Abstract:

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.