Abstract:

Among the storage problems of maize and beans, insect pests are a major
constraint. In order to minimize storage losses, farmers usually apply
different methods which include physical or inert materials, cultural means,
botanical and chemicals to control storage pests. However, none of the
individual control method has been successful.
Diatomaceous earth (DE), a natural mineral product from fossilized silica shell
or remains of unicellular organisms is reported to be effective on storage
pests. Of recent, DE has been found in Nebbi district in Uganda. The efficacy
of DE of Uganda origin is not well known. A study was therefore conducted to
evaluate the effectiveness of DE of Uganda origin against S. zeamais and A.
obtectus, the major storage pests of maize and beans respectively.
Three experiments were conducted; first was to determine the DE optimum
dosage rate, the second one was to study the effect of DE on hatchability and
survival of A. obtectus and third was to compare efficacy of DE, selected
botanicals and pesticides against the two storage insect pests. Optimum DE
application rate was determined by investigating six different dosage rates of
0.5,1,2,3,4 and 5% weight per weight basis. The control was untreated grain.
The treated and non treated grains were artificially infested with 5 males and
10 females of each of the species obtained from routine stock culture, each
treatment replicated four times. 2-3 days old A. obtectus and 14 days old S.
zeamais were used. The insects were allowed to mate and oviposit for seven
days and then removed and discarded. To count the eggs of S. zeamais, the
grains were treated with acid fuschin to stain the eggs. For Acanthoscelides
obtectus, eggs were separated from DE-bean mixture and counted under a binocular microscope After counting, the eggs together with the predetermined
DE dosages and grains were put back to their respective
containers and incubated.
The infested cultures were placed in shelves in laboratory under prevailing
ambient conditions until the F4 progeny started to emerge. The adults that
emerged were counted daily, recorded and then discarded.
Results showed that there were significant differences (P< 0.05) in number of
eggs laid and adult emergents in grains treated with DE compared to the
controls. The untreated grain registered the highest number of eggs laid and
adult insect emergence while grains treated with DE at 5% dosage rate
recorded the least number of eggs and insect emergence. However, there
were no significant differences (P> 0.05) in the mean number of emergent
adults in grains treated with DE at different dosage rates.
The highest hatchability percentage and survival of A.obtectus was in the
controls and lowest in grains treated with DE at 5% dosage level.
The effect of botanicals, DE (0.5, 1, and 2%) and insecticides (actellic
super1%, malathion 2% dust) were evaluated. Botanicals tested included;
tobacco leaf and seed powder, neem leaf and seed powder. The same
protocol used in experiment 1 was adopted.
Neem leaf and tobacco seed powder were not toxic to S. zeamais and A.
obtectus. Tobacco leaf and neem seed extracts were effective against the
two candidate storage insect pests. The mean number of S. zeamais and A.
obtectus emergence in maize and beans treated with DE at 2% and 1%
respectively were not statistically different from those treated with synthetic
pesticides. There were no significant differences (P< 0.05) in mean number
of adult that emerged and seed damage in grains treated with DE, tobacco
leaf and neem seed powder.
The incorporation of DE in integrated pest management of S.zeamais and
A.obtectus and their application on-farm is recommended. On-farm trials,
should however, precede recommendation for wider use.