Directory UMM :Journals:Journal of Stored Products Research:Vol 36.Issue2.Apr2000:

Journal of Stored Products Research 36 (2000) 177±185
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Occurrence of Beauveria bassiana on insect pests of stored
maize in Kenya
G.I. Oduor a,*, S.M. Smith a, E.A. Chandi a, L.W. Karanja a, J.O. Agano a,
D. Moore b
a

CABI Africa Regional Centre, P.O. Box 633, Village Market, Nairobi, Kenya
CABI Bioscience UK Centre (Ascot), Silwood Park, Ascot, Berks, SL5 7TA, UK

b

Accepted 27 July 1999

Abstract
Surveys for fungal pathogens of maize storage pests were conducted in Kenya between February and
August 1997. A total of 124 farms in 12 districts located in Lower Highland, Upper Midland and Lower
Midland agroecological zones (AEZs) were sampled. Sitophilus zeamais constituted at least 75% of the
pests in the 3 AEZs. Prostephanus truncatus (Larger Grain Borer), introduced into Kenya in 1983 was

found in stored maize only in Taita Taveta district. Insects infected with Beauveria spp were
encountered rarely (in 12 of the 124 farms sampled) but had a wide distribution (in 8 of the 12 districts
visited). The fungus occurred at a very low prevalence of between 0.08 and 0.94% of the total insects
collected, mainly infecting S. zeamais but also found on Tribolium sp and Carpophilus spp. # 2000
Elsevier Science Ltd. All rights reserved.
Keywords: Entomopathogenic fungi; Beauveria spp; Sitophilus zeamais; Prostephanus truncatus; Survey; Kenya

1. Introduction
Maize is the staple food of over 80% of Kenya's population of over 24 million. Although it
is grown on large farms, especially in the Rift Valley province, more than 90% of this crop is
produced on small scale farms (Odhiambo, 1988). Both local and hybrid maize are grown.
Insect pests are one of the major constraints on maize production in Kenya, and attack
* Corresponding author. Tel.: +254-2-522150 3450; fax: +254-2-522150/521001.
E-mail address: g.oduor@cabi.org (G.I. Oduor).
0022-474X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 2 - 4 7 4 X ( 9 9 ) 0 0 0 4 1 - 7

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G.I. Oduor et al. / Journal of Stored Products Research 36 (2000) 177±185

maize both in the ®eld (pre-harvest) and while in storage (post-harvest). The main post-harvest
pests in Kenya include Sitophilus zeamais Motsch. (maize weevil), Sitotroga cerealella (Olivier)
(Angoumois grain moth), Prostephanus truncatus (Horn) (larger grain borer±LGB) and
Tribolium spp (¯our beetles). Post-harvest yield losses vary widely (Schulten, 1988), but losses
are generally around 4±5% per annum of maize stored on-farm in Africa (McFarlane, 1988).
Attempts to minimise losses caused by storage pests have relied mainly on cultural and
chemical methods. Cultural methods involve early harvesting of maize as soon as it attains
physiological maturity, and ensuring general cleanliness in and around the store. Chemical
methods, which started with the use of DDT in 1948, have repeatedly been faced by problems
of pesticide resistance in Kenya. These pests have developed resistance to chemicals that were
considered to be more e€ective than DDT, including lindane (Giles and Ashman, 1971) and
malathion (Champ and Dyte, 1976), and this has also been reported in other countries
including USA (Haliscak and Beeman, 1983), Uganda (Evans, 1985), Israel (Navarro et al.,
1986), Argentina (Picollo de Villar et al., 1987), Philippines (Sayaboc and Acda, 1990) and
Australia (White and Lambkin, 1990). A common control measure for infestation of S.
zeamais and P. truncatus in Kenya is to treat grain with a dust formulation containing a
mixture of an organophosphate (OP) and a pyrethroid insecticide. However, P. truncatus is
tolerant of OPs and is likely to develop resistance to pyrethroids, which are not particularly
e€ective in controlling Sitophilus spp (Barbosa et al., 1995; Golob et al., 1990, Haubruge,

1990). This, together with the high cost of the pesticides, environmental risks and human
poisoning, has led to the need to ®nd alternative, safer and environment friendly control
strategies.
The use of fungal pathogens to control stored product pests has not received much
attention, probably because of the belief that the maize storage environment is too hostile for
the survival and ecacy of entomopathogenic fungi. However, studies involving the use of
these fungi against a number of pests have produced encouraging results. Lourenc° aÄo et al.
(1993) reported that between 98.4 and 100% of S. zeamais adults infesting maize grains which
had been treated with conidia of Beauveria bassiana (Balsamo) Vuillemin died within 14 days.
Rodrigues and Pratissoli (1990) showed that maize mixed with conidia of B. brongniartii
(Sacc.) Petch and inoculated with S. zeamais, was still protected from damage after a period of
6 months. Adane et al. (1996) also showed that although pirimiphos-methyl killed adult S.
zeamais faster than the spores of B. bassiana, the damage the weevil caused to the maize
grains, after a 14-day storage period, was not signi®cantly di€erent between the 2 treatments.
A project to investigate the use of entomopathogenic fungi as control agents of storage pests
began in Kenya in December 1996. P. truncatus and S. zeamais were selected as the primary
target pests for insect pathogens. Initial work (S. Smith, personal communication, 1998)
showed P. truncatus to be very susceptible to attack by non-speci®c isolates of Beauveria spp
obtained from soil samples by baiting with larvae of the wax moth Galleria mellonella L.
(Zimmermann, 1986), while S. zeamais was much less susceptible. However, those isolates that

were most pathogenic to S. zeamais were also the most pathogenic to P. truncatus (Smith et
al., in press). It was hypothesised that isolates obtained from S. zeamais might be more
e€ective against this species as well as being e€ective against P. truncatus. Where the intention
is to develop a product that can be used in maize stores, the speci®city of fungal isolates is
important. An isolate virulent against a wide variety of insects could be used alone but may

G.I. Oduor et al. / Journal of Stored Products Research 36 (2000) 177±185

179

have e€ects on non-target organisms, whereas isolates speci®c to one pest species may need to
be used in combination against a pest complex. To our knowledge, no isolates of con®rmed
entomopathogenic fungi had previously been isolated from storage pests in East Africa. Thus
the aim of this study was to survey the maize growing regions of Kenya and assess the
occurrence of Beauveria spp and other entomopathogenic fungi in ®eld populations of pests of
stored maize. These isolates could then be assessed for their pathogenicity to S. zeamais and P.
truncatus in subsequent studies.
2. Materials and methods
The surveys for pathogens in maize storage pests in the di€erent AEZs in Kenya were
carried out between February and August 1997 with the assistance of the local District

Agricultural Ocers in the respective districts. According to Jaetzold and Schmidt (1982) the
following agroecological zones (AEZs) are the most suitable for the cultivation of maize.
Lower Highland zone (LH): 1800±2400 m above sea level (masl). Moderately cool with an
annual mean temperature of 15±188C. The main maize growing zones here are LH1 which is
humid and with an annual average precipitation at least 80% of the potential evaporation,
LH2 which is subhumid and with an annual average precipitation 65±80% of the potential
evaporation and LH3 which is semi-humid with an annual average precipitation 50±65% of
the potential evaporation.
Upper Midland zone (UM): 1300±1900 masl. Temperate with annual mean temperature of
18±218C. The maize growing zones here are the humid UM1, sub-humid UM2, semi-humid
UM3 and the transitional UM4.
Lower Midland zone (LM): 800±1500 masl. Warm with annual mean temperatures of 21±
248C. Similarly, the main maize growing zones here are the humid LM1, sub-humid LM2,
semi-humid LM3 and transitional LM4.
Lowland zone (L): 0±800 masl. Hot with an annual mean temperature of more than 248C.
Here maize does well in the sub-humid L2, semi-humid L3 and transitional L4.
In choosing districts, an attempt was made to visit all the AEZs where maize is grown. Most
districts were visited both in the wet and dry seasons, except when stored maize was not
available when districts were visited only once. Some districts, where previous severe drought
had led to extremely poor maize harvests, e.g. in the Lowland AEZ, were not visited. A total

of 124 farms in 12 districts were sampled. The locations of the districts visited and their zones
are shown in Fig. 1.
In each district, between 1 and 6 farms were selected in each AEZ where the maize storage
structures were sampled and the farmers interviewed. Maize was stored either unshelled (on the
cob and with or without the husks) or shelled. For maize on the cob, 5 heavily infested maize
cobs were selected from the storage structures at each farm and kept in 10 cm diameter  20 cm
high glass jars with perforated lids. Shelled maize was ®rst sieved using two 20 cm diameter
sieves (upper and lower sieves of 3 mm2 and 1 mm2 mesh sizes, respectively). Maize grains
collected from the ®rst sieve, and the pests, which collected on the lower sieve, were then
brushed into a separate glass jar similar to the above. Some grains of maize were thereafter
added to the jars to serve as food for the sampled pests. All material was returned to the
CABI-ARC laboratories for sorting.

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G.I. Oduor et al. / Journal of Stored Products Research 36 (2000) 177±185

Fig. 1. Agroecological zones of Kenya and location of districts where maize stores were sampled. AEZs: LH Ð
Lower Highland; UM Ð Upper Midland; LM Ð Lower Midland; L Ð Lowland; Districts sampled: 1 Ð Busia; 2
Ð Siaya; 3 Ð Migori; 4 Ð Kisii; 5 Ð Gucha; 6 Ð Kuria; 7 Ð Nyamira; 8 Ð Lugari/Malava; 9 Ð Uasin Gishu;

10 Ð Trans Nzoia; 11 Ð Nakuru; 12 Ð Taita-Taveta.

Work in the laboratory focused only on the adult insects. The samples were sorted by
species and dead and live insects were counted. After the ®rst 28 samples had been processed it
became clear that counting all individuals collected per store was an excessively time
consuming activity, so for subsequent samples a maximum of 600 dead and 600 live individuals
per species were sorted out from each sample. Living individuals were reared on ground dried
maize in 9 cm diameter  9 cm high jam jars and observed for dead individuals fortnightly for
a period of 4 weeks. To check whether individuals died from mycosis, they were surface
sterilised by dipping in 5% sodium hypochlorite for 2 min, rinsed in sterile distilled water and
placed on moist ®lter papers in 9 cm diameter plastic Petri dishes. Dead insects were
maintained on a bench at room temperature and observed every other day (over a period of 10
days) for the presence of fungal growth.
The entomopathogenic fungi arising from the cadavers were isolated and inoculated onto

G.I. Oduor et al. / Journal of Stored Products Research 36 (2000) 177±185

181

Sabouraud Dextrose Agar (SDA) medium with antibiotics (penicillin and streptomycin), to

limit the growth of contaminants. Subsequent subcultures were grown on SDA without
antibiotics.

3. Results
The most common pests found in the stores were S. zeamais, Tribolium spp, and in Taita
Taveta District only, P. truncatus. Others included Rhyzopertha dominica (F.), Oryzaephilus
surinamensis (L.), Carpophilus spp and Cryptolestes ferrugineus (Stephens). High numbers of S.
cerealella were also frequently observed, especially in the higher altitude zones, however,
population sizes were not recorded due to the diculties in counting the highly mobile adults.
The composition of the pests in the di€erent AEZs is shown in Table 1a. S. zeamais was by far
the dominant pest insect species. In the districts from which the entire sample of insects were
counted, i.e. those taken during the ®rst dry season (Table 1a), S. zeamais accounted for
between 77.7% and 94.7% of the insects collected. However, in the Taveta region of Taita
Taveta district, S. zeamais accounted for only 5.0% of the insects in the samples and P.
truncatus made up 75.6% of the total. S. zeamais constituted 85.6% and Tribolium spp 8.6%
of all the insects collected during the ®rst dry season. Proportions of insects collected were not
expressed for the subsequent samples (Table 1b and c) because of the maxima of 600 live and
600 dead insects per species counted.
S. zeamais had the highest proportion of dead individuals; 38.7% of the insects were dead
when collected and a further 4.2% of the reared insects died within two weeks of collection.

The only pathogen found and isolated from cadavers was Beauveria bassiana. Insects infected
by B. bassiana were found in 12 of the 124 farms (9.7%) visited. A total of 33,416 cadavers of
S. zeamais were examined for pathogen infection (94% of all cadavers), and 26 of the 29
insects from which B. bassiana was isolated (89.7%) were S. zeamais (Table 1). Two cadavers
of infected Tribolium spp were found and one from Carpophilus spp, although the fungus
isolated from the latter failed to become established in culture.
The fungus was widespread, occurring in 8 of the 12 districts visited (Table 1). It was also
found in all the AEZ sampled i.e. on 6 farms in LH (two each in LH1 and LH2 and four in
LH3), 4 farms in UM (two each in UM1 and UM3) and 2 farms in LM (one each in LM1 and
LM3). Although the fungus had a wide distribution, it was sparse in occurrence, infecting less
than 1.0% of the insects in the stores where it was recorded. The highest prevalence (0.94%)
was recorded at a store in Nyamira and the lowest (0.08%) in Trans Nzoia. Of the 29 insects
from which the fungus was isolated, 23 (79.3%) were dead at the time of collection. An isolate
was de®ned as the occurrence of Beauveria sp. from one store. Where it was found in the same
store infecting more than one species, e.g. in Nyamira and Kisii, it was presumed to be the
same isolate, although separate cultures were kept from each host.
The infested maize from which the samples were collected was stored in di€erent ways. In 11
of the 12 stores (92%) where the fungus was found, the maize was shelled, and stored in sacks
whereas maize was stored in this way at only 76.8% of the farms visited. Seven of the twelve
isolates were collected from samples taken during the dry season. Five of these were found

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G.I. Oduor et al. / Journal of Stored Products Research 36 (2000) 177±185

Table 1
Total number of insects collected (and those from which B. bassiana was isolated) from sample groups according to
season, district and AEZ. (a) shows the percentage of each species in the sample group in brackets
Number of insects collected
(live and dead)
(percentage of insects in sample group)
District

AEZ

Farms

S. zeamais

(a) Dry season I: early February±mid March 1997

Taita Tavetaa
UM
6
2970 (77.7)
LM
2
26 (5.0)
Lugari Malava
UM
5
8641 (94.7)
LM
1
1151 (86.7)
Siayaa
LM
8
4433 (82.2)
Busia
LM
6
2930 (87.1)
Sub-totals
28
20,151 (85.6)
Infected insects
3
(b) Wet season: mid March±mid July 1997
Nakuru
LH
3
2017
Uasin Gishua
LH
10
7806
Nyamiraa
LH
3
1813
Trans Nzoia
LH
3
2020
UM
6
5188
Kuriaa
UM
3
1114
Taita Taveta
UM
5
2326
LM
6
746
Lugari Malava
UM
5
4268
Kisiia
UM
2
667
LM
1
720
Migori
LM
2
1055
Siaya
LM
6
2749
Busia
LM
2
1109
Sub-totals
57
33,598
Infected insects
12
(c) Dry season II: mid July±end August 1997
Nyamira
LH
1
484
Nakurua
LH
6
3297
Uasin Gishua
LH
6
5354
Trans Nzoiaa
LH
5
4239
UM
4
3494
Gucha
UM
6
3427
Kuria
UM
4
2690
LM
2
1934
Migori
LM
5
2398
Sub-totals
39
27,317
Infected insects
11
Totals
124
81,066
Total infected insects
26
a

Tribolium

P. truncatus

779 (20.4)
102 (19.4)
379 (4.2)
176 (13.3)
405 (7.5)
174 (5.2)
2015 (8.6)
1

47 (1.2)
394 (75.6)
±
±
±
±
441 (1.9)
±

±
656
197
48
590
3
216
464
451
±
±
±
95
8
2728
1

±
±
±
±
±
±
64
3551
±
±
±
±
±
±
3615
±

155
81
1837
415
879
5
26
3
3401
±
8144
2

Indicates districts from where B. bassiana was recovered.

±
±
±
±
±
±
±
±
±
±
±
4056
±

Others

25 (0.7)
±
101 (1.1)
±
556 (10.3)
259 (7.7)
941 (4.0)
±
±
62
12
4
101
3
22
5
431
±
28
±
37
2
707
1
±
27
60
731
4
35
27
13
±
897
±
2545
1

Infected insects

3
±
±
±
1
±
4
±
2
6
±
±
3
±
±
±
1
2
±
±
±
14
±
1
9
±
1
±
±
±
±
11
29

G.I. Oduor et al. / Journal of Stored Products Research 36 (2000) 177±185

183

from samples collected at the beginning of the dry season, i.e. after the long rains of midMarch to mid-July.

4. Discussion
S. zeamais was the predominant species overall and was so in almost every sample, but there
were marked di€erences between samples. For example, although S. zeamais constituted 75.5%
of the pests in the Lower Midland AEZ, this ®gure was found to be higher (e.g. in Migori
district) or lower (e.g. in Taveta region of Taita Taveta district) in the same AEZ, showing the
non-uniformity of the dominance of S. zeamais even within the same AEZ. The low numbers
of S. zeamais in the Taveta region of Taita Taveta district may be due to competition from P.
truncatus. Other factors such as season may play a signi®cant role in the pest composition at a
given locality. Indeed, De Lima (1977) found that the dominance of S. zeamais over S.
cerealella and vice versa depended on the time of the storage season. Consequently it is
essential to attempt control of the complex of pests rather than of individual species.
Pheromone traps placed at di€erent sites in Kenya reveal that the distribution of P. truncatus
is widening and now includes Nyamira, Migori, Busia and Nakuru districts (J. Mbugua,
personal communication, 1998). Pheromone traps use chemical cues and are more ecient at
detecting low levels of pests than physical examination methods. It could be just a matter of
time before P. truncatus is detected in the stores, where it is presently recorded as absent.
Despite the misconception that the environment in cribs and other structures used to store
maize is too hostile to support growth of entomopathogenic fungi, Beauveria bassiana was
found to be widespread in Kenya, and was isolated from S. zeamais, Tribolium and
Carpophilus species (occurrence in these species may indicate that the application of a
Beauveria spp formulation has potential to control a wide range of storage pests).
Until recently there were relatively few records of entomopathogenic fungi from Africa, for
example, Shah et al. (1997) stated that before major surveys were conducted only three isolates
of Metarhizium spp were recorded from Orthoptera in Africa. Their surveys produced 112
isolates from West Africa, of which 81 were from the Republic of Benin alone which was the
most extensively searched. The present work adds support to the view that absence of isolates
is due to absence of searches and that a vast array of potentially valuable isolates waits to be
discovered. Smith et al. (in press) showed that the S. zeamais-derived isolates are more virulent
to S. zeamais than non-speci®c isolates obtained from soil samples or other pest species. Such
studies should include improved application and formulation techniques. The prevalence of
Beauveria bassiana among populations of storage pests of maize was found to be very low.
This can be increased through the development of mycopesticides. Pham et al. (1995) showed
that populations of stored pests of cereals and beans could be reduced by 61.1% by spraying
conidia of B. bassiana onto the surface of bags in which these products were stored. Adane et
al. (1996) evaluated the virulence of 10 isolates of B. bassiana to S. zeamais and found that one
of the isolates (at a low dose of 104 conidia/ml) caused 88.0% mortality within 8 days.
Formulations can increase the ecacy of fungal pathogens used against storage pests (Hidalgo
et al., 1998).
More isolates of B. bassiana were obtained from dry season samples suggesting that the

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G.I. Oduor et al. / Journal of Stored Products Research 36 (2000) 177±185

more humid conditions during the wet season may have had little impact on the incidence of
entomopathogenic fungi in stores. However, it could be argued that as 5 of the 12 isolates were
found in the dry season after the long rains, the incidence of B. bassiana infection may have
been initiated in the wet season but not detected until the dry season.
That all but one of the isolates of B. bassiana was obtained from S. zeamais was
unsurprising given the numbers of this species collected, but the prevalence of B. bassiana was
higher for Tribolium sp. with two isolates obtained from 1155 cadavers. Although the
proportion of isolates obtained from shelled maize was higher than the proportion of stores
with shelled maize the number of isolates is too low to confer any signi®cance. Shelled maize is
more susceptible to attack from S. zeamais than maize stored on the cob (Golob and Hanks,
1990), which is probably why nearly all insects infected with fungal pathogens came from
shelled maize.
Since most of the B. bassiana was isolated from insect cadavers that were dead when
collected rather than those that died in the laboratory, and from maize which was shelled and
stored in sacks, subsequent surveys for fungi should target this storage method and only dead
insects should be collected.
At present it is not known how many distinct isolates were collected during this survey, as
this entomopathogenic fungus still needs to be characterised. The fungus collected in di€erent
locations will be characterised and the most suitable isolate (human safety, virulence to target
pests, non-target safety, persistence, etc.) will be selected for further studies.

Acknowledgements
This publication is an output from a research project funded by the Department for
International Development of the United Kingdom. However, the Department for
International Development can accept no responsibility for any information provided or views
expressed. DFID Project R6773 Crop Post Harvest Research Programme. Assistance given by
the sta€ of the di€erent District Agricultural Oces is also gratefully acknowledged.

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