Survey Of Insects Associated With Oil - Seed Rape (Brassica Napus L. And B. Campestris L.) In Kenya, With Reference To Susceptibility Of Six Rape Varieties To Insect Attack.
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Rape seed farming in Kenya is gaining popularity among farmers in the wheat growing areas where the crop fits well in rotation with cereals like wheat and barley. The factories processing the seeds for oil extraction are strategically situated in Nakuru and have a high milling capacity to cope with increasing rapeseed production. Therefore, the rapeseed industry in Kenya may undergo a rapid economic growth. Insect pests are likely to impede this growth and for this reason this study was undertaken to: 1. To determine the insect pests attacking rape. 2. To determine the sequence of infestation of various insect pests. 3. To determine the natural enemies associated with the insect pests of rape. 4. To identify the important insect species which help in the pollination of this crop. 5. To find out the varietal susceptibility to insect pests and whether the insect attacks on these varieties are reflected in plant growth and yield. A general survey of insects was carried out on commercial farms in four different localities in medium and high altitude areas. Detailed faunistic and varietal susceptibility studies were carried out on experimental plots planted in November 1976 and April 1977 at the National Plant Breeding Station, Njoro. Insects were collected using sweep net, motorised sunction sampler (JLO Type L35L), sticky traps and hand collection. The data obtained was used for qualitative (in general survey) and quantitative (in planned experiments) assessments. About 90 different species of insects were recorded from the two aspects of this study. About 60 of these were phytophagous species and only a few of them were cons1dered serious pests of this crop. Agrotis segetum, Athalia sjostedti, Heliothis armigera and Brevisoryne brassicae were among the most notorious pests recorded. Some of the insects that moderately attacked the- crop could become serious pests in their outbreak years. Plusia oricnalcea and a weevil of the genus nematoccrus belonged to this group. At each stage (seedling, . vegetative and flowering ) " several pest species were found attackinq rape. Among the above mentioned pests A. se1etum and A. sjostedti attacked seedlings, H. armigera and other lepidopterous caterpillars occurred in vegetative stage while aphids, B. brassicae was most destructive to the flowering rape. The other 30 species included the beneficial insects and a few species of undertermined role. Ichneumonidae dominated the parasitic population while Coccinellidae were the most aLundant predators. The honey bee, Apis mellifera displayed superiority over the other pollinators. Some of the varieties examined suffered nore insect attack than others and losses in yield due to insect damage was in some cases very significant. Torpe was the most susceptible variety while yield losses due to insect damage in midas and turget were the lowest among the six varieties examined •