in males, while fertilized, i.e. diploid, eggs result in females), a female has
perfect control over the sex of the eggs it lays, and it can produce precise
sex ratios.
In solitary parasitoids, parasitoid larvae within a single host are
aggressive to each other and generally only a single adult emerges. In
gregarious parasitoids, the larvae share the host. Hence, solitary para-
sitoids usually lay a single egg (but not always (see above)) and gregarious
species lay a number of eggs in a single host. This clutch size is strongly
affected by the size of the host, even in koinobiont species. In idiobiont
species, larger clutches in larger hosts are due to the trade-off between
number and size of offspring from a host (with smaller offspring having a
lower fitness (Visser, 1994)). In koinobionts, there may be an additional
ultimate factor that determines clutch size. We may find smaller clutches
in small hosts, because investing in smaller hosts is more risky in terms
of lower survival, due to predation or starvation, of the host individual
(Vetet al., 1994).
The clutch size is also affected by the encounter rate with hosts.
Within a searching bout, clutch size is rarely constant (Ikawa and Suzuki,
1982; Visser, 1996). Apparently, the female adjusts her clutch size as she
balances the number of eggs she has available with the number of hosts
she is likely to encounter during the remainder of her life. If she encoun-
ters more hosts than expected, the female’s clutch size will decrease,
while, if the opposite holds, it will increase. As most experiments are
carried out at relatively high host densities, the former is more commonly
found.
The sex ratio produced in a host is often biased. In species where
mating takes place at the location where the offspring emerge from the
host, sex ratios are often female-biased. This is because the sons are in
competition with each other, rather than with unrelated males, for their
sisters (local mate competition) (Hamilton, 1967). In that case, a mother
should produce only just enough males to fertilize her daughters. With an
increasing probability that other females will produce offspring at the
same location, the optimal sex ratio approaches 50 : 50 rather quickly.
When only a single egg is laid in a host, frequently sons are produced on
small hosts and daughters on larger hosts. The reason for this is that host
size determines offspring size and that being large is more important for
daughters than it is for sons (Trivers and Willard, 1973). The sex of the egg
laid is thus affected by the size of the host. However, parasitoids do not
use absolute measures for large and small host sizes, but relative ones.
Whether they lay a male offspring in a host of a certain size depends on
the range of host sizes encountered (van der Assem, 1971; Charnovet al.,
1981). When a femaleLariophagus distinguendusis presented with hosts
of a single size (1.4 mm), it lays 15% male eggs. However, if it is given
alternating large and small hosts and the 1.4 mm are the smallest, it will
lay 30% males. If, in contrast, they are the largest, it lays 2%. Clearly, the
parasitoids use a relative size rule, allowing them to optimally adjust their
behaviour to varying circumstances.Flexibility in Host-search and Patch-use Strategies 51