Occurrence of the diamondback moth (Plutella xylostella L.) and its parasitoids in Ethiopia: influence of geographical region and agronomic traits

Abstract

Surveys were carried out in 2001 in brassica-producing areas of Ethiopia to determine the geographic distribution and severity of diamondback moth (DBM) and occurrence of indigenous parasitoids. Principal component analysis was used to analyse the importance of nine parameters -DBM density, parasitism by different parasitoid species (Oomyzus sokolowskii, Diadegma spp., Apanteles sp. and overall parasitism), cropping systems, pesticide use, frequency of pesticide application and altitude. A total of 194 brassica fields in 13 different areas were visited. Principal component analysis showed that the first two of the nine principal components (largely DBM density and pesticide use) accounted for 70% of the variation in the data set. Higher DBM numbers were associated with pesticide usage and higher overall parasitism with intercropping. Eight parasitoid species were recorded of which three species were important both in distribution and level of parasitism. These include Oomyzus sokolowskii (Hym., Eulophidae), Diadegma spp. (Hym., Ichneumonidae) and Apanteles sp. (Hym., Braconidae). Overall parasitism ranged from 3.6% to 79.5% with big differences between areas. Apanteles sp. and Diadegma spp. were largely confined to the south-western part of Ethiopia where insecticide use is minimal. Oomyzus sokolowskii is mainly distributed in areas of the Rift Valley and northern Ethiopia with relatively heavy insecticide use. This study indicated a huge impact of repeated insecticide treatment and agronomic practices on DBM severity and its parasitoids.

Tri-Trophic Insecticidal Effects of African Plants against Cabbage Pests

“…Until the early 1960s when large scale application of synthetic insecticides was introduced to commercial vegetable farming, P. xylostella was not a major pest in China [43]. The use of pesticides that are harmful to the third trophic level need to be minimised in favour of less harmful plant protection compounds to allow biological control to play a role in integrated pest management [44]. In particular, an active natural enemy fauna can reduce the frequency with which insecticides applications are required and kill survivors of insecticide application to prevent the development of resistance in the pest population.…”

Determinants of parasitoid assemblages of the diamondback moth, Plutella xylostella, in cabbage farmer fields in Senegal

“…Low numbers of O. sokolowskii in south-western Ethiopia were attributed to probable competition with Apanteles sp. (Ayalew and Go, 2006). Similarly, Liu et al (2000) reported a negative relationship in parasitism level between O. sokolowskii and C. vestalis, which could explain partly that parasitism due to C. vestalis was greater during the second part of dry season.…”

“…In the present study, high interfield variability of parasitism rates (0e50%) indicates that crop management, especially insecticide use, but possibly also landscape context, eg semi-natural habitats or crops providing food or alternative hosts to parasitoids, could influence biocontrol by natural enemies (Bianchi et al, 2008). Surveys conducted in 2001 in brassica-producing areas of Ethiopia (194 brassica fields, 13 areas) revealed high variability among areas, with overall parasitism ranging from 3.6 to 79.5% (Ayalew and Go, 2006). Parasitism by indigenous parasitoid species was low (13.2%) and DBM density was high in the Rift Valley, the major brassica-producing area.…”

A stochastic model for predicting the stage emergence of Plutella xylostellaunder field conditions

“…Recent and conservative estimates show that the total annual costs associated with the worldwide management of P. xylostella are approximately US$ 4 billion (Zalucki et al , ). Among the reasons usually mentioned to explain the high incidence of diamondback moth are the absence of effective natural enemies, as well as its high reproductive potential and ability to rapidly develop resistance to insecticides (Lim, ; Talekar & Shelton, ; Ayalew & Ogol, ; Ayalew et al , ; Furlong et al , ; Sayed & Teilep, ). Combination of these factors has hampered the control of P. xylostella , making the economical production of brassicas impracticable in certain regions of the world (Sarfraz et al , ).…”