Abstract:

For effective use of the entomopathogen Beauveria bassiana in the management of
the banana weevil, high inoculum levels and appropriate formulations are mandatory.
Equally important is the development of a suitable formulation that is persistent within
the target environment, in this case the field soil ecosystem. Evaluations of Beauveria
bassiana were carried out from Kawanda, Uganda on conidial yield from selected
substrates, efficacy of different formulations against the banana weevil, and, efficacy
and persistence of the fungus under organic and inorganic soil amendments. The
substrates evaluated were cracked maize, maize bran, “machicha”, bagasse, cotton
spent yeast
and they were formulated with clay or loam soils. The soil amendments evaluated
were un-amended soils, coffee husks, decomposed cowdung manure and inorganic
fertilizers. The results indicated that cracked maize grains were the most ideal for B.
bassiana conidia production while the dried formulation of the same media was also
the most effective against the banana weevil under laboratory conditions. The
formulation gradually lost its efficacy as storage period increased. Overall, the soil
amendments significantly reduced the efficacy and persistence of the
entomopathogenic fungus over time.
Laboratory evaluation of the substrates was based on conidia counts using the
improved haemocytometer. Cracked maize grains were the best substrate with 3.2
x109 conidia per gram, even though it was not significantly different from that of maize
bran (3.1 x109). Evaluation of the B. bassiana formulations against the banana weevil
ix
husks, maize bran + bagasse, maize bran + cotton husks, and bagasse +
under laboratory conditions resulted in cracked maize followed by cracked maize
formulated with clay soil as the most effective with over 80% weevil mortality in 30
days. Following storage under laboratory conditions for 90 days, recorded weevil
mortality after exposure to the cracked maize grains formulation was > 85% in 30
days, but this declined to 20% following 180 days of storage.
In the pots, soil: amendment ratios were 100%, 50%, 33.3%, 25% and 0% (eg. un
amended soil, coffee husks alone or cow dung alone to give the 100%, 1 part of soil:
1part of amendment to give the 50% by volume, etc.). One hundred grams (100g) of
cracked maize B. bassiana formulation were applied in each pot at the same time.
Ten weevils of the same age, mixed sex (1:1 ratio) were released in each pot for 30
days, and this was repeated every other 30 days. Weevil mortality was monitored
from pots and from B. bassiana culture picked from pots and tested in the laboratory
at 5-day intervals. Moisture content, temperature, soil invertebrates, substrate
colonisers and decomposers, and other amendment characteristics were monitored
from pots during the course of the experiments. Results indicated that Beauveria
applied in un amended soils had the highest efficacy of 55% weevil mortality for the
first release and 27% after 90 days. However, this result was not significantly different
(P>0.05), for the first release, for all the soil amendments. Significant differences were
observed in the pots after 60 days. By 90 days after application, the B. bassiana
formulation had significantly lost efficacy and persistence in all the treatments in the
pots.
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