Seasonality of major cabbage pests and incidence of their natural enemies in Central Kenya

The population dynamics of major brassica pests and occurrence of their natural enemies were monitored weekly for a period of 9 months on cabbage (var. Copenhagen Market) at a research station plot (no pesticides applied) and a farmer’s field (weekly application of Permethrin 10 EC or Dimethoate 40 EC) in Central Kenya. Pest infestations occurred within 14 days after the seedlings were transplanted and persisted thereafter. Diamondback moth (DBM) (Plutella xylostella) and cabbage aphid (Brevicoryne brassicae) were the major pests at the two sites. At the station, the average population of DBM encountered were mainly larval-pupal parasitoids including Diadegma sp. and Oomyzus sokolowskii whose combined parasitism rarely exceeded 20% at any one time. The mean combined parasitism for the three growing seasons was about 7% with Diadegma sp. being more dominant (60.5%). Mean infestation by cabbage aphid for three seasons was about 41%. The mean rate of parasitism of the aphid by Diaretiella rapae was about 1.2%. In the farmer’s field, the average population of DBM larvae and pupae per plant were 1.12 and 1.2, respectively. Natural enemies encountered included Diadegma sp., O. sokolowskii, bacteria and entomopathogens whose mean combined parasitism for the two seasons was about 7%. B. brassicae was also the most common aphid with mean infestation and parasitism of about 26 and 0.8%, respectively, for the two seasons. Other commonly observed pests included leafminers (Liriomyza brassica), Thrips sp., loopers (Plusia sp.) and foliar diseases. No parasitoids of DBM eggs were observed at the two sites.