lipoprotein (capsid) (Tanada and Kaya,
1993). The simplest viruses can be seen only
with the aid of the electron microscope; how-
ever, some viruses are occluded in proteina-
ceous bodies that can be detected by light
microscopy (Thomas, 1974).
More than 20 groups of viruses consist of
known insect pathogens (Martignoni and
Iwai, 1981). The Baculoviridae are the most
common and widely studied group of insect
viruses and are found exclusively in arthro-
pods. This family consists of two major sub-
groups: the nuclear polyhedrosis viruses and
the granulosis viruses. The former group has
been reported from more than 400 insect
species, whereas the latter have been found
exclusively in lepidopteran hosts. The
Polydnaviridae are associated with parasitic
wasps and are categorized into two major
groups based on viral structure and wasp-
family association. Some polydnaviruses are
pathogenic, whereas others form mutualistic
relationships with their hosts (Tanada and
Kaya, 1993).
In general, infection occurs after viruses
are ingested but transmission may occur
transovarially or through spiracles or
wounds (Thomas, 1974; Tanada and Kaya,
1993). Viruses are able to organize their own
replication within suitable host cells. This
involves the adsorption, uptake and uncoat-
ing of virus particles, followed by the replica-
tion of the viral genome and production of
viral progeny (Tanada and Kaya, 1993).
Diagnostic features that are considered to be
general characteristics of infection include:
dead larvae found hanging or lying on leaf or
plant surfaces, a very fragile cuticle, which
may rupture easily when touched to release
the liquefied body contents, and white
masses of fat body, which are visible through
the cuticle. The host body has no filamentous
structures on the cuticle, which may acquire a
bluish or bluish-purple iridescence (Thomas,
1974). Infection may induce prominent
behavioural changes in the host, including
changes in the level of activity (wandering
behaviour) and/or changes of microhabitat
preference (elevation-seeking behaviour or
‘tree-top’ diseases, movement to exposed
locations and diurnal behaviour of nocturnal
insects) (Horton and Moore, 1993).
Viruses are reported infrequently in nat-
ural enemies and are documented for
Bombus (MacFarlane et al., 1995), Cotesia
(Hamm et al., 1985) and Phytoseiulus
(Sut’áková and Rüttgen, 1978). ̆Diseases caused by bacteriaBacteria are the most common microorgan-
isms associated with insects (Tanada and
Kaya, 1993); however, relatively few bacteria
are capable of infecting or killing their insect
hosts. Most bacteria are non-pathogenic but
may become pathogenic if conditions are
favourable (Thomas, 1974; Tanada and Kaya,
1993). Such facultative pathogens are mainly
found in the genera Enterobacter, Serratia,
Pseudomonas and Proteus(Tanada and Kaya,
1993). Transmission of bacterial pathogens
occurs mostly through the mouth and diges-
tive tract. Less often, they are transmitted
through the egg or through wounds in the
integument (Thomas, 1974; Andreadis, 1987;
Tanada and Kaya, 1993). Upon ingestion by a
susceptible host, pathogenic bacteria multi-
ply and produce enzymes or toxins that
degrade the cells of the gut. Bacterial
pathogens may cause septicaemia, whereby
they invade the haemocoel, multiply, and
produce toxins that kill the host. Some bacte-
ria may cause toxaemia, a condition resulting
from the production of bacterial toxins, while
the bacteria themselves are often confined to
the gut lumen (Tanada and Kaya, 1993).
In general, insects that are killed by bacte-
ria rapidly become dark in colour and are
often very soft. Symptoms are usually more
pronounced in larval stages. While the
integument remains intact, internal tissues
and organs degenerate and the body
becomes viscid. The infected host may stop
feeding and excrete diarrhoea-like faeces.
Advanced infection is often accompanied by
a putrid odour. The cadaver often shrivels
and becomes dry and hard (Thomas, 1974;
Tanada and Kaya, 1993).
Reports of bacterial diseases among nat-
ural enemies used for biological control are
scarce. However, the facultative pathogen
Serratia marcescensis common among labora-
tory-reared insects and can be responsiblePathogens of Natural Enemies and Pollinators 135