predatory hemipterans (Bugg et al., 1991),
predacious beetles (Larochell, 1990),
lacewings (Limburg and Rosenheim, 2001)
and predacious wasps (Beggs and Rees, 1999).
Feeding on pollen or nectar can enable these
species to bridge periods of low prey avail-
ability (Limburg and Rosenheim, 2001). When
combined with prey feeding, plant-derived
foods can increase predator fitness over prey
feeding alone (van Baalen et al., 2001). A sec-
ond category of natural enemies are obliga-
tory consumers of plant-derived foods, at
least during part of their life cycle. This cate-
gory includes many ant species (Porter, 1989;
Tobin, 1994), syrphid flies (Lunau and Wacht,
1994) and parasitoids (Jervis et al., 1996).
As the nutritional ecology of predators
has been extensively covered elsewhere, the
focus of this chapter will be on issues con-
cerning feeding by adult parasitoids. I shall
stress that sugar feeding represents an inte-
gral part of parasitoid biology and that
insight in this topic is essential to our under-
standing of parasitoid ecology, as well as
their efficacy as biological control agents.
Nutritional Requirements of Parasitoids
During their development from parasitic lar-
vae to free-living adults, the dietary require-
ments of parasitoids take an equally marked
turn. While parasitoid larvae are strictly car-
nivorous, virtually all adult parasitoids
require carbohydrates as a source of energy
(Jervis et al., 1996), especially for flight
(Hoferer et al., 2000).
While predators can often utilize both liq-
uid and solid plant substrates (pollen, food
bodies), by far the majority of parasitoids are
restricted to feeding on sugar-rich solutions,
such as nectar and honeydew. This group
includes those species that emerge with a
full complement of mature eggs (so-called
preovigenic species), as well as species that
continue to mature eggs during their adult
life (synovigenic).
Some (usually synovigenic) parasitoid
species retain a level of carnivory during their
adult life, as they may feed on host
haemolymph in addition to sugar feeding
(Jervis and Kidd, 1986). Due to their different
nutritional composition, haemolymph and
nectar or honeydew are only partly inter-
changeable and they are believed to cover sep-
arate requirements. Sugar-rich nectar or
honeydew primarily provides for the para-
sitoid’s energetic needs. While these food
sources usually contain low levels of amino
acids, proteins and lipids, they might never-
theless contribute to physiological processes,
such as egg maturation. Host haemolymph,
on the other hand, is usually a relatively poor
source of energy. In part, this can be explained
by the fact that haemolymph in general con-
tains relatively low levels of carbohydrates
(Kimura et al., 1992). An additional limitation
lies in the fact that trehalose as the main
haemolymph sugar is rather poorly metabo-
lized by parasitoids (Wäckers, 2001). Instead,
haemolymph constitutes a primary source of
protein for physiological processes, such as
egg maturation (Rivero and Casas, 1999).
Those synovigenic species that do not engage
in host feeding draw upon the protein and fat
reserves transferred over from the larval stage.Effects of Sugar Feeding on Parasitoid
Fitness Parameters
Parasitoids emerge with a limited supply of
energy. The nutrients transferred from the
larval stage often cover no more than 48 h of
the parasitoid’s energetic requirements. This
period is extremely brief, considering the
fact that these species usually have the
potential to live for weeks when suitable
food is available. Part of this brief period
covered by larval food reserves cannot be
used to search for hosts, as parasitoids often
require a preoviposition period for the matu-
ration of their eggs. The reproductive success
in the remaining narrow time-window is fur-
ther limited by lack of experience, resulting
in an initially slow and inefficient search
(Turlings et al., 1993; Vet et al., 1995).
Sugar feeding can considerably increase
the parasitoid’s lifespan. Taking the pre-
oviposition period and experience into
account, the effective impact will be even
more significant. This means that parasitoids
that fail to replenish their energy reserves
through sugar feeding will suffer severe fit-60 F.L. Wäckers