Full text of DOI:10.1079/BER200199

Bulletin of Entomological Research (2001) 91, 255–264
DOI: 10.1079/BER200199
Family planning in a stemborer
parasitoid: sex ratio, brood size and size-
fitness relationships in Parallorhogas
pyralophagus (Hymenoptera: Braconidae),
and implications for biological control
J.S. Bernal*, P.O. Gillogly and J. Griset
Biological Control Laboratory, Texas A&M University, College Station,
TX 77843-2475, USA
Abstract
Various aspects were studied of the brood size and sex allocation strategies, and
of size-fitness relationships in Parallorhogas pyralophagus (Marsh), a gregarious
ectoparasitoid of Eoreuma loftini Dyar. Brood size was significantly correlated with
host size; larger hosts were allocated larger broods. Brood sex ratios were fixed
precisely at 1 male per 4 females, and eggs were likely to be deposited in that
order; differential mortality did not contribute to this precise sex ratio. The sex
allocation strategy of P. pyralophagus is likely to conform to strict, i.e. single
foundress, local mate competition. Adoption of this strategy is probably influenced
by a limited insemination capacity of males; a smaller proportion of females (0.09
vs. 0.21) remained virgin in broods with precise or higher sex ratios (≥ 0.20 males)
relative to broods with lower than precise sex ratios (< 0.20 males). Moreover, all
females were inseminated in most broods (60%) with precise or higher sex ratios,
whereas this did not occur in broods with lower than precise sex ratios. The
hypothesized occurrence of strict local mate competition in P. pyralophagus was
supported also by observations that: (i) offspring brood sex ratios were
independent of maternal brood sex ratios and number of parental females
concurrently allocating offspring to a group of hosts, and; (ii) the rate of superpara-
sitism under no-choice conditions was low (~20%), suggesting that rates of
outbreeding in the field are low. Other results suggested that fitness in P.
pyralophagus was correlated with adult size; longevity and reproductive capacity
both increased with adult size in males and females. However, adult size may be
more important for females than for males because the differences in reproductive
capacity between the largest and smallest individuals was up to 7.3 times greater
in females versus < 2 times in males.
imported to Texas, USA, in 1981 for biological control of
Introduction
Mexican rice borer, Eoreuma loftini Dyar (Lepidoptera:
Pyralidae) (Smith et al., 1987). Eoreuma loftini is the key pest
of sugarcane in the Rio Grande Valley of Texas (Meagher et
al., 1998), and an important pest of maize in southern Texas
and north-eastern Mexico (Rodríguez del Bosque et al., 1988;
Youm et al., 1988). Parallorhogas pyralophagus also parasitizes
the sugarcane borer Diatraea saccharalis Fabricius
(Lepidoptera: Pyralidae), another important pest of
Parallorhogas pyralophagus (Marsh) (Hymenoptera:
Braconidae) is
a gregarious external parasitoid of
gramineous stemborers that was discovered in Mexico and
*Fax: (409) 845 7977
E-mail: julio-s-bernal@tamu.edu

256
J.S. Bernal et al.
sugarcane in south Texas and elsewhere, but field parasitism
rates are typically low (Dasrat et al., 1997; Meagher et al.,
1998). Other hosts of P. pyralophagus include various species
of Diatraea, Tryporyza and Chilo (Smith et al., 1987).
biology that had not been considered in previous studies
such as: (i) the effects of adult size and food availability on
female and male partial fecundity; (ii) the effects of adult
size on female and male longevity; and (iii) whether females
avoid superparasitizing their hosts.
Laboratory studies conducted following importation of P.
pyralophagus documented its biology and life history (Melton
& Browning, 1986; Smith et al., 1987). Among other
biological characteristics, these studies showed that P.
pyralophagus is an idiobiont parasitoid; females paralyse
their hosts, which are concealed in their feeding tunnels
within plant stems, prior to oviposition. Moreover, P.
pyralophagus reproduces by arrhenotokous parthenogenesis
and sex determination is haplo-diploid; females are hence
able to determine the sex of offspring through selective
fertilization of eggs. Thus, P. pyralophagus females could
gauge host quality (size) at the time of oviposition, and
allocate optimal numbers of male and female offspring to
each host. Early observations suggested that P. pyralophagus
females adjusted brood size (number of parasitoid offspring
allocated per host) according to host size, and that sex ratios
were strongly female-biased (Melton & Browning, 1986;
Smith et al., 1987; personal observations).
Female-biased sex ratios are frequently displayed by
gregarious parasitoids as matings predominantly occur
between siblings in a brood and males compete against one
another for mates; this phenomenon is referred to as local
mate competition (Hamilton, 1967; Godfray, 1994). Offspring
sex ratio optima under local mate competition are expected
to vary according to the number of parental females
contributing to a group of interbreeding offspring and the
prevalence of matings between individuals from different
groups (Hamilton, 1967; Godfray, 1994; Hardy et al., 1998). If
a single female produces each offspring group and matings
between groups are rare, i.e. local mate competition is strict,
then it is expected that each offspring group should include
only enough males to inseminate all females. If more than
one female contributes to an offspring group, then the
proportion of male offspring increases asymptotically until it
equals that of female offspring (Hamilton, 1967; Godfray,
1994; Hardy et al., 1998). Because P. pyralophagus is a
gregarious parasitoid and its hosts typically do not occur in
dense aggregations in agricultural crops (Meagher et al.,
1996; personal observations), it was expected that in the field
offspring groups would be produced by individual females
and that matings would occur largely among individuals
from a single brood. Thus, it was expected that local mate
competition would be strong in P. pyralophagus. However,
local mate competition is thought to be constrained in the
Ichneumonoidea, including Braconidae, by single-locus
complementary sex determination (Butcher et al., 2000a,b).
In the light of these observations, the studies reported
herein addressed whether P. pyralophagus females adjust the
sex and number of offspring allocated per host based on host
size. In addition, because P. pyralophagus is a gregarious
parasitoid, the possibility was examined that female P.
pyralophagus adjust their offspring sex ratio (proportion
males) in response to local mate competition (Hamilton,
1967). Moreover, the sex ratios of individual P. pyralophagus
broods were analysed to determine whether they displayed
a ‘precise’ sex ratio pattern (sensu Green et al., 1982), and
whether parental females incur a fitness cost by producing
broods with lower than the ‘precise’ sex ratio. Finally, in
addition to examining the sex allocation strategy of P.
pyralophagus, these studies addressed other aspects of its
Material and methods
Host and parasitoid cultures
All experiments were conducted using E. loftini as the
host. Environmental conditions for rearing and for all
experiments were set at 25 ꢀ 1°C, 14L:10D photoperiod, and
50–70% relative humidity, except as noted. Eoreuma loftini
and P. pyralophagus were reared following previously
described methodology; rearing of the former was
accomplished using artificial diet (Martinez et al., 1988;
Smith et al., 1993).
Adult parasitoids were produced by placing individual
E. loftini larvae, between 28 and 32 days old, inside 2 cm
long waxed paper straw segments (6 mm diameter). The
straws were plugged at both ends with artificial diet, and
then sealed by dipping each end in melted paraffin wax.
Single larvae inside straws were offered for 24 h to
individual mated female parasitoids inside 12 ꢁ 35 mm
glass shell vials plugged with cotton. After 24 h, the female
was removed and the larva within the straw was incubated
inside the shell vial. Emerging male–female parasitoid
pairs were isolated inside 9 ꢁ 30 mm shell vials plugged
with cotton and provided with honey. Females were used
in experiments when they were 2–4 days old, except as
noted.
Influence of host size and maternal brood sex ratio on offspring
brood size and sex ratio
Individual female parasitoids were offered a single host
larva inside a 2 cm straw for 24 h. For each female, the
weight (mg) of each host larva, and the size and sex ratio of
the maternal brood (i.e. the female’s brood of origin) were
recorded. Sex ratios were estimated as the proportion of
males relative to total individuals. Maternal broods with
atypically high sex ratios (i.e. weaker female bias) were
obtained by adding adult males (2–4 days old) from other
broods; males were added to maternal broods as soon as the
first individual emerged, typically a male, and females were
never removed or added for manipulating brood sex ratios.
In general, all maternal broods with > 40% males used in
these experiments were produced by this method, whereas
none of the broods with < 30% males were manipulated. The
number and sex ratio of adult parasitoids emerging from
each straw were recorded. This procedure was repeated 188
times, and yielded 142 broods. The resulting data were used
to explore relationships between: (i) host size and brood size;
(ii) maternal brood sex ratio and offspring brood sex ratio;
and (iii) offspring brood size and sex ratio. Potential
relationships between these variables were explored via
linear regression analyses (Zar, 1996). Offspring broods of <
5 wasps (4/142 broods) were excluded from the analyses
because sex ratios were found to conform to a pattern of
1 male:4 females (see Results). All sex ratio estimates were
transformed to their arcsine values, with Anscombe’s
transformation, prior to analyses (Zar, 1996). Unless
indicated otherwise, all proportions and rates are presented

Precise sex ratio and fitness of Parallorhogas pyralophagus
257
followed by their standard deviation, and means followed
by their standard error.
numbers of parasitoids emerging per host were counted and
their sex determined. Six trials involving one foundress, six
trials involving four foundresses, and four trials involving
eight foundresses were conducted; trials not yielding adult
parasitoids or with broods consisting solely of males were
excluded from the analyses. The sex ratios corresponding to
each trial were arcsine transformed and means were
compared via single factor analysis of variance (Zar, 1996).
A second experiment assessed the relationship between
host size and brood (clutch) size based on numbers of
parasitoid eggs allocated per host. The protocol was as
described above, except that the straws containing the host
were cut open immediately after exposure to females and
the number of P. pyralophagus eggs within was recorded.
This procedure was repeated 110 times and yielded 75
broods. The potential relationship between host size and
brood size, based on P. pyralophagus eggs, was examined via
linear regression analysis (Zar, 1996).
Differential pre-imaginal mortality and brood sex ratio
Clear gelatin capsules (size 0) containing individual hosts
were exposed to individual female parasitoids for 24 h
inside 24 ꢁ 35 mm glass shell vials plugged with cotton. The
gelatin capsules were examined under a stereomicroscope
after 24 h, and the number of P. pyralophagus eggs within was
recorded. The hosts inside the gelatin capsules were
incubated, and the number and sex of parasitoids emerging
from each host were recorded and compared against the
corresponding number of eggs. Broods in which all eggs
successfully developed into adults were combined into one
group (no pre-imaginal mortality), and broods in which at
least one egg failed to develop into an adult were combined
into a second group (some pre-imaginal mortality). Sex
ratios of broods with no and some pre-imaginal mortality
were compared via a log-likelihood contingency table test
(Zar, 1996).
Precise sex ratios
The precision of P. pyralophagus brood sex ratios was
determined in two ways. Firstly, the mean sex ratio from 142
individual broods produced by isolated females (see above)
was tested for consistency with a hypothesized sex ratio of
precisely 1 male per 4 females (ꢂ 0.20) via a log-likelihood
goodness of fit test (Zar, 1996). Secondly, the expected
numbers of males in broods of 5–20 wasps were calculated
under two assumptions: male eggs laid first, or male eggs
laid last, in a sequence of five eggs. The binomial test (Green
et al., 1982) was then used to examine whether the number of
broods observed with exactly the expected number of males
was significantly greater than predicted if brood sex ratios
were binomially distributed; i.e. the test provides
a
Superparasitism
conservative estimate of the probability of observing as
many as k (or more) broods with exactly the predicted
number of males. This probability was estimated as:
Individual (first) females were offered a single larva (35–65
mg) inside a 2 cm straw for 24 h. The female was removed
after 24 h and the larva was examined for signs of parasitism;
parasitized larvae are completely paralysed or lethargic. All
parasitoid eggs were counted and removed from paralysed
host larvae with a fine paintbrush. Paralysed larvae were
transferred to new 2 cm straws and offered to a different
(second) female for 24 h. After 24 h, each larva was again
inspected for P. pyralophagus eggs to determine whether it had
been parasitized by the second female, and the number of
eggs was recorded. It was hypothesized that females do not
discriminate between paralysed and healthy hosts, and thus
all paralysed larvae would be superparasitized by second
females. The observed superparasitism rate was compared
against the expected rate, i.e. 100%, via a log-likelihood
goodness of fit test (Zar, 1996). The mean and variance of the
second female’s brood size were compared against those of
the first female using a paired-sample t-test and Bartlett’s test
for homogeneity of variances, respectively (Zar, 1996).
n
n
^
^
n−x
α =
Px 1− P
(
)
∑
x
x=k
where k is a random variable with a binomial distribution,
with parameters n and P, when P is no larger than P. The
parameter P is the maximum possible probability of having
exactly the expected number of males per brood (1 male per
4 females in this study). P was calculated as
^
N−1
P = 1−1/N
(
)
where N ꢂ brood size (Green et al., 1982).
Influence of the number of parental females exploiting a
common group of hosts on offspring sex ratios
Groups of 1, 4 or 8 newly emerged females and a
corresponding number of males were held for two days
inside 15 ꢁ 45 mm shell vials with honey; each of the
females in each group was obtained from a different brood
and thus were not siblings. After two days, the males were
removed, and the females (i.e. foundresses) were transferred
to a covered arena where they were offered eight straw
segments (2.5 cm length), each containing one host larva
(35–65 mg), for 24 h. The arena consisted of two Petri dish
bottoms (140 mm diameter ꢁ 17 mm deep) placed one atop
the other, facing inward, and held together with rubber
bands. The straw segments were secured in an upright
position, in an evenly spaced circular distribution in the
arena, by pressing the still-warm waxed end of the straw
against the bottom of the arena. After 24 h, the females were
removed from the arena and the straws were placed inside
individual 12 ꢁ 35 mm shell vials plugged with cotton. The
Female virginity in relation to brood sex ratio
The proportion of virgin females in broods with precise
or higher sex ratios, i.e. ≥ 1 male per 4 females, was
compared against that in broods with lower than precise sex
ratios, i.e. < 1 male per 4 females. Individual females were
offered a single host larva inside a 3 cm straw segment for
24 h. Sixteen days later, each straw was transferred to an
emergence chamber designed to discriminate females that
were inseminated by a common-brood male sibling prior to
dispersing from the emergence site (straw), from females
that dispersed uninseminated. Straws were placed upright
inside a 12 ꢁ 35 mm shell vial secured to the bottom of a
Fisher™ tri-cornered polypropylene beaker (400 ml, 80 mm
upper diameter). An inverted Pyrex™ 60° angle funnel

258
J.S. Bernal et al.
(75 mm diameter, 75 mm stem) was fastened to the beaker
with poster putty and a 25 ꢁ 95 mm shell vial placed over
the funnel’s stem to collect dispersing parasitoids. The
emergence chambers were maintained at room temperature
(21–23°C) under artificial lighting between 08:00 h and 17:00
h, and monitored at intervals < 15 min to collect dispersing
parasitoids. Dispersing females were immediately dissected
in a droplet of physiological saline to determine whether
their spermathecae contained sperm, an indication that they
had been inseminated prior to dispersing. Trials were
terminated after seven days if no parasitoids or only males
remained within the chamber, and broods in which a female
coincided with a male inside the collection vial were
discarded. The sex ratios of six broods were manipulated by
removing the first-emerged male; males were removed only
if they emerged prior to any females. This was necessary
because P. pyralophagus broods with sex ratios < 0.20 are
uncommon. Proportions of virgin females were estimated
separately for broods with sex ratios ≥ 0.20 and those with
sex ratios < 0.20, and were compared via a log-likelihood
contingency table test (Zar, 1996).
regression slopes were not significantly different (P ꢂ 0.50,
t ꢂ 1.97, 213 df). Offspring brood sex ratios were not
influenced either by offspring brood sizes (fig. 2a) or
maternal brood sex ratios (fig. 2b).
Precise sex ratios
Individual brood sex ratios (pooled sex ratio ꢂ 0.196
0.010, n ꢂ 1506) were not significantly different from a
hypothesized sex ratio of 0.20, i.e. 1 male:4 females (P >
0.999, G ꢂ 13.47, 141 df). Broods with exactly the predicted
number of males occurred more often than expected under
the assumption of binomial variance for several brood sizes
(table 1); brood size frequencies were small or nil for broods
in which the predicted number of males did not deviate
significantly from expected. Precise sex ratios were evident
both when the male egg was presumed to be deposited first
(table 1a) or last (table 1b) in a sequence of five eggs.
However, the precise sex ratio pattern appeared to be more
prevalent in the case where male eggs were presumed to be
deposited first (9/10 broods where n ≥ 4) versus last (7/10
broods where n ≥ 4) in a sequence of five eggs.
Adult parasitoid size versus egg load, insemination capacity
and longevity
Potential relationships between parasitoid adult size and
egg load, insemination capacity and longevity were
explored using the length of the hind tibia as an index of
adult size. An adult size–egg load relationship was assessed
for female parasitoids of different ages in the presence and
absence of food: (i) newly emerged (< 24-h-old); (ii) 4-day-
old, either provided with or deprived of honey; and (iii)
7-day-old either provided with or deprived of honey.
Females were dissected in a droplet of 70% ethyl alcohol and
the number of ovarian eggs was counted. An adult size-
insemination capacity relationship was assessed in male
parasitoids using the emergence chambers and protocol
described previously. In this case, only broods with single
males and lower than precise sex ratios (< 0.20) were
included in the analyses. Thirty-four trials were conducted,
which yielded 13 broods suitable for analysis. The
Influence of the number of parental females exploiting a
common group of hosts on offspring sex ratios
Single females and groups of females produced offspring
with similar sex ratios (fig. 3). The trend toward increased
sex ratio with number of parental females was not
significant (P ꢂ 0.15)
Differential pre-imaginal mortality and brood sex ratio
Thirty-four of 40 hosts offered to individual females were
parasitized, nine broods suffered no pre-imaginal mortality,
and 25 suffered some pre-imaginal mortality. The sex ratio of
broods suffering no pre-imaginal mortality (0.14 ꢀ 0.03) was
not significantly different from that of broods suffering some
pre-imaginal mortality (0.18 ꢀ 0.03) (P ꢂ 0.49, G ꢂ 0.813, 1
df). This indicated that the observed sex ratio patterns were
not influenced by differences in pre-imaginal mortality rates
between males and females.
proportion of inseminated females in
a brood was
determined through dissection in physiological saline. Adult
parasitoid size–longevity relationships in the absence of
food were examined for females and males. Newly emerged
parasitoids of each sex were individually isolated in shell
vials (9 ꢁ 30 mm) plugged with cotton, and inspected daily
until their death. Relationships between adult size and egg
load, insemination capacity (arcsine transformed
proportions), and longevity were explored via linear
regression analyses; t-tests were used to compare the rates of
gain in these fitness parameters with adult size between
females and males (Zar, 1996).
Superparasitism
One hundred and six of the 149 larvae exposed to
parasitism by first females were successfully parasitized
and thus subject to superparasitism. Most females
avoided parasitizing previously paralysed hosts; only
19.8% ꢀ 0.04% of second females offered parasitized hosts
superparasitized these hosts. This rate was lower than
expected under the hypothesis that females do not
discriminate previously parasitized hosts (P < 0.001, G ꢂ
172.25, 1 df). The mean number of eggs allocated to super-
parasitized hosts by second females (5.2 ꢀ 1.1) did not
differ from that allocated by first females (6.7 ꢀ 0.6) (P ꢂ
0.267, t ꢂ 1.141, 20 df). However, greater variance was
associated with second-female broods (s2 ꢂ 23.89) relative
to first-female broods (s2 ꢂ 7.21) (P ꢂ 0.010, F ꢂ 0.302, 40
df).
Results
Influence of host size and maternal brood sex ratio on offspring
brood size and sex ratio
Parasitoid brood size was significantly influenced by host
size (fig. 1). Larger hosts yielded larger broods both based on
the number of adult parasitoids emerging per host (fig. 1a)
and number of eggs allocated per host (fig. 1b). Adult brood
size appeared greater than egg clutch size but their

Precise sex ratio and fitness of Parallorhogas pyralophagus
259
24
18
12
6
(a)
(b)
P < 0.001, r2 = 0.164, n = 75
P << 0.001, r2 = 0.308, n = 142
0.05 0.10
0
0.00
Host size (g)
0.05
0.10
0.15
0.00
0.15
Fig. 1. The influence of Eoreuma loftini host size on Parallorhogas pyralophagus brood size based on (a) the number of P. pyralophagus
adults emerging per host (y = 5.40 + 101.84x), and (b) the number of P. pyralophagus eggs allocated per host (y = 4.65 + 60.30x).
1.0
(a)
(b)
P = 0.889, r2 = 0.0001, n = 138
P = 0.309, r2 = 0.008, n = 138
0.8
0.6
0.4
0.2
0.0
24
0
6
18
12
0.0
0.2
0.4
0.6
0.8
1.0
Offspring brood size
Parental brood sex ratio
Fig. 2. (a) The sex ratio (proportion males) of Parallorhogas pyralophagus broods in relation to brood size (y = 34.13 + 0.06x). (b) The sex
ratio of P. pyralophagus broods in relation to the sex ratio of the maternal brood (i.e. the parental female’s brood of origin) (y = 25.19 +
0.03x). Arcsine-transformed sex ratio values were used for analyses, and untransformed values are shown in both (a) and (b).
Female virginity in relation to brood sex ratio
Adult parasitoid size versus egg load, insemination capacity
and longevity
Twenty-five of 64 trials yielded broods with sex ratios ≥
0.20 and 15 with sex ratios < 0.20, while 24 were discarded
due to coincidence of males and females in the collection
vial. A smaller proportion of females remained virgin in
broods with precise or higher sex ratios (sex ratio ≥ 0.20)
relative to broods with lower than precise sex ratios (sex
ratio < 0.20) (fig. 4). Moreover, in none of the broods with
lower than precise sex ratios were all females successfully
inseminated, whereas all females were successfully
inseminated in most (60%) broods with precise or higher sex
ratios. In addition, there was no consistent relationship
between virginity and brood size (P ꢂ 0.09, r2 ꢂ 0.07, b ꢂ
1.14 ꢀ 0.66, df ꢂ 1, 39).
Large females had greater egg loads than smaller
females independent of age and access to food (fig. 5). Egg
load appeared to increase at a greater rate with female size
for older females, but to increase at similar rates with
adult size for females provided food relative to those
denied food. Regression estimates showed that the largest
females had egg loads between 3.8 times (4- and 7-day-old
old females without honey) and 7.3 times (7-day-old
females with honey) greater than those of the smallest
females.
Large males inseminated a greater proportion of their
common-brood siblings relative to smaller males (fig. 6). The

260
J.S. Bernal et al.
Table 1. Distribution of Parallorhogas pyralophagus brood sizes, and , the upper bound of the probability of observing k
or more broods with the expected number of males under a binomial distribution of sex ratios.
Expected no. of
males
No. with expected
no. of males (k)
Brood size (N)
Frequency (n)
(a) The male egg is deposited first in a sequence of five eggs
5
6
3
1
1
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
1
0
1.000
1.000
7
12
18
15
15
13
15
12
6
7
0.003
8
9
13
9
7 ꢁ 10ꢃ7
3 ꢁ 10ꢃ4
2 ꢁ 10ꢃ6
0.002
10
11
12
13
14
15
16
17
18
19
20
11
5
4
0.023
4
0.009
2
0.062
7
5
3 ꢁ 10ꢃ5
0.121
4
1
2
1
0.060
0
0
1.000
0
0
1.000
2
0
1.000
(b) The male egg is deposited last in a sequence of five eggs
5
6
3
1
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
1
0
0.794
1.000
1.000
1.000
1.000
2 ꢁ 10ꢃ6
0.002
0.006
0.030
0.007
3 ꢁ 10ꢃ5
0.025
0.128
1.000
1.000
1.000
7
12
18
15
15
13
15
12
6
2
8
9
5
3
10
11
12
13
14
15
16
17
18
19
20
11
7
7
5
4
7
5
4
2
2
1
0
0
0
0
2
0
largest males used in these trials were ~1.5 times more
successful than the smallest males.
superparasitism was not entirely absent, the observed rate
(~20%) can be considered low given that females in the
experiment were both inexperienced and exclusively offered
previously parasitized hosts, factors known to increase the
incidence of superparasitism (Godfray, 1994). In the field,
avoidance of superparasitism will reduce the likelihood of
outbreeding in a gregarious species such as P. pyralophagus.
The observed lack of influences of maternal brood sex
ratio and increasing foundress number on offspring sex
ratios, together with the above results suggest that the sex
ratio strategy of P. pyralophagus conforms to a strict (i.e.
single foundress) local mate competition strategy (Hamilton,
1967; Hartl, 1971; Godfray, 1994; Hardy et al., 1998). Under
this hypothesis, most matings in P. pyralophagus would be
expected to occur between wasps belonging to a single
brood. An apparent limitation of the insemination capacity
of males may be an important consideration for sex
allocation in P. pyralophagus as exactly enough sons to
inseminate all daughters should be allocated to each brood,
leading to a constant and precise sex ratio independent of
maternal brood sex ratio, foundress number and brood size.
This contrasts with some other studies on gregarious
parasitoids, which show that sex ratios decrease with brood
size (e.g. Green et al., 1982; Hardy et al., 1998). In addition,
Large parasitoids, both female and male, lived longer
than smaller ones (fig. 7). The largest females lived ~2.7
times longer than the smallest ones, while the largest males
lived ~2.4 times longer than the smallest ones. Longevity
increased with adult size at similar rates in females and
males.
Discussion
Sex allocation strategy of P. pyralophagus
Overall, the results of this study suggest that female P.
pyralophagus: (i) allocate more offspring to larger hosts
relative to smaller ones; (ii) deposit precisely one male egg
per every four female eggs; (iii) are likely to deposit male
eggs first in each sequence of five eggs; and (iv) that this
ratio and sequence of male and female eggs ensures that
most to all daughters are inseminated by their common-
brood siblings soon after emergence and prior to dispersing.
In addition, P. pyralophagus females tended to avoid
attacking previously paralysed hosts, which suggests that
superparasitism is likely to be infrequent. Although

Precise sex ratio and fitness of Parallorhogas pyralophagus
261
solely of females, and ~3% of all females had zero
opportunity to mate at the natal site (data not shown).
The results of this study indicating the occurrence of a
precise sex ratio and strict local mate competition sex ratio
strategy in P. pyralophagus are unique because sex ratio
strategies in which levels of sib-mating are expected to be
high are thought to be constrained in the Ichneumonoidea.
Single-locus complementary sex determination is thought to
be an ancestral condition in the Ichneumonoidea, and some
studies strongly support this hypothesis for specific
subfamilies (Butcher et al., 2000a,b). Under single-locus
complementary sex determination, intense sib-mating
rapidly leads to strongly male-biased sex ratios. The results
of this study indicate that single-locus complementary sex
determination does not occur in P. pyralophagus; strict local
mate competition as suggested to occur in this parasitoid
leads to a precise and female-biased sex ratio. Moreover, the
results support the notion that more data than currently
available are required to confirm the hypothesis that single-
locus complementary sex determination is the ancestral
condition in the Ichneumonoidea (Butcher et al., 2000a,b).
0.3
0.2
0.1
0.0
4 females
8 females
1 female
Parental females
Fig. 3. The sex ratio (proportion males) of Parallorhogas
pyralophagus offspring groups in relation to the number of
parental females contributing offspring to each offspring group
(P = 0.148, F2, 13 = 2.22).
Adult size and fitness in P. pyralophagus
Parallorhogas pyralophagus adult size was positively
correlated with two fitness parameters, reproductive capacity
and longevity, in both males and females. The largest females
had egg loads up to 7.3 times greater than the smallest ones,
insemination capacity of the largest males was < 2 times
greater than that of the smallest ones, and differences in
longevity between the largest and smallest individuals were
slightly greater for females (2.7 times) than for males (2.4
times). Overall, our results suggested that adult size may be
more important for female than for male P. pyralophagus.
These results are consistent with those obtained for
Heterospilus prosopidis Viereck (Hymenoptera: Braconidae)
and Lariophagus distinguendus Förster (Hymenoptera:
Pteromalidae) (Jones, 1982; van den Assem et al., 1989).
However, the results of a field study with Trichogramma
pretiosum Riley (Hymenoptera: Trichogrammatidae) showed
that differences in reproductive success between large and
small individuals were less for females than for males
(Kazmer & Luck, 1995). The fitness parameters evaluated in
this study were measured under laboratory conditions and
prior studies show that parasitoid size-fitness relationships
assessed under laboratory conditions may not closely reflect
those relationships under field conditions (West et al., 1996).
Therefore, the relationships found in this study should be
viewed with caution.
0.4
P = 0.009, 1 df,
n = 40 trials
0.3
0.2
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Brood sex ratio
Implications for biological control
Fig. 4. The proportion of Parallorhogas pyralophagus virgin
females in broods with a lower than precise sex ratio, i.e. < 0.20
males (circles, left of broken line), versus broods with a precise
or higher sex ratio, i.e. ꢄ 0.20 males (squares, right of broken
line). Mean proportions ( sd) are indicated by upright and
inverted triangles, respectively.
The results showed that sex allocation by P. pyralophagus
females is precise, and that the apparent limited
insemination capacity of males may contribute to the
precision of the sex ratio. Moreover, the results suggested
that the sex ratio strategy of P. pyralophagus conforms to strict
local mate competition, characterized by intense sib-mating,
and that very few opportunities may be available for
outbreeding as low rates of superparasitism are expected in
the field. Finally, the results showed that adult size is an
important factor influencing the fitness of P. pyralophagus.
These findings have significant implications for various
aspects of biological control efforts involving P. pyralophagus
the results did not support the prediction that virginity and
brood size are negatively correlated under strict local mate
competition (Heimpel, 1994). In contrast, the results were
consistent with the prediction that virginity is low in
parasitoids in which strict local mate competition is a major
component of the mating structure (Godfray & Hardy, 1993);
only eight (small) broods (1–7 individuals) of 142 consisted

262
J.S. Bernal et al.
40
30
20
10
P << 0.001, r2 = 0.79
(a)
Newly emerged
0
40
P << 0.001, r2 = 0.63
(b)
(c)
30
20
10
P << 0.001, r2 = 0.70
4 d old, w/honey
4 d old, w/o honey
0
40
P << 0.001, r2 = 0.67
P << 0.001, r2 = 0.73
(d)
(e)
30
20
10
0
7 d old, w/o honey
0.7 0.8 0.9 1.0 1.1 1.2 1.3
Parasitoid size (hind tibia length in mm)
7 d old, w/honey
0.7 0.8 0.9 1.0 1.1 1.2 1.3
Fig. 5. The influence of adult size (length of hind tibia in mm) of female Parallorhogas pyralophagus on the egg load: (a) newly-emerged
females (y = 40.62x – 22.07); (b) 4-day-old females provided with honey (y = 68.06x – 42.39); (c) 4-day-old females denied hone y (y =
47.69x – 24.89); (d) 7-day-old females provided with honey (y = 70.37x – 44.39), and; (e) 7-day-old females denied honey (y = 4 3.30x –
22.43). The rates of gain in egg load are similar between (b) and (c) (P > 0.500, t = 0.440, 82 df) and between (d) and (e) (P > 0.500, t =
0.436, 64 df).
and other species with similar ecological characteristics.
For example, some colonization efforts with imported
parasitoids are believed to have failed due to Allee effects,
low encounter rates between males and females and the
resulting high virginity rates, when parasitoids are
released in small numbers or too sparsely over extensive
areas (Hopper & Roush, 1993). However, relative to other
parasitoids (e.g. solitary parasitoids of isolated hosts), this
may be less important for P. pyralophagus given its
gregariousness, precise sex ratio and propensity for sib-
mating. The propensity for sib-mating in P. pyralophagus,
however, raises two other concerns, inbreeding depression
and diploid males. Genetic loads and inbreeding
depression are commonly thought to be low in species
with haplo-diploid sex determination, such as P.
pyralophagus. Although this may be the case for males, in
which deleterious mutations may be rapidly eliminated
due to their haploidy, it is less so for females. Diploid

Precise sex ratio and fitness of Parallorhogas pyralophagus
263
evident changes in the sex ratio over > 4 years (~18
generations per year) of continuous rearing in the laboratory
(J.W. Smith, Jr., personal communication; personal
observations), suggest that single-locus complementary sex
determination does not occur in P. pyralophagus. Sex ratio
manipulation to increase female parasitoid yields during
rearing (e.g. Heinz, 1998; Bernal et al., 1999) seems gratuitous
and difficult with P. pyralophagus. In addition, any effects of
inbreeding may be ameliorated by pooling individual
broods produced by different females and concentrating
their emergence in small cages to foster encounters
between unrelated males and females. Finally, prior studies
have shown that larger parasitoid females are more
reproductively successful in the field than smaller females,
and hence may be more effective augmentative biological
control agents (Visser, 1994; Kazmer & Luck, 1995; West et
al., 1996; Bennett & Hoffman, 1998; Ellers et al., 1998). The
results of this study also suggested that larger P.
pyralophagus females may be more reproductively
successful than smaller females, but this remains to be
verified in the field. However, opportunities for
manipulating the size of P. pyralophagus females reared for
field release were not exposed in this study. Improving the
quality of gregarious parasitoids reared for field release by
manipulating their adult size is a question that has
received relatively little attention but that merits further
investigation.
1.0
0.8
0.6
0.4
0.2
0.0
P = 0.012, r2 = 0.451, n = 13
0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85
Parasitoid size (hind tibia length in mm)
Fig. 6. The influence of adult size (length of hind tibia in mm) of
male Parallorhogas pyralophagus on the proportion of common-
brood female siblings that they inseminate in broods comprised
of a single male and a variable number of females (y = 1.28 +
86.83x). Arcsine-transformed sex ratio values were used for
analyses, and untransformed values are shown.
Acknowledgements
Special thanks are due to Drs Jim Smith and Bob Wharton,
both at Texas A&M University, for critical reading of an early
version of the manuscript and for much valuable insight to
the biology and ecology of P. pyralophagus; Alan Gillogly, also
at TAMU, provided thoughtful criticism of earlier versions of
the manuscript. The statistical advice and comments of Dr M.
Sétamou (TAMU) and two anonymous reviewers contributed
to improving the manuscript. Dr Jim Smith is acknowledged
also for the supply of parasitoid and host cultures.
females under chronic inbreeding will have genetic loads,
and thus will suffer inbreeding depression in female-only
traits similar to those observed in diploid species (Werren,
1993; Antolin, 1999). Single-locus complementary sex
determination, which is more prevalent in ichneumonoids
relative to other parasitoid groups, may rapidly lead to
strongly male-biased sex ratios and extinction in the
laboratory in species where it is known to occur (Butcher et
al., 2000a,b). The results of this study, along with the lack of
20
(a)
(b)
P < 0.001, r2 = 0.195, n = 61
15
10
5
P << 0.001, r2 = 0.702, n = 61
0
0.4
0.6
0.8
1.0
1.2
1.4 0.4
0.6
0.8
1.0
1.2
1.4
Parasitoid size (hind tibia length in mm)
Fig. 7. The influence of adult size (length of hind tibia in mm) on longevity (days) in Parallorhogas pyralophagus (a) females (y = 19.19x –
7.38) and (b) males (y = 13.02x – 1.52). The rate of gain in longevity with adult size is similar between females and males (P > 0.500, t =
0.351, 118 df).

264
J.S. Bernal et al.
Jones, W.T. (1982) Sex ratio and host size in a parasitic wasp.
References
Behavioral Ecology and Sociobiology 10, 207–210.
Kazmer, D.J. & Luck, R.F. (1995) Field tests of the size-fitness
hypothesis in the egg parasitoid Trichogramma pretiosum.
Ecology 76, 412–425.
Martinez, A.J., Bard, J. & Holler, T.A. (1988) Mass rearing
sugarcane borer and Mexican rice borer for production of
parasites Allorhogas pyralophagus and Rhacontus rosilensis.
USDA-APHIS-PPQ, APHIS 83–1.
Antolin, M.F. (1999) A genetic perspective on mating systems
and sex ratios of parasitoid wasps. Researches in Population
Ecology 41, 29–37.
Bennett, D.M. & Hoffmann, A.A. (1998) Effects of size and
fluctuating asymmetry on field fitness of the parasitoid
Trichogramma carverae (Hymenoptera: Trichogrammatidae).
Journal of Animal Ecology 67, 580–591.
Bernal, J.S., Luck, R.F. & Morse, J.G. (1999) Host influences on
sex ratio, longevity, and egg load of two Metaphycus species
parasitic on soft scales: implications for insectary rearing.
Entomologia Experimentalis et Applicata 92, 191–204.
Butcher, R.D.J., Whitfield, W.G.F. & Hubbard, S.F. (2000a)
Single-locus complementary sex determination in Diadegma
chrysoctictos (Gmelin) (Hymenoptera: Ichneumonidae).
Journal of Heredity 91, 104–111.
Butcher, R.D.J., Whitfield, W.G.F. & Hubbard, S.F. (2000b)
Complementary sex determination in the genus Diadegma
(Hymenoptera: Ichneumonidae). Journal of Evolutionary
Biology 13, 593–606.
Meagher, R.L., Jr., Wilson, L.T. & Pfannenstiel, R.S. (1996)
Sampling Eoreuma loftini (Lepidoptera: Pyralidae) on Texas
sugarcane. Environmental Entomology 25, 7–16.
Meagher, R.L., Jr., Smith, J.W., Jr., Browning, H.W. & Saldaña,
R.R. (1998) Sugarcane stemborers and their parasites in
southern Texas. Environmental Entomology 27, 759–766.
Melton, C.W. & Browning, H.W. (1986) Life history and
reproductive biology of Allorhogas pyralophagus
(Hymenoptera: Braconidae), a parasite imported for release
against Eoreuma loftini (Lepidoptera: Pyralidae). Annals of
the Entomological Society of America 79, 402–406.
Rodríguez del Bosque, L.A., Smith, J.W., Jr. & Browning, H.W.
(1988) Damage by stalkborers (Lepidoptera: Pyralidae) to
corn in northeastern Mexico. Journal of Economic Entomology
81, 1775–1780.
Smith, J.W., Jr., Browning, H.W. & Bennet, F.D. (1987)
Allorhogas pyralophagus [Hym., Braconidae], a gregarious
external parasite imported into Texas, USA, for biological
control of the stalkborer Eoreuma loftini [Lep., Pyralidae] on
sugarcane. Entomophaga 32, 477–482.
Smith, J.W., Jr., Wiedenmann, R.N. & Overholt, W.A. (1993)
Parasites of lepidopteran stemborers of tropical gramineous
plants. 89 pp. Nairobi, ICIPE Science Press.
van den Assem, J., van Iersel, J.J.A. & Los-den Hartogh, R.L.
(1989) Is being large more important for female than for
male parasitic wasps? Behaviour 108, 160–195.
Visser, M.E. (1994) The importance of being large: the
relationship between size and fitness in females of the
parasitoid Aphaereta minuta (Hymenoptera: Braconidae).
Journal of Animal Ecology 63, 963–978.
Werren, J.H. (1993) The evolution of inbreeding in haplodiploid
organisms. pp. 42–94 in Thornhill, N.W. (Ed.) The natural
history of inbreeding and outbreeding. Chicago, University of
Chicago Press.
West, S.A., Flanagan, K.E. & Godfray, H.C.J. (1996) The
relationship between parasitoid size and fitness in the field:
Dasrat, B., Rajkumar, A., Richards-Haynes, C., Quashie-
Williams, C.
& Eastwood, D. (1997) Evaluation of
Allorhogas pyralophagus Marsh (Hymenoptera: Braconidae)
for the biological control of Diatraea spp. (Lepidoptera:
Pyralidae) in sugar-cane in Guyana. Crop Protection 16,
723–726.
Ellers, J., van Alphen, J.J.M. & Sevenster, J.G. (1998) A field
study of size-fitness relationships in the parasitoid Asobara
tabida. Journal of Animal Ecology 67, 318–324.
Godfray, H.C.J. (1994) Parasitoids: behavioral and evolutionary
ecology. 473 pp. Princeton, Princeton University Press.
Godfray, H.C.J. & Hardy, I.C.W. (1993) Sex ratio and virginity
in haplodiploid insects. pp. 402–417 in Wrensch, D.L. &
Ebbert, M.A. (Eds) Evolution and diversity of sex ratio in
insects and mites. New York, Chapman & Hall.
Green, R.F., Gordh, G. & Hawkins, B.A. (1982) Precise sex
ratios in highly inbred parasitic wasps. American Naturalist
120, 653–665.
Hamilton, W.D. (1967) Extraordinary sex ratios. Science 156,
477–488.
Hardy, I.C.W., Dijkstra, L.J., Gillis, J.E.M. & Luft, P.A. (1998)
Patterns of sex ratio, virginity and developmental mortality
in gregarious parasitoids. Biological Journal of the Linnaean
Society 64, 239–270.
Hartl, D.L. (1971) Some aspects of natural selection in
arrhenotokous populations. American Zoologist 11, 309–325.
Heimpel, G.E. (1994) Virginity and the cost of insurance in
highly inbred Hymenoptera. Ecological Entomology 19,
299–302.
Heinz, K.M. (1998) Host size-dependent sex allocation
behaviour in a parasitoid: implications for Catolaccus
grandis (Hymenoptera: Pteromalidae) mass rearing
programmes. Bulletin of Entomological Research 88, 37–45.
Hopper, K.R. & Roush, R.T. (1993) Mate finding, dispersal,
number released, and the success of biological control
introductions. Ecological Entomology 18, 321–331.
a
study of Achrysocharoides zwoelferi (Hymenoptera:
Eulophidae). Journal of Animal Ecology 65, 631–639.
Youm, O., Browning, H.W. & Gilstrap, F.E. (1988) Population
dynamics of stalk borers attacking corn and sorghum in the
Texas Rio Grande Valley. Southwestern Entomologist 13,
199–204.
Zar, J.H. (1996) Biostatistical analysis. 662 pp. + Apps. Upper
Saddle River, Prentice Hall.
(Accepted 19 March 2001)
© CAB International, 2001