The diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is a
cosmopolitan insect pest of Brassica crops. In South Africa, there are no action thresholds
for its chemical control which makes it difficult for growers to make informed decisions on
when to apply insecticides and how frequently to apply them in order to achieve optimal
crop yield. To contribute towards optimum application of insecticides against P. xylostella,
this study compared the impact of weekly and bi-weekly applications of a selective
insecticide Dipel® (Bacillus thuringiensis Berliner var. kurstaki) applied at 250 g/ha, and a
broad-spectrum insecticide Dichlorvos (an organophosphate) applied at 1 ml/L against
biological control (Control) on the pest population density on cabbage during October–
December 2011 and March–May 2012. The use of both selective and broad-spectrum
insecticides for experiments enables us to understand if efforts to optimise cabbage yield
depend mainly on effective suppression of P. xylostella densities. Furthermore,
investigations were carried out to determine the impact of these chemicals on parasitism
rates of P. xylostella and species richness of its primary parasitoids.
During the October–December 2011 growing season, the lowest infestation of P. xylostella
occurred on cabbage plots that received weekly application of Dipel and the highest on
untreated control plots. Cabbage weights were negatively related to infestation levels,
implying that weekly application of Dipel yielded bigger cabbage heads. During March–
May 2012, P. xylostella infestations were again higher on the control followed by weekly
and bi-weekly treatments of Dichlorvos, then weekly and bi-weekly applications of Dipel.
Despite the significant differences observed, infestation levels were much lower (< 1 P.
xylostella per plant on average) in all treatments during this season. Consequently no
significant differences in cabbage weights were observed among the treatments. The lower
infestation levels were attributed to higher parasitism levels (≥50 %), especially during the
early stages of crop development.
A total of four parasitic Hymenoptera species were recorded from P. xylostella larvae and
pupae during October–December 2011, while three species were recorded during March–
May 2012. However, Cotesia vestalis (Haliday) (Braconidae) accounted for >80 % of total parasitism levels in all treatments. Parasitism levels were not significantly different among
the treatments in both seasons. Parasitoid species richness was highest on the control.
Although two parasitoid species were recorded in all Dipel and Dichlorvos treatments
during October–December 2011, only one parasitoid species was recorded in the Dipel
treatments during March–May 2012 compared to two species in Dichlorvos treatments.
Although weekly applications of Dipel ensured good yield and crop quality during
October–December, weekly applications of the chemical did not lead to better quality crop
during March–May crop growing season. Thus, it is not necessary to apply insecticides
during periods in which natural mortality of P. xylostella is high due to parasitoids. Since
P. xylostella abundance was a determining factor of crop quality, these results imply that
insect pest management should focus mainly on suppressing its numbers. Furthermore,
there was no evidence that application of either insecticide type had a negative impact on
parasitism rates of P. xylostella. The lower parasitoid species richness on Dipel treated
plots was the consequence of its higher efficiency in suppressing the pest population which
substantially reduced availability of potential hosts for parasitoids, hence only the efficient
C. vestalis was recorded at low host densities / Agriculture and Animal Health / M.Sc. (Agriculture)
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:unisa/oai:umkn-dsp01.int.unisa.ac.za:10500/13605 |
| Date | 04 July 2014 |
| Creators | Bopape, Malesela Jonas |
| Contributors | Modise, D. M., Nofemela, R. S. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | 1 online resource (xi, 59 leaves) |
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