Parasitism Of Plutella Xylostella (L.) (Lepidoptera: Plutellidae) By Cotesia Plutellaekurdjumov (Hymenoptera: Braconidae) And Parasitoid Development On Brassica Oleracea L. Sprayed With Insecticides
Parasitism of Plutellaxylostella by Cotesia plutellae was determined on Brassica oleracea L. var capitata under insecticidal treatment. The variety KK cross was planted in three seasons at the Weija Irrigation Company Limited, Weija. The insecticides applied were Lambda-Cyhalothrin marketed as Karate 2.5 EC (4.66 - 9.321/ ha at 2.4 ml. / litre of water), Bacillus thuringiensis wettable powder (1.94 - 3.88 kg / ha at 1.0 g / litre of water) and neem seed water extract (194.1kg / ha at 50 g / litre of water). The insecticides were applied after the first sampling of insects and thereafter, every two weeks. There were four replicates per treatment and each had 21 plants. Larvae of P xylostella and pupae of C. plutellae were sampled from the seven central plants three weeks after transplanting and thereafter every week till harvest. Laboratory studies were carried out to determine the direct and residual effects of the insecticides on the adults and the pupae of the parasitoid at a mean temperature of 23 + 2 °C and relative humidity of 45 ± 1%. The neem product used was 1.0 ml. Neem-azal / litre of water. Each treatment was replicated four times and each had 10 individuals. Field parasitism rates were higher on the Karate treated plots but these were not significantly different (p > 0.05) from the other treatments. The proportion of C. plutellae adults that eventually emerged from field parasitized P. xylostella was not significantly different (p > 0.05) between the control and Karate treated plots. In the laboratory, Karate caused 100% mortality within a minute of direct exposure. A significantly higher mortality of 31% (p < 0.05) was recorded in the pupae exposed directly to Neem Azal as spray whereas Karate caused only 3.6%. However; it was only adult C. plutellae emerging from Karate treated pupae that sometimes died in the process of eclosion or lived for only a day. Both neem products adversely affected the ability of the parasitoid larva to spin its cocoon. The results suggest that an Integrated Management strategy based on either lower doses of the neem products or Bacillus thuringiensis and C. plutellae can be developed against P xylostella on cabbage.