.marginata 439
tunities under natural conditions compared to them to become workers. In course of time
those egg-layers in the experiment that took a the colony will have a large worker force and
very long time to start laying eggs. Both these' also require less effort in expanding the nest.
forms of pre-imaginal caste bias appear to be Besides, at such a time the queen, either on
mediated at least partly by larval nutrition. account of old age and weakness or for the
The higher the rate at which larvae are fed purpose of producing reproductives, may slow
in a nest, the greater is the probability that down her rate of egg laying leading to accumu-
eclosing females from that nest will become. lation of empty cells and soon to a reduction in
egg-layers and the smaller is the time required the number of larvae to be fed relative to the
for them to start laying eggs. To the extent that amount of food available. Larvae at this time
these laboratory results can be extrapolated to may thus be better nourished and hence be
~ natural conditions, the results presented here programmed to develop into egg-layers. Such a
not only provide some support to the roles of hypothesis is consistent with what is known
parental manipulation (Alexander, 1974) and about colony cycles (Jeanne, 1972), variation in
sub-fertility (West-Eberhard, 1975) in promot- larval and adult nutrition through colony devel-
.1
ing the origin of eusociality (Gadagkar, 1990, opment (West-Eberhard, 1969; Wilson, 1971;
1991) but also suggest a mechanism by which Wheeler, 1986) and with the postulated role of
such factors may operate (Hunt, 1991). nutrition in social evolution (Pardi & Marino
In previous studies it was shown that nests Piccioli, 1970, 1981; Hunt, 1984, 1989, 1990,
that ~ave more empty cells are likely to produce 1991; Hunt et al., 1987; Rossi & Hunt, 1988). The
a greater proportion of egg-layers, and nests time is clearly ripe for more direct laboratory
that had fewer empty cells were likely to produce experimentation to examine the link between
a greater proportion of non-egg-layers. A link larval nutrition and adult caste in primitively
was hypothesized between the state of the queen eusocial wasps.
and the number of empty cells that she may
leave behind such that a young and healthy
queen was not expected to leave behind too Acknowledgments
many empty cells while an old and weak one was
expected to do so. But the nature of the link, if It is a pleasure to thank J. H. Hunt, R. L. Jeanne
any, between the number of empty cells left and D. E. Wheeler for a critical reading of an
behind by the queen and her ability or otherwise earlier version of this paper and J. H. Hunt for
to prevent her daughters from developing into letting us see an unpublished version of Hunt
potential reproductives was not clear. Indeed, it (1991). Supported in part by a grant from the
was suggested that the number of empty cells Indian National Science Academy.
was merely 'an indirect consequence of the .
same set of processes that lead to the production
of reproductives or workers and not to be caus- References
ally related to caste determination (or biasing)'
t (Gadagkar et al., 1990, page 184). Alexander, R.D. (1974) The evolution of social be-
In the light of the results presented here, havior. Annual Review of Ecology and Systematics,
however, an attractive hypothesis causally 5,325-383,
t linking the number of empty cells with caste Brian, M. V, (1980) Social control over sex and
biasing, a hypothesis also independently pro- caste in bees, wasps and ants. Biological Reviews,
posed by Hunt (1991), suggests itself. Nests 55,379-415.
begin with relatively young and healthy queens Engelmann,. F, (1970) The Physiology of Insect
..Reproduction. Pergamon Press, Oxford.who may leave behind few or no empty cells. G d gk R (1990)0 ' . d I t ' fa a ar,. ngln an evoulon 0 eu-Su~h nests may also h~ve a modest work~r force sociality: a perspective from studying primitively
which furthermore will be busy expandmg the eusocial wasps. Journal of Genetics, 69, 113-125.
nest to match the queen's egg-laying rate. The Gadagkar, R. (1991) Belonogaster, Mischocyttarus,
food that is brought to the nest at this time and Parapolybia nd independent-founding Ropalidia.
shared between the adults and the larvae may The Social Biology of Wasps (ed. by K. G. Ross and
therefore be limited, leading to a bias in the R. W. Matthews), pp. 149-190. Cornell University
nature of larval development thus programming Press, Ithaca, New York.



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The role of larval nutrition in pre-imaginal biasing of caste in the primitively eusocial wasp Ropalidia marginata (Hymenoptera: Vespidae)

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