POPULATION DYNAMICS AND PARASITISM OF BRASSICA INSECT PESTS IN ZIMBABWE WITH EMPHASIS ON THE DIAMONDBACK MOTH, PLUTELLA XYLOSTELLA (L.) (LEPIDOPTERA: PLUTELLIDAE)

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is a major insect pest of brassicas in Zimbabwe. Field surveys were conducted to assess brassica pest management practices at large scale farms (≈2.0-10.0 ha) and smallholder farms (\u3c2.0 ha). Insecticides used on large scale farms include λ-cyhalothrin, fenvalerate, dimethoate, malathion, diazinon, dichlorvos, kelthane and lufenuron. Smallholder farmers predominantly used dimethoate, malathion, diazinon, methamidophos, dichlorvos, fenvalerate, carbaryl and methomyl. Six out of seven smallholder farmer clusters were ranked into an Intermediate Category between Integrated Pest Management (IPM) practices and Conventional Insecticide use. Smallholder farmers in the Chinamhora area were ranked into a Conventional Insecticide use category. Five out of seven large scale farms were ranked into a Conventional Insecticide use category while two of the seven farms were ranked into an Intermediate Category. Diamondback moth larval incidence was high in the hot-dry season in October and November of both 2007 and 2008 and reached a density of 15.58 larvae per plant at Nyanga in November 2008. The major larval endoparasitoid was Cotesia plutellae (Hymenoptera: Braconidae) and parasitism reached 95.51% at a host density of 2.83 larvae per plant at Harem in early summer (October and November) of 2008. The larval-pupal endoparasitoid Diadegma mollipla (Hymenoptera: Ichneumonidae) was recorded at three high altitude sites (\u3e1,400 m) with parasitism levels of 5.56% at ART, 4.65% at Nyanga and 5.26% at Eden. The larval endoparasitoid Apanteles sp. (Hymenoptera: Braconidae) reached a parasitism level of 0.93% at Eden. The pupal parasitoid Oomyzus sokolowskii (Hymenoptera: Eulophidae) reached parasitism levels of 1.31% at Africa University and 2.41% at Hartzell. Field edge effects on DBM density were tested at four large scale farms. There were significantly higher (p\u3c0.05) DBM larval and pupal populations in the field interior compared to the 5 field edge rows at two farms. The other two farms had significantly higher (p\u3c0.05) DBM larval and pupal populations in the 5 field edge rows compared to the field interior. Studies on unsprayed plots of covo (Brassica oleracea var. acephala), rape (Brassica napus), cabbage (Brassica oleracea var. capitata), Ethiopian mustard (Brassica carinata) and Indian mustard (Brassica juncea) showed that DBM and aphids (Brevicoryne brassicae) infested cabbage and rape. Flea beetles (Phyllotreta sp.) infested Indian mustard only. There was no significant difference (p\u3e0.05) in DBM larval density between cabbage only and cabbage intercropped with Indian mustard. The entomopathogen Zoophthora radicans caused 98.68% mortality on small (1st-2nd instar) DBM larvae in vitro and 21.34% mortality on large (3rd-4th instar) DBM larvae six days after treatment. Zoophthora radicans was not effective against larvae of Helicoverpa armigera and cabbage looper (Trichoplusia ni). There was no significant difference (p\u3e0.05) in adult C. plutellae emergence from cocoons treated with Z. radicans and cocoons sprayed with water. There was 95% emergence of C. plutellae adults from cocoons treated with Dimethoate 40 EC. No adults of C. plutellae emerged from cocoons treated with Carbaryl 75 WP, Malathion 25 WP and Malathion 50 EC