Journal of Invertebrate Pathology 73, 309–314 (1999)
Article ID jipa.1998.4841, available online at http://www.idealibrary.com on
Entomopathogenic Potential of Verticillium and Acremonium Species
(Deuteromycotina: Hyphomycetes)
Tove Steenberg*,† and Richard A. Humber‡
*Danish Pest Infestation Laboratory, Skovbrynet 14, DK-2800 Lyngby, Denmark; †Section of Zoology, The Royal Veterinary and Agricultural
University, Bulowsvej 13, DK-1870 Frederiksberg C., Denmark; and ‡USDA-ARS Collection of Entomopathogenic Fungal Cultures,
Plant Protection Research Unit, US Plant, Soil and Nutrition Laboratory, Tower Road, Ithaca, New York 14853-2901
Received June 19, 1998; accepted December 18, 1998
Hyphomycetes with conidia formed in slimy heads
from hyaline mycelium were isolated from a range of
insect hosts. Isolation on artificial medium and microscopic examination revealed that these fungi in many
cases were not Verticillium lecanii despite a superficial resemblance to this common entomopathogen.
The fungi were identified as Verticillium fusisporum,
Verticillium psalliotae, Verticillium lamellicola, and
species of Acremonium. Isolates of these fungi were
bioassayed against the sweet-potato whitefly (Bemisia
tabaci) and against the housefly (Musca domestica) to
examine their entomopathogenicity. A test was also
conducted with a coleopteran (lesser mealworm, Alphitobius diaperinus) to further evaluate the host range
for some of the fungi. V. lamellicola was not pathogenic
to the two species of insects treated, while varying
levels of pathogenicity were shown for the other species. In general, V. lecanii was the most pathogenic
species. Immature whiteflies appeared to be more
susceptible to fungal infection than adult houseflies,
and the host range for several of the fungi also included lesser mealworm. r 1999 Academic Press
Key Words: Acremonium; Verticillium fusisporum; V.
lamellicola; V. lecanii; V. psalliotae; Alphitobius diaperinus; Bemisia tabaci; Musca domestica.
INTRODUCTION
Verticillium lecanii (Zimmermann) Viégas is a wellknown pathogen of arthropods. The host range of the
species is wide and includes homopteran insects as well
as a range of other arthropod groups. V. lecanii is
considered to be a species complex that includes isolates with very variable morphological and biochemical
features (Gams, 1971; Jun et al., 1991). Conidia of V.
lecanii are short ovoid to cylindrical, but they are never
fusiform, falcate, or curved. Among the nine entomogenous species of Verticillium listed by Gams (1971),
some form conidia with more or less pointed tips, e.g., V.
fusisporum W. Gams. Occasionally some of these spe-
cies are isolated from arthropod cadavers (Balazy et al.,
1987; Kalsbeek et al., 1995). However, V. lecanii seems
to be by far the most common Verticillium species
isolated from arthropods. Information on the pathogenicity against insects of the other species of Verticillium
is scarce. However, Ekbom and Åhman (1980) studied
the effect of an isolate of V. fusisporum against different
homopteran glasshouse pests and found that the isolate was highly pathogenic to insects. The genus Acremonium Link also is reported to contain species with
pathogenicity to insects and other arthropods. Acremonium species have been isolated from arthropods (Petch,
1931, 1938; Gams, 1971; Balazy et al., 1987) but there
are very few reports verifying the pathogenicity of
these fungi for insects (Riba, 1983). Acremonium was
monographed with Verticillium and a range of other
related fungi by Gams (1971).
Fungi which on first inspection resembled V. lecanii
were isolated from diseased insects on several occasions. After in vitro isolation and microscopic examination it became clear that these isolates did not belong to
V. lecanii, despite the morphological variation found
within this species complex. In several cases the proportion of ‘non-lecanii’ isolates among the total number of
fungi isolated from the diseased host population was
surprisingly high. Therefore, this study was initiated in
order to identify the fungi and to test their pathogenicity against insects.
MATERIALS AND METHODS
Origin of Fungal Isolates
Diseased insects were collected from epizootics in the
sweet-potato whitefly Bemisia tabaci on poinsettia in a
glasshouse and in laboratory rearing cages of B. tabaci
on tobacco, Myzus persicae on pepper, and Thrips tabaci
on beans. All isolates from a particular host population
were made from separate cadavers attached to the
same leaf. Isolates from B. tabaci were from adults;
isolates from M. persicae were from adults and nymphs;
309
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310
STEENBERG AND HUMBER
and isolates from T. tabaci were from larvae. Live adult
houseflies (Musca domestica) were collected with a
sweep net from stables at two different farms and
incubated in cardboard cages for a week. Occasionally,
a few specimens died with signs of fungal infection by
Verticillium species, and these fungi were isolated.
From each farm there was one isolate that could not
readily be identified as one of the common entomopathogenic species. In all instances, isolates were cultured on
2% Sabouraud dextrose agar (SDA). A total of 33
isolates were obtained (Table 1); all isolates were from
host cadavers covered by fluffy, hyaline mycelium.
Preliminary inspection in a stereo microscope revealed
that all isolates produced conidia in slimy heads on
phialides that were placed singly, in pairs, or in whorls
on the mycelium.
Identification of Fungi
For macromorphological studies, isolates were inoculated in the center of petri dishes with 2% SDA and
incubated in constant light at 20°C. Culture characteristics assessed after 3 weeks were colony growth rate,
color and height of colony, and pigmentation of the
reverse. The micromorphology was studied in mycelium samples taken from 3-week-old cultures. The
arrangement of the phialides on the mycelium and the
shape and size of the conidia were noted for all isolates.
In addition, for eight isolates that were clearly not V.
lecanii, the dimensions of 50 conidia and 25 phialides
were measured. For isolates that produced a mixture of
spores, e.g., small ellipsoidal conidia and curved conidia, 50 conidia of each type were measured. The
majority of the isolates were subsequently deposited in
the USDA-ARS Collection of Entomopathogenic Fungal Cultures (ARSEF, Ithaca, NY) (Table 1).
Bioassay against Bemisia tabaci
All fungal isolates originating from glasshouse pests
were tested in a preliminary bioassay to assess their
pathogenicity. The method used was a slightly modified
version of the method of Hall (1984). Spores were
harvested from 3-week-old cultures with 0.05% Triton
X-100 and adjusted to 106 conidia/ml. For each isolate,
nine excised leaf discs (10 mm diameter) of tobacco (cv.
Samsun) each with 10–20 second- to third-instar larvae
of B. tabaci were immersed in a petri dish in 20 ml
spore suspension for 10 s. Following treatment, excess
moisture was removed by drying the leaf discs on
sterile filter paper, and the leaf discs were then transferred to square incubation chambers with 25 compartments containing a layer of 2% water agar (see Hall,
1984). After 7 days at 20°C in constant light, infection
was verified by isolating fungus from cadavers supporting sporulation and comparing the morphological characteristics with those of the isolate used. Fourteen
isolates that had been found to be pathogenic in the
preliminary test were then bioassayed against B. ta-
baci as described above. Controls were treated with
sterile 0.05% Triton X-100. All treatments were replicated three times. The number of individuals per leaf
disc was counted after mounting the discs in the
incubation chambers. After 7 days, dead individuals
were recorded. This included individuals with mycelium growing from the cadaver.
Bioassay against Musca domestica
Eight fungal isolates from glasshouse pests and three
isolates from M. domestica were tested against adult
M. domestica. Female flies (,4 days old) of strain DPIL
772a (field-collected in a Danish pig stable in 1989 and
since reared repeatedly in the laboratory) were anesthetized with carbon dioxide, and groups of 10 flies where
immersed in spore suspensions of 107 conidia/ml for 10
s. Spore suspensions were prepared with 0.05% Triton
X-100; controls were treated with sterile 0.05% Triton
X-100. After drying off excessive moisture on filter
paper, the flies were placed in cardboard cages (9 cm
diameter) supplied with 2% sucrose and dried milk
powder and incubated for 24 h at 23°C in plastic bags
lined with damp tissue paper. The cages where then
removed from the bags and placed at 23°C, 40–60% RH,
with a photoperiod of 12:12 (L:D). Each treatment was
replicated five times. Dead flies were recorded daily for
21 days and were transferred to moist chambers to
induce fungus sporulation.
Infectivity against Alphitobius diaperinus
Eleven isolates listed in Table 1 were tested for
pathogenicity against larvae of lesser mealworm (Alphitobius diaperinus Panz.). These larvae were maintained as a laboratory colony and for each assay, 10
late-instar larvae of A. diaperinus were placed in petri
dishes with 3-week-old sporulating fungal culture for
10 min and then incubated separately at 26°C, 70% RH
for 10 days in 30-ml plastic vials with ventilated lids.
The larvae were provided with water-soaked cotton
plugs. Control insects were placed in petri dishes with
2% SDA for 10 min. Dead insects were removed after 7
and 14 days and placed in moist chambers for 5 days to
induce fungal sporulation.
Statistical Analysis
Percentage mortality was arcsine transformed, and
means were separated using Tukey’s test (a 5 0.05) in
the General Linear Models procedure (SAS Institute,
1996).
RESULTS
Identification of Fungi
The 33 isolates were identified using the keys and
species descriptions in Gams (1971) (see Table 1).
Fifteen isolates were identified as V. lecanii; the remain-
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ENTOMOPATHOGENIC POTENTIAL OF VERTICILLIUM AND ACREMONIUM
TABLE 1
Fungal Isolates: Origin, Identification, and ARSEF Accession Numbers
Host insect
Bemisia tabaci
Location
Poinsettia (in rearing cage), Lyngby
Tobacco (in glasshouse), Lyngby
Myzus persicae
Green pepper (in rearing cage), Lyngby
Thrips tabaci
Beans (in rearing cage), Lyngby
Musca domestica
a
Stable, Roskilde
Stable, St. Karleby
Stable, St. Karleby
Strain
Fungus
BP2
BP3
BP4
BP5
BT2
BT3
BT4
BT5
BT6
BT10
BT11
BT12
BT13
BT14
BT15
MP1
MP2
MP3
MP5
MP6
MP7
MP9
MP10
MP11
TT1
TT2
TT3
TT4
TT5
TT6
625
626
615
Acremonium sp.
Acremonium sp.
Acremonium sp.
Acremonium sp.
V. lecanii
V. lecanii
V. lecanii
V. psalliotae
V. lecanii
V. lamellicola
V. lecanii
V. lecanii
V. lecanii
V. lecanii
V. lecanii
V. fusisporum
V. lecanii
V. lecanii
V. lecanii
V. fusisporum
V. fusisporum
V. fusisporum
V. fusisporum
V. lecanii
Acremonium sp.
Acremonium sp.
Acremonium sp.
V. lecanii
Acremonium sp.
Acremonium sp.
V. fusisporum
V. fusisporum
V. lecanii
ARSEF
accession number
4058 a
4059
4064
4065
5582 a
4067
4068 a
4069
4070 a
4071
4072
4074
4075
4076
4077
4078 a
4079
4062 a
5580 a
4063 a
5578 a
5579 a
Denotes isolates tested against lesser mealworm (A. diaperinus).
ing isolates included 7 of V. fusisporum, 1 each of V.
psalliotae and V. lamellicola, and 9 attributed to the
genus Acremonium but not identified to the species
level, as we were not able to find a good fit for it in Gams
(1971).
Macromorphological Features
Most of the 33 isolates produced a high cottony, white
colony on SDA. However, the V. lecanii isolates isolated
from B. tabaci (BT-isolates) produced a flat, more
velvet-like colony. This was also the case for V. lecanii
(615) from M. domestica. The only isolate that did not
form a white colony was V. psalliotae (BT5), which
produced a pale yellow mycelium. The majority of
non-lecanii species were characterized by a tendancy to
grow in radially irregular colonies unlike the much
more uniformly circular colonies of V. lecanii isolates.
Acremonium isolates conversely grew as uniformly
circular colonies, but differed from V. lecanii in producing areas with brown pigmentation in the reverse of the
petri dishes. Some Acremonium isolates also produced
brown areas in the center of the colonies with maturity.
V. fusisporum (625 and 626) from houseflies produced a
brownish pigment in the reverse; this was not seen in V.
fusisporum from M. persicae in which the reverse for all
five isolates was without any appreciable pigmentation. V. lamellicola (BT10) likewise did not produce a
pigment but was clearly distinguished from the remaining isolates by a very low growth rate. V. psalliotae also
grew less abundantly than the other isolates and,
furthermore, produced a red-brown pigment that
quickly colored the agar in the whole dish. It, furthermore, produced a distinct, mouldy smell.
Micromorphological Features
The Acremonium isolates (see Table 2) were distinguished by producing long and slender phialides that in
some cases were placed at right angles on the mycelium. In contrast to the Verticillium species, these
phialides never occurred in pairs or in whorls. The
conidia of the Acremonium isolates were rather small
and subglobose to ellipsoidal. The ‘non-lecanii’ isolates
of Verticillium produced a mixture of conidial forms, in
which one part of the conidia was relatively small and
312
STEENBERG AND HUMBER
TABLE 2
Measurements of Conidia and Phialides for Different Species of Verticillium and Acremonium
Small conidia
Fungus
V. psalliotae
V. lamellicola
V. fusisporum
V. fusisporum
Acremonium sp.
Acremonium sp.
Acremonium sp.
Acremonium sp.
Mean
Mean
Mean
Strain length Range width Range length
BT5
BT10
MP6
MP1
TT5
TT3
BP2
BP4
4.7
2.9
3.6
4.5
2.9
2.9
3.2
3.0
3.6–5.6
2.4–4.0
3.2–4.8
3.2–4.8
2.4–4.4
2.4–3.6
2.8–4.0
2.4–4.0
2.4
1.4
1.6
1.6
1.6
1.6
1.7
1.6
Phialides
Large conidia
2.4–2.8
0.8–1.6
1.6–2.0
NC
NC
1.2–2.0
1.6–2.4
2.4–4.0
11.9
6.5
8.0
10.6
Range
8.8–15.2
4.8–8.8
6.4–10.4
7.2–15.2
Mean
Mean Range Mean
width Range L/W
L/W length
2.4
1.3
1.8
2.0
1.6–3.2
1.2–1.6
1.6–5.4
1.6–2.4
5.0
5.0
4.5
5.2
4.3–6.5
3.5–6.7
3.0–5.5
3.7–7.6
31.3
18.8
16.6
20.8
33.7
53.7
30.0
30.6
Range
25.6–36.0
13.6–22.4
11.2–23.2
12.8–27.2
15.6–46.4
32.0–78.4
19.2–41.6
20.8–46.7
Mean
basal
width Range
2.1
0.9
1.5
1.6
0.9
1.6
1.4
1.4
1.6–2.4
0.8–1.2
1.2–1.6
1.6–2.0
0.8–1.2
NC
1.2–1.6
NC
Note. For each isolate, 25 phialides and 50 conidia were measured at 4003 magnification. For isolates which produced conidia with different
shapes (e.g., small ellipsoidal and large curved conidia), 50 conidia of each group were measured. All data are given in µm. NC (not calculated)
denotes that all values were identical.
ellipsoidal, and another part was large and fusiform or
curved. In some cases, the conidial dimensions differed
from those given in the species description; e.g., isolate
MP1 was identified as V. fusisporum although the
fusiform conidia were longer than stated in Gams
(1971). In contrast, V. lecanii isolates produced only
cylindrical or ellipsoidal conidia.
Bioassay against Bemisia tabaci
In the preliminary test, 29 of 30 isolates isolated
from glasshouse pests were pathogenic to B. tabaci
immatures. Fungus-killed immatures did not always
support fungus outgrowth 7 days after treatment
but, based on the white body color, they could readily
be distinguished from live individuals and from those
that died from other causes. Only V. lamellicola (BT10)
did not cause mortality. In the bioassay, there were
highly significant differences between treatments
(F 5 12.64, df 5 14, P . 0.001) (Table 3). All isolates
caused mortalities that differed significantly from those
in the control. The overall control mortality on day 7
was 6.2%.
TABLE 3
Bioassays against Bemisia tabaci Immatures and Adult Musca domestica
Fungus
Strain
Acremonium sp.
Acremonium sp.
Acremonium sp.
Acremonium sp.
V. lecanii
Acremonium sp.
V. lecanii
V. psalliotae
V. lecanii
V. lecanii
V. lecanii
V. lecanii
V. lecanii
V. lecanii
V. lecanii
V. fusisporum
V. lecanii
V. lecanii *
V. fusisporum
V. fusisporum
V. lecanii
Control
BP2
BP3
BP4
TT3
TT4
TT5
BT3
BT5
BT12
BT13
BT14
BT15
MP2
MP3
MP5
MP6
MP11
VL10
625
626
615
Bemisia tabaci,
% mortality
Day 7
71.6 b,c,d
77.2 a,b,c
41.3 h
64.6 c,d,e,f
46.5 f,g,h
94.1 a,b
87.3 a,b
50.7 e,f,g,h
88.2 a
29.7 h
46.8 c,d,e,f,g
68.8 c,d,e
60.5 c,d,e,f,g
55.2 d,e,f,g
6.2 i
Musca domestica, % mortality
Day 21
Musca domestica,
% sporulation
Day 7
Day 14
27.8 a
42.6 d
66.7 b
50.0 b
25.4 a
21.8 a
15.7 a
42.4 d
50.9 b,c,d
39.2 d
64.4 b,c
70.9 b
66.7 b
47.5 b,c
34.5 b
29.4 b
22.0 a
32.0 a
42.0 c,d
82.0 a,b,c
66.0 b
98.0 a
30.0 b
88.0 a
12.0 a
16.0 d
26.0 d
22.0 d
28.0 a
18.0 a
22.0 a
32.0 a
14.0 a
92.0
30.0 d
36.0 d
84.0 a,b
16.0 d
100.0 a
44.0 b,c,d
44.0 b,c,d
96.0 a
22.0 d
94.0 a
18.0 b,c,d
24.0 b,c,d
82.0 a
0.0 d
Note. Letters within each column indicate Tukey groupings (a 5 0.05).
* Denotes an isolate from an unidentified aphid. Total % sporulation for M. domestica was measured 26 days after inoculation.
ENTOMOPATHOGENIC POTENTIAL OF VERTICILLIUM AND ACREMONIUM
Bioassays against Musca domestica
The mortality caused by the 10 isolates tested against
houseflies did not differ significantly at day 7 postinoculation. By day 14 and day 21 the differences
between isolates were highly significant (day 14:
F 5 10,24, df 5 11, P . 0.001; day 21: F 5 23,09,
df 5 11, P . 0.001). Likewise, the difference in percentage sporulation between isolates was highly significant
(F 5 24,78, df 5 11, P . 0.001). At day 21 there were
no differences in the mortalities of the three isolates of
V. fusisporum and the control mortality (Table 3). The
other isolates were pathogenic, including 3 isolates of
Acremonium, V. psalliotae, and V. fusisporum. In general, the isolates of V. lecanii were more pathogenic
than those of the other species. By day 21 a group of
isolates with pathogenicity intermediate to that of
houseflies could be distinguished. This group included
V. psalliotae (BT5) and three isolates of Acremonium
(TT3, TT5, and BP2).
Infectivity against Alphitobius diaperinus
Patent infection of A. diaperinus larvae was obtained
with the three Acremonium isolates tested, with one
out of three isolates of V. fusisporum (BP2), and with
the three isolates of V. lecanii tested. V. psallioae and V.
lamellicola were not pathogenic to this host.
DISCUSSION
Fungi Isolated from Diseased Insects
Fifteen of the 33 isolates studied here are identified
as V. lecanii using the information in Gams (1971).
These identifications, however, must be regarded as
provisional since Gams (1971) treatment of V. lecanii
includes a wide range of morphological variability and
this taxon is now widely agreed to be an unresolved
species complex. Molecular studies of a wide range of
isolates of Verticillium from insects (Humber, unpublished) indicate that the use of V. lecanii will have to be
restricted to a narrow range of fungi with a distinctive
set of molecular profiles coming primarily from Southeast Asia (Zimmermann described this fungus from
coffee green scale collected in Bogor, Indonesia). While
the great majority of collections of V. lecanii as recognized by Gams (1971) fall outside of V. lecanii in this
narrowly restricted sense, the V. lecanii species complex does contain several distinctive taxa defined by
their molecular profiles and traditional taxonomic characters that will be recognized as separate species once
it can be determined whether they must bear names
currently treated as synonyms of V. lecanii or be
described as entirely new taxa (Humber, unpublished).
In the meantime, the only reasonable course of action is
to still use the name V. lecanii in the broad sense of
Gams (1971), as has been done here.
A high proportion of the fungal isolates isolated from
313
epizootics in populations of glasshouse pests proved not
to be V. lecanii despite a superficial resemblance to this
entomopathogen (Table 1). From M. persicae, five out of
nine isolates were V. fusisporum. One group of isolates
from B. tabaci were all Acremonium species (BPisolates), while another group consisted not only of V.
lecanii but also contained V. psalliotae and V. lamellicola (BT-isolates). At two different farms the only
hyphomycetes found to affect houseflies were V. fusisporum and V. lecanii. In contrast to the epizootics in
glasshouse pests, only a few of the live-collected houseflies were killed by fungi. These results show the need
for thorough examination of the micromorphology of
entomopathogenic fungi in order not to overlook less
well-known species.
Species of Acremonium have previously been associated with mortality in insects and have occasionally
been listed in studies on the natural occurrence of
entomopathogenic fungi (Petch, 1931, 1938; Balazy et
al., 1987; Sanchez-Peña, 1990). However, Gams (1971)
regarded only A. larvarum (Petch) W. Gams to be an
entomopathogen. Nevertheless, the infectivity assays
in this study clearly demonstrate that species of Acremonium are entomopathogens with some potential as
biocontrol agents, at least against B. tabaci immatures.
V. lamellicola was isolated from B. tabaci but did not
cause mortality when tested against immatures of this
species or against A. diaperinus. Balazy et al. (1987)
likewise reported an isolate of V. lamellicola (isolated
from a mite) that had no pathogenicity for bark beetle
larvae. The fungus is known to be a parasite of mushroom cultures and fruit bodies of some ascomycetes and
basidiomycetes; the recovery of this species on dead
arthropods probably reflects merely its capability as a
saprophyte. This species has straight fusiform conidia
like those of V. fusisporum, but the conidial tips of V.
lamellicola appear more pointed, and the ratio between
the length and width is often greater.
A high proportion of the conidia produced by V.
lamellicola were small and ellipsoidal rather than long
and narrow fusiform, and this tendency to produce a
mixture of conidia with different shapes and dimensions was found in V. psalliotae as well as in the three
isolates of V. fusisporum. That such variable conidial
shapes were also found in single-spore isolates made
from these cultures (Steenberg, unpublished data)
tended to disprove the possibility that these cultures
were mixed isolates. Such a phenomenon was also
described for V. psalliotae by Gams (1971). In some
isolates, the small and ellipsoidal conidia dominate
completely, and unless an isolate produces a strong
pigment, a fungus may well be misidentified as V.
lecanii by virtue of the large amount of morphological
variability within this species complex.
V. psalliotae is mainly known as a parasite of mushrooms and rust fungi but has also been isolated from
dead mites and insects (Domsch et al., 1980; Balazy et
314
STEENBERG AND HUMBER
al., 1987). The isolate BT5 matches the characterization of this species by Gams (1971), although the
sickle-shaped conidia are larger than those noted by
Gams. Furthermore, the isolate produces a distinct,
mouldy smell, a characteristic that does not seem to be
common in this species (Gams, 1971; Domsch et al.,
1980).
V. fusisporum was described by W. Gams (1971) from
isolates from soil and dead leaves. Ekbom and Åhman
(1980) isolated it from whiteflies and showed that the
isolate was pathogenic to several homopteran insects.
According to these references, V. fusisporum produces a
red pigment in the medium. When grown on SDA in
this study, only the two V. fusisporum isolates from
houseflies produced a pigment. However, the pigment
was brown and colored the reverse but not the agar.
Thus, this macroscopic character may be of little use in
the discrimination between the closely related species
of entomopathogenic Verticillium since the observed
differences in pigment production may depend entirely
upon the growth medium used. The conidial dimensions for V. fusisporum in this study were larger than
described previously for this species. Although V. fusisporum, V. psalliotae, and V. lamellicola were never
observed to produce more than one conidial type within
the categories sickle-shaped, narrow fusiform, and
broad fusiform, the deviations of the morphologies of
these fungi from the descriptions in Gams (1971) shows
the need for a revision of the entomopathogenic Verticillium species.
Entomopathogenic Potential of the Different Fungi
The bioassay against B. tabaci showed that, except
for V. lamellicola, all isolates tested were pathogenic to
this host (Table 3). Among the four most pathogenic
isolates were three of V. lecanii from B. tabaci and one
isolate of Acremonium also from the homologous host.
However, V. lecanii isolates were also among the least
pathogenic cultures; so, this species may not necessarily be the best choice for control of Bemisia immatures
and, as with nearly all fungal entomopathogens, the
expected level of control of a particular host can be
highly dependent on the isolate used. In the bioassay
against adult houseflies, the mortality in general occurred very late, and none of the fungi seem to have any
control potential against adult houseflies, a pest that
has an estimated mean life span of 3–4 days in stables
(Rasmussen and Skovmand, 1984). Interestingly, none
of the three isolates of V. fusisporum, including the two
from houseflies, caused mortality that differed statistically from that in the control. While V. fusisporum MP6
was pathogenic to lesser mealworm larvae, the two
isolates from houseflies were not pathogenic. Because
no pathogenicity for insects was shown for these isolates it is, therefore, probable that they were present as
contaminants on the cuticle or in the guts of the
houseflies from whose cadavers they were isolated as
successfully colonized saprophytes. A high proportion of
the cadavers that had been treated with the three
nonpathogenic V. fusisporum isolates as live flies developed sporulating fungus growth anyway. Thus the
fungus is capable of growing as a saprophyte on fly
cadavers. However, V. fusisporum isolate MP6 from B.
tabaci was pathogenic to other B. tabaci individuals
and also to lesser mealworm and must be considered a
true entomopathogen even though the host range of
this isolate does not appear to include M. domestica.
In conclusion, the results of this study indicate that
Verticillium species other than V. lecanii and even
species of Acremonium can act as primary pathogens in
nature and their pathogenicity for a range of insects
can be confirmed in bioassay tests. It does appear that
among the entomopathogenic species of Verticillium
and Acremonium the greatest potential for use in
applied microbial biocontrol is to be found within the V.
lecanii species complex, but the other taxa within these
genera found here can also show appreciable pathogenicity and should not necessarily be ruled out for use in
applied fungal control against some insect pests.
ACKNOWLEDGMENTS
This study was funded by the Danish Veterinary and Agricultural
Research Council and by the Danish Ministry of Food, Agriculture,
and Fisheries. Minna Wernegreen, Henriette Hansen, and Arne
Kirkeby-Thomsen are thanked for their technical assistance and Dr.
Annie Enkegaard for supplying Bemisia tabaci.
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