The goals of the study were to confirm establishment of two exotic parasitoids, Cotesia vestalis (Haliday) (Hymenoptera: Braconidae) and Diadromus collaris (Grav.) (Hymenoptera: Ichneumonidae), released in Lusaka Province, Zambia, during the 1970s and 1980s, to control the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), a serious pest of vegetables in Zambia, and to assess the effectiveness of one of the released parasitoids, C. vestalis, as a biological control agent of the pest. Confirmation of the establishment of the exotic parasitoids was through field sampling and collection of various developmental stages of the parasitoids, in and around the release sites in Lusaka Province. The assessment of the effectiveness of C. vestalis, as a biological control agent of the crop pest was through laboratory experiments.
The specific objectives were to: i) Sample and collect various developmental stages of the parasitoids, C. vestalis and D. collaris, in and around the 1977-1984 parasitoids release sites in Lusaka Province, Zambia; ii) determine the types of pesticides used by local farmers in the Chongwe, Kafue and Lusaka districts on common vegetable pests and their impacts on the exotic parasitoids, C. vestalis and D. collaris; iii) determine the phenology of P. xylostella and identify its parasitoids in the selected study area; and iv) assess the effectiveness of C. vestalis as a biological control agent of P. xylostella under laboratory conditions.Vegetable farms, in and around the original release sites of parasitoids in Lusaka west and in the Makeni area were selected for the study. Parasitoid presence was the criterion used to confirm establishment. The sampled vegetable fields were geo-referenced using a Global Positioning System (GPS) and the information obtained was used to construct a pest/parasitoid field sampling map. The nearest plant after every 2 m in the campus direction on a 33.2 m diagonal line transect was sampled for the diamondback moth larvae and pupae.A structured questionnaire was employed to collect information on pesticide usage by local farmers in the Chongwe, Lusaka and Kafue district on common vegetable pests in their areas, while the assessment of the impacts of pesticides on C. vestalis involved use of a stock culture of the parasitoid established in the insectary in the School of Agricultural Sciences at the University of Zambia. Determination of the phenology of Plutella xylostella involved establishment of a pesticide-free, hand-weeded, drought and disease tolerant hybrid cabbage plot (cultivar Pannar?, Star 3308) at the University of Zambia, School of Agricultural Sciences, Field Station, following normal agronomic practices. The effectiveness of C. vestalis as biological control agent was tested using different second larval instar densities (5, 10, 25, 50 and 100 per adult female) of P. xylostella under laboratory conditions. The impact of pesticides on C. vestalis was assessed using four insecticides namely, acephate at 10, 30, 100, 300, 500, 1000, 1500, 1600 and 1700 ppm, cypermethrin at 2, 4, 6, 8, 10 ppm, ?-cyhalothrin at 2, 4, 6, 8, 10 and 50 ppm, and dichlorvos at 4, 6, 8, 8.5, 9, 9.5 and 10 ppm. Ten unsexed parasitoids were put in individual Petri dishes containing the different bioassays treatments. Parasitoid mortality was recorded after 12, 24, 36, 48, 60 and 72 h of exposure.
The exotic parasitoids, C. vestalis and D. collaris, were confirmed to have established themselves in the Lusaka Province, Zambia, in this study. Both immature stages and adults of both species of parasitoids were found and collected from the field. Cotesia vestalis was collected from five farms (Liempe, Malambo, Miller, Mwenya and Uzakulweni) while D. collaris was recorded only at Malambo farm. The level of parasitism by C. vestalis ranged from 4.4% to 54.9% while D. collaris was 14.3%. The other parasitoid recorded was the hyper-parasitoid of C. vestalis, namely, Aphanogmus sp. (Hymenoptera: Ceraphronidae). The hyper-parasitoid, Aphanogmus sp. is being reported for the first time in Zambia by this study. Its parasitism rate was 15.3% under field conditions.A survey on pesticide use in Chongwe, Kafue and Lusaka Districts of Zambia in this study revealed that the insecticides used to control vegetable pests belonged to the pyrethroids, (43.5%), organophosphates (40.6%), carbamates (3.3%) and insect growth regulators (2.4%). Aphids were perceived by farmers (33.4%) to be the most important vegetable pests, followed by spider mites (19.8%) mainly on tomato, P. xylostella (13.7%) on brassicas and bollworm (12.9%) on tomato.The larval populations of P. xylostella were recorded from April to December and the pupal populations were recorded from June to early December. The larval populations of P. xylostella reached the highest peak in mid-September while the pupal populations reached the highest peak in late September. The local parasitoid species of P. xylostella found was O.sokolowskii.Plutella xylostella larvae were parasitized predominantly by the larval parasitoid C.vestalis, which peaked in late June, and the larval-pupal parasitoid, O.sokolowskii, which was most abundant from September to October. Other parasitoids found were the larval-pupal parasitoid D. collaris, which occurred in small numbers from July to November and the pupal parasitoid D. mollipla, recorded from June to October. An unidentified species of Aphanogmus, a hyper-parasitoid of C. vestalis, was also recorded.Toxicity tests of four insecticides on C. vestalis in this study, indicated that the pyrethroid, cypermethrin and the organophosphate, dichlorvos were the most toxic, while another pyrethroid, ?-cyhalothrin was intermediate and the organophosphate, acephate was the least toxic after 12 to 72 h of exposure. At the recommended field rate for each insecticide, i.e. acephate 75% WP (3g/L), cypermethrin 20% EC (1ml/L), dichlorvos 100% EC (3ml/L) and ?-cyhalothrin 20% EC (1ml/L), there was 100% mortality of C. vestalis adults under laboratory conditions. Cotesia vestalis was found to have high potential for use as biological control agent of P. xylostella. Laboratory tests on its effectiveness as a biological control agent indicated a type II Holling disc functional response, that is, the proportion of the parasitized second instar larvae of P. xylostella over 24 h period decreased exponentially as the density was increased from 5 to 100 per Petri dish.
Failure of the established parasitoids to control the diamondback moth in Lusaka District is discussed. The following conclusions were drawn from this study: i) The exotic parasitoids, C. vestalis and D. collaris, established themselves in Lusaka district, ii) Though established, the population densities of the two parasitoids were not high enough to effectively control the diamondback moth in the district, iii) One possible contributing factor to the ineffectiveness of the parasitoids to control the diamondback moth was the use of highly toxic pesticides by farmers to control local vegetable pests such as dichlorvos, iv) Acephate was more friendly insecticide to C. vestalis adults than cypermethrin, dichlorvos and ?-cyhalothrin, v) C. vestalis was an effective biological control agent of the diamondback moth in laboratory bioassays.The following recommendations were made regarding the future of the biological control programme of the diamondback moth in Lusaka Province: i) Assessment of the potential of P. xylostella parasitoids should also be done under field conditions, ii) Parasitism and functional responses of parasitoids need to be investigated under field conditions, to further the development of a biological control programme for P. xylostella, iii) There is need to mass rear C. vestalis for release in selected farms to augment the existing populations of this parasitoid, iv) The impact of all insecticides used against P. xylostella on its key parasitoids need to be investigated, v) Acephate should be recommended for use by vegetable farmers to control P. xylostella for it has been shown that it is less toxic to C. vestalis, and vi) Since the phenologies of O. sokolowskii parasitoid and the diamondback moth larva were found to be in synchrony and that of O. sokolowskii is also density dependent it is recommended that future studies on the diamondback moth take this parasitoid into consideration as biological control agent.