immune system does not recognize specific entities, but it is able to
recognize classes of foreign objects, using pattern-recognition receptors
(PRRs) (Medzhitov and Janeway, 1997). For example, certain PRRs
recognize lipopolysaccharides that are ubiquitous on the surface of
bacteria but absent from the surface of most eukaryotic cells. Insects are
also able to recognize parasitoids and other pathogenic eukaryotes, such
as nematodes. Genetic analysis suggests that host resistance to parasitoids
is determined by one or only a few genes and that resistance is restricted
to certain types of parasitoids as opposed to all parasitoids (Benassiet al.,
1998; Hitaet al., 1999; Felloweset al., 1999). However, the identity of
the molecules on parasitoids that allow hosts to recognize them as foreign
and the PRRs involved in recognition are currently unknown (Schmidt
et al., 2001). What is known is that the receptors involved in recognition
of parasitoids are present both in haemolymph and on the surface
of specific types of haemocytes (Lavine and Strand, 2001). In the
lepidopteranPseudoplusia includens, a particular class of haemocytes,
called granular cells, are specifically responsible for recognition of many
foreign targets. Once granular cells have attached to the foreign invader,
they release cytokines, which induce a second type of haemocyte, called
plasmatocytes, to attach to the target and form the capsule (Pech and
Strand, 1996; Clarket al., 1997).
The ability to encapsulate parasitoids is affected by other conditions,
including host age, temperature and stress (Salt, 1970). For example,
parasitoids in the genusMetaphycusare readily encapsulated by older
instars of soft brown scale but are not encapsulated by younger instars
(Blumberg and DeBach, 1981). Encapsulation responses to the same
parasitoid species can also vary among strains of the same host species, as
well as among closely related host species (Kraaijeveld and Godfray,
1997; Felloweset al., 1998). Why parasitoid larvae die in capsules is
not well understood, although asphyxiation and secretion of cytotoxic
molecules have both been implicated as killing mechanisms (Salt, 1968;
Strand and Pech, 1995; Nappi and Ottaviani, 2000). In addition, while
encapsulation is the main defence against parasitoid larvae, non-cellular
immune responses can also kill parasitoids (Vinson, 1990; Carton and
Nappi, 1997). For example, parasitoid eggs and larvae fail to develop in
some lepidopteran and aphid hosts, even though they are never encapsu-
lated or melanized (Henter and Via, 1995; Trudeau and Strand, 1998).
Whether this is due to a humoral molecule, nutritional incompatibility or
some other factor is unknown.Evasion of host defence responses
Many parasitoids avoid encapsulation passively by developing either
on hosts that lack a functional immune system or in a location that is
inaccessible to host immune cells or effector molecules. The best example
of the former are parasitoids that attack hosts when they are still in the eggInteractions between Larval Parasitoids and Their Hosts 135