0851996884.pdf

Taste perception

To test the gustatory response of Cotesia
glomerata, food-deprived parasitoids were
presented with highly concentrated (2M)
solutions of individual sugars. Of the 14
sugars tested, only eight elicited feeding
(Wäckers, 1999). Six sugars, including the
honeydew sugar raffinose, did not elicit any
feeding response in the food-deprived para-
sitoids. Both raffinose and mannose showed
a deterrent effect when mixed with low
molar solutions of sucrose. Parasitoids
showed highest gustatory sensitivity (lowest
acceptance threshold) to the common nectar
sugars sucrose, glucose and fructose, as well
as the honeydew sugar erlose.
Based on the extensive work on honey-
bees and ants, we know that these social
Hymenoptera show distinct preferences for
particular sugars (von Frisch, 1934; Vander
Meer et al., 1995; Tinti and Nofre, 2001), as
well as certain sugar concentrations (Wykes,
1952; Waller, 1972; Baker and Baker, 1982a).
In sharp contrast to this body of research, we
know little or nothing about sugar prefer-
ences in parasitoids. This omission is in part
due to methodological problems in assessing
sugar preferences in parasitoids, as the estab-
lishment of preference requires that the test
organism shows an inclination to sample,
and feeds in repeated bouts. While these
conditions are met in social Hymenoptera,
whose foragers continuously collect food for
the entire colony, solitary parasitoids feed
infrequently, as their food foraging is
restricted to their individuals needs. The
number of parasitoid feeding events is fur-
ther restricted by the fact that they can ingest
and store sugar meals of up to a third of their
body weight. Upon encountering a food
source of sufficient quantity and quality, a
hungry parasitoid will typically feed until
saturation, rather than sample the food site
and continue foraging for alternative sugar
sources. The level of food consumption may
differ depending on the sugar offered
(Wäckers, 2001). However, this is at best an
indirect measure of preference, as parasitoids
are not making a choice based on complete
knowledge of the alternatives.

Effects on longevity

To obtain a more comprehensive overview of

the metabolic utilization of sugars by

Hymenopteran parasitoids, the same 14 sugars

were subsequently tested in respect of their

effect on parasitoid longevity (Wäckers, 2001).

Here again, considerable differences among

sugars were found. Those sugars to which par-

asitoids were most sensitive in the gustatory

experiment increased the parasitoid’s lifespan

by a factor of 15–16. A range of other sugars

had a less distinct or only marginal effect.

Lactose and raffinose did not significantly raise

parasitoid longevity, while rhamnose actually

reduced the parasitoid’s lifespan significantly.

The information obtained from these

studies can be of relevance to our under-

standing of the (un)suitability of the broad

range of naturally occurring sugar sources.

For instance, the poor performance of C.

glomerataon honeydew-specific sugars might

explain previous reports showing that hon-

eydew can be an inferior food source com-

pared with honey or sucrose (Leius, 1961;

Avidov et al., 1970; Wäckers, 2000).

Trade-offs Between Feeding and

Reproduction

Even though oviposition and feeding repre-

sent separate behavioural categories, they can

be interdependent. Obviously, feeding extends

a parasitoid’s reproductive lifespan, while the

energetic costs of host search, oviposition and

egg maturation can take toll of parasitoid

longevity. Various other types of interactions

between reproduction and longevity can occur

as well, representing distinct trade-offs

between these two fitness parameters (Fig.

5.1). Parasitoids have evolved a range of

strategies to optimize these fitness conflicts.

Two basic trade-offs between oviposition and

feeding will be discussed below.

Host-feeding versus oviposition

In many parasitoid species, host-feeding and

reproduction are mutually exclusive, as host-

feeding leaves the host unsuitable for larval

development. For host-feeding species, this

Food Ecology and Mass Rearing in Biocontrol 65