Biological control of gastrointestinal nematodes of small ruminants, using Bacillus thuringiensis (Berliner) and Clonostachys rosea (Schroers).
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Date
2011
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Abstract
Gastrointestinal nematode parasites cause great losses in the production of small ruminants
through reduced productivity and the cost of preventive and curative treatments. Because of the
threat of anthelmintic resistance, biological control of sheep nematodes has been identified as an
alternative to anthelmintic drugs. Bacillus thuringiensis (Bt) (Berliner) and Clonostachys rosea
(Schroers) have been widely studied as biocontrol agents. B. thuringiensis has been used for the
biocontrol of insects and C. rosea has been successfully used as biocontrol agent of Botrytis
cinera (De Bary) in plants.
B. thuringiensis and C. rosea strains were isolated from soil collected from the Livestock Section
at Ukulinga Research Farm, University of KwaZulu Natal, Pietermaritzburg. Twenty-five strains
of Bt and 10 strains of C. rosea were successfully isolated. The Bt colonies were identified by
their circular, white, flat and undulate character, and the gram-positive and rod-shaped
endospores. C. rosea was identified by white colonies on Potato-dextose agar and the
characteristic conidiophores, which were branched and showed phialides at the tips.
In vitro screening of the isolates was undertaken to select the best isolates. The isolates that
caused significantly greater mortality were Bt isolate B2, B10 and B12 and C. rosea isolates P1,
P3 and P8. These isolates caused substantial nematode mortality in both faeces and water
bioassay. Nematode counts were reduced by 28.5% to 62% and 44% to 69.9% in faecal bioassay
for Bt and C. rosea, respectively. In the water bioassay, nematode counts were reduced by 62%
to 85% for Bt and by 62.7% to 89.3% for C. rosea.
The best inoculum level at which the best isolates were most effective, and the optimum
frequency of application were determined. The trial was conducted using bioassays with faeces
and water. Inoculum levels of 10(6), 10(8), 10(10), 10(12) spores ml-1 for Bt and 10(6), 10(8) and 10(10)
conidia ml-1 for C. rosea was used in the faecal bioassay. The inoculum levels tested in water
bioassay were 10(6), 10(8), 10(10) and 10(12) spores ml-1 for Bt and 10(9), 10(10), 10(11), 10(12) conidia ml-1 for
C. rosea. In the faecal bioassay, B2 was the most effective Bt isolate at an inoculum level of 10(10)
spores ml-1. Isolate P3 was the best C. rosea isolate at 10(8) conidia ml-1. In the water bioassay,
Isolate P3 caused a mortality of 85% at inoculum levels of 10(9), 10(10) and 10(11) conidia ml-1.
The performance of biological control agents in the field is sometimes inconsistent. Combining
different biocontrol agents may be a method of improving their reliability and performance.
However, the combination of most of the isolates was antagonistic, with efficacy less than that of
either individual biocontrol agent. In particular, Isolate P3 was more effective when used alone
than when combined with any other isolates. Therefore, the combination of biocontrol agents
does not always result in synergistic interaction. There were some additive interactions between
two bacterial isolates, and with one bacterial and fungal combination.
The effect of feeding the best of the biocontrol agents, or diatomaceous earth (DE), was
evaluated in sheep. Two doses of Bt (1g and 2g kg-1BW) and C. rosea (1g kg-1BW) reduced the
numbers of L3 nematode larvae in sheep faeces. The DE product (at 15% of feed) also
reduced L3 numbers but it was less effective than either the Bt or the C. rosea products.
Nematode counts were reduced by 74.6%, 75.1%, 84.6%, 68.5% and 27.5% for Bt 1g kg-1BW,
Bt 2g kg-1BW, C. rosea (1g kg-1 BW), DE and control, respectively.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.
Keywords
Ruminants--Diseases., Sheep--Diseases., Ruminants--Parasites--Biological control., Sheep--Parasites--Biological control., Nematodes--Biological control., Biological pest control agents., Bacillus thuringiensis., Antihelmintics., Veterinary helminthology., Theses--Plant pathology.