Population Ecology of Diamondback Moth, Plutella Xylostella L. (Lepidoptera: Plutellidae) and its Parasitoid in Ethiopia

Abstract

The objective of this study was to generate information on the ecology of Diamondback moth (DBM) (Plutella xylostella L.) and the parasitoids associated with it with emphasis on parasitism for the purpose of strengthening biocontrol in Integrated Pest Management (IPM) program. Prior to this study no or little information was available in Ethiopia on DBM despite its importance in brassica production. A series of studies were conducted between 2001 and 2002, including a survey covering the most important crucifer growing areas, within field distribution of larvae and pupae, yield loss assessment, suitability of wild and cultivated crucifers for DBM development and population dynamics (spatial and temporal) of DBM and its parasitoids.

Survey conducted in thirteen brassica producing areas showed that DBM density and level of damage were higher in the vegetable production system of central Rift valley regions and Arsi high land. Seven hymenopteran parasitoid species from five families were recorded. Of these, Oomyzus sokolowskii (Hymenoptera: Eulophidae), Diadegma sp. (Hymenoptera: Ichneumonidae) and Apanteles sp. (Hymenoptera: Braconinae) were the dominant ones accounting for more than 90 per cent of the parasitoid complex. Level of parasitism by the indigenous parasitoid species was low in major brassica producing areas of the country, the Arsi highland and the rift valley areas.

Yield loss was assessed using natural infestation in a Randomized Complete Block Design with nine treatment. Treatments were pesticides, Bacillus thuringiensis (Bt) and karate, applied at different growth stages of head cabbage, seedling, pre-heading, and heading. Yield loss ranged between 36.1 and 85.2 percent corresponding to 12 to 27.6 tons per hectare.

The developmental period of DBM on four cultivated brassica crops and one wild crucifer, Erucastrum arabicum Fisch and Mey was studied for two generations in the laboratory in ambient conditions (Temperature ranged between 23 and 300C). Life table statistics including development periods indicated that cabbage was the most suitable host and the wild crucifier was as suitable as some of the cultivated crucifers.

Within field distribution of larvae and pupae was contagious in cabbage and kale. In Ethiopia mustard, only the young larvae showed contagious distribution. Spatial analysis showed that DBM is aggregated in field of its host plants. The influence of weather variables on aggregation varied with locations. In highland areas of Arsi, maximum temperature showed a significant influence (R2 = 0.407, p < 0.05). In the lowland areas of Wonji, the aggregation index was significantly influenced by rainfall (R2 = 0.603, p < 0.05).

Studies on temporal dynamic showed two to three generations per head cabbage growing season in the highland brassica production area and three to five generations in the lowland. Population fluctuated between 0 to 15.7 and 0 to 1.7 insects per plant in November/December and April planted trials, respectively, at the highland site; and between 0 to 3.2 and 0 to 8.5 at the lowland site. Lower DBM density was associated with higher rainfall and lower maximum temperature in the highland experimental site. At both locations, three species of parasitoids were recorded. These were Oomyzus sokolowskii, Diadegma sp., and Apanteles sp. Average overall apparent (and potential) parasitism was 13.6 (14.0 ) % and 46.3 (51.7) % at the highland site; and 4.6 (4.6) % and 25.6 (26.3) % at the lowland site in the November/December and April planted experiments, respectively. O. sokolowskii was the dominant species at the low land site and Diadegma sp. at the highland site. One or more factors including host density, season of planting, location, and environmental variables, showed significant influence (P < 0.5) on parasitism level by the different parasitoid species.

Biocontrol of DBM in Ethiopia needs to target areas of major brassica production with higher level of DBM damage and low parasitism: the Arsi highland and the central rift vallet region. In addition, currently used pesticides in these areas need to be replaced with safer alternatives with minimal effect on bio-control agents such as the use of neem based products and microbial pesticides such as Bt