Cruciferous crops comprise one of the major crop divisions and contribute
significantly to global crop production. Insect pests associated with cruciferous crops
have the potential to destroy harvests. The diamondback moth, Plutella xylostella
(L.) (Lepidoptera: Plutellidae) and cabbage aphids, Brevicoryne brassicae (L.) and
Lipaphis erysimi (Kaltenbach) (Homoptera: Aphididae) are considered the most
important cabbage pests in South Africa. Pest control in cabbage is still heavily
reliant on use of insecticides even though insecticide application usually result in
increased pest status of P. xylostella. Use of integrated pest management (IPM)
strategies may yet prove invaluable as a means of suppressing the more serious
cabbage pests. Global interest in cultural control strategies, that includes
intercropping of unrelated crop plants, has been rekindled because of problems
experienced with pesticide use. A study that involved the planting of two field trials
with cabbage and carrot plants in different ratios, as a substitutive intercropping
system, was undertaken during the 2002/03 and 2003/04 planting seasons. Field
trials had five treatments viz. control (cabbage monoculture sprayed with
mercaptothion), cabbage monoculture (without chemical application), 1:1
intercropping (cabbage:carrots), 1:3 intercropping and 1:5 intercropping, replicated
six times in a randomized block design. Insects were sampled weekly by removing
the third-youngest open leaf, from one plant per plot (six plants per treatment).
Feeding damage to cabbage leaves was assessed visually by rating 14 plants per plot
according to a numeric scale based on damage severity. P. xylostella and aphid
parasitism levels were determined. Prominence values were calculated for P.
xylostella larvae and aphids for each treatment per sampling date. P. xylostella
infestations and parasitism levels were significantly (P ≤ 0.05) lower in control
treatments. Intercropping ratio did not have a significant impact on pest infestations.
P. xylostella infestations and parasitism on intercropping treatments did not differ
significantly (P > 0.05) from that on monoculture plots (except for 1:5 intercropping
during the 2003/04 season). Cotesia plutellae (Hymenoptera: Braconidae) was the
dominant parasitoid and parasitism levels were significantly correlated with P.
xylostella infestation levels. Prominence values showed a decline in P. xylostella
infestations on intercropping treatments towards the last sampling date during both
seasons. Feeding damage severity was lowest on the control treatment but similar for
other treatments for both seasons. Aphid infestations were significantly (P ≤ 0.05)
lower on the control treatment, but did not differ significantly between intercropping
treatments. Aphid prominence values indicated possible suppression of infestations at
low population levels during the early part of the season. Aphid parasitism was low
during both seasons. It seems more likely that the reduction in insect infestation
levels reported from the literature are related more to spatial arrangement of plants
than cropping ratios. The observed suppression of aphid infestation levels on
intercropped plots early in the season may be valuable in reducing the need for early
chemical applications. The limiting of early insecticide application against aphids
may result in a reduced pest status of P. xylostella, due to the conservation of C.
plutellae populations. / Thesis (M. Environmental Science))--North-West University, Potchefstroom Campus, 2007.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/1662 |
| Date | January 2007 |
| Creators | Weeks, William James |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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