ABSTRACT
Twenty-two strains of entomopathogenic fungi were tested in the laboratory to determine their
pathogenicity to adult Megalurothrips sjostedti (Trybom). All the fungal strains were pathogenic
to the insect. Beauveria bassiana and Metarhizium anisopliae strains caused mortality ranging
between 29 to 100% and 54 to 100%, respectively. In contrast, VerticiIlium lecanii and
Paecilomycesfumosoroseus were less virulent, causing mortality ranging between 29 to 68% and
13%, respectively. The LT50 of the most virulent B. bassiana andM. anisopliae strains was 2.7 and
2.4 days, respectively. The LC50 for the most active B. bassiana strain was 7.9 x 106 conidia ml”1.
The LC50 for the most active strain of M anisopliae was 1.3 x 106 conidia ml”1. Two strains of B.
bassiana and four strains of M anisopliae were found to be highly pathogenic to M. sjostedti,
which suggests a potential for their use in biological control of the pest
The effect of temperature on germination, radial growth and pathogenic activity of two
strains of B. bassiana and four strains of M. anisopliae selected during the screening against M
sjostedti, was studied in the laboratory. Germination, radial growth and pathogenic activity were
low for all strains at 15°C. Optimum temperature for germination, radial growth and pathogenic
activity ranged between 25-30°C. The fastest growing strain at 25-30°C wasM anisopliae IC1PE
69. Compared to other strains, ICIPE 69 also appeared to have a broad temperature range of
pathogenic activity against the pest.
The effect of M anisopliae strain ICIPE 69 was tested against M sjostedti developmental
stages. All stages of M sjostedti were susceptible to infection by A/, anisopliae. However, larval
and pupal stages were less susceptible to fungal infection than the adult stage. Mortality in all stages
was dose-dependent with the highest mortality occurring at 1 x 108 conidia ml”1. Daily number of
pollen emptied in infected thrips was reduced within 24 h in adults but more slowly in larval thrips
Fecundity, egg fertility and longevity in adult surviving larval infection were significantly reduced
compared to the control treatment.
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The susceptibility of M sjostedti to M. anisopliae, strain ICIPE 69, when reared on
varieties of cowpea that are susceptible (ICV 2), tolerant (ICV 7), and moderately resistant
(ICV 8) to M. sjostedti was evaluated at different temperatures in the laboratory. Mortality
was significantly higher on the moderately resistant variety at all temperatures compared to the
susceptible and tolerant varieties. Correspondingly, lethal time and lethal concentration values
were significantly shorter and lower, respectively on the resistant variety compared to the
other varieties, thus indicating that the two control methods could be a compatible integrated
pest management strategies. The tolerant variety incurred a significantly low level of mortality.
Observation on the effects of volatiles and crude extracts of this variety revealed an inhibitory
effect on germination and colony forming units of the fungus. This suggest the existence of
some antifungal substances in the tolerant variety
Field experiments were conducted at ICIPE Mbita Point Field Station (MPFS), western
Kenya, for two seasons to evaluate the potential of the entomopathogenic fungus, M. anisopliae
for biological control of M. sjostedti on cowpea. An Ultra-low volume (ULV) oil/aqueous
formulation and a High volume (HV) aqueous formulation of conidia were applied thrice each at
two concentrations of 1 x 1011 and 1 x 1013 conidia ha”1. Compared with the controls, both
formulations significantly reduced thrips population and plant damage in both seasons. Adult insects
collected from fungal treated plots and reared in the laboratory showed high mortality due to
mycosis. Flower and pod production was significantly higher in treated plots compared to the
control plots. HV formulation was superior to ULV formulation in reducing tlirips population and
plant damage, and in increasing flower and pod production. During the first season, plots treated
with HV formulation at 1 x 1013 conidia ha”1 produced yields which were significantly higher than
other treatments. No significant difference in grain yield was found between the fungal treated plots
and the chemical insecticidal treatment, Lambda-cyhalothrin (Karate 17.5 EC) during the second
season. The results of these experiments suggest that M anisopliae is a potential candidate for the
management ofM. sjostedti on cowpea.
Field experiments were conducted during two cropping seasons in 1997 to assess the
combined effects of intercropping cowpea with maize and application of M. anisopliae or synthetic
insecticide on the density of M. sjostedti and cowpea grain yield. Thrips population density and
damage were significantly lower in the intercrop treated with fungus compared to the control.
Mortality due to mycosis was significantly higher in cowpea intercrop compared to the monocrop.
Flower and pod production were significantly higher within the intercrop treated with M.
anisopliae than in the monocrop. In both seasons, grain yields in cowpea intercrop treated with M
anisopliae were not significantly different from yields within cowpea intercrop treated with Karate.
M. anisopliae had no adverse effect on the populations of non-target organisms. The results of this
study suggest thatM anisopliae is a promising candidate for the management ofM. sjostedti,
especially within a cowpea/maize intercropping systems.
Field experiments were conducted for two seasons to evaluate the timing of application of
hi. anisopliae for the control of M. sjostedti on cowpea. One application of M. anisopliae given at
flower bud stage and another at flowering stage did not protect cowpea yield as does chemical
insecticide. Instead, one application given at flower bud stage and two applications given at
flowering were required to keep M. sjostedti density in check through these stages, which are very
sensitive to thrips, damage. Studies of persistence showed thatM anisopliae remained active in the
field for 3-4 days.
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