Effects of selected cultivated crucifers in Kenya on fecundity, development and parasitism of diamondback moth, Plutella xylostella l., (lepidoptera: plutellidae) by parasitoid Diadegma semiclausum hellen (hymenoptera: ichneumonidae)

Abstract

Crucifers form an important component of the livelihood of small-scale farmers in East Africa but diamondback moth (DBM) pest constrains their production. Attempts to control the pest by use of host plant resistance (HPR) have been modest especially in cultivars expressing a whitish appearance (normal wax bloom). Conversely, crucifers characterized by their shiny dark green leaves (Glossy) have shown some resistance to DBM. As a consequence, several seed companies claim resistance attributes in their novel cultivars. Thus, this work sought to investigate the effect of some Brassica oleracea cultivars grown in Kenya on fecundity, development and parasitism of DBM. The performance of DBM was compared on seven cultivars in both laboratory and Greenh use trials. Laboratory investigations were conducted on oviposition preference (choice, no-choice) and survivorship of DBM on the test cultivars and on the basis of these data, life tables were constructed. Egg to adult survival, plant damage as well as cultivar effect on parasitism by Diadegma semiclausum were studied in a plastic house. Leaf wax quantity and structure of the adaxial leaf surfaces were evaluated via scanning electron microscopy (SEM). While DBM did not discriminate any of the test cultivars for oviposition in the choice test, more eggs were laid on the cultivars “Collard Georgia” and “Green Challenger” in the no-choice test. Besides, the pest tended to oviposit a higher percent of its eggs away from the plant while probing on “Green Challenger” in both the choice and no - choice tests. Larval period was more than a day longer on “Riana” “Green Challenger” and “Thousand Head” than on “Gloria” and “Collard Georgia” in the Laborator trial, and larval survival on “Green Challenger” was greatly reduced. Pupal weight of DBM raised on “Green Challenger” (4.3 mg) in the laboratory was significantly lower (P<0.05) than on all cultivars except “Copenhagen Market” (4.6 mg). Consequently there was a significantly lower (P<0.05) fecundity by DBM raised on “Green Challenger” and “Copenhagen Market” than on the rest of the cultivars. Presence of parasitoid caused a higher death of DBM larvae on “Thousand Head” than on “Blue Dynasty” in the greenhouse. Consequently, more percent parasitoids emerged on larvae reared on “Blue Dynasty” than on the former. The net reproductive rate and the intrinsic rate of population increase were lower on “Green Challenger” and higher on “Thousand Headed”, “Blue Dynasty” and “Riana”. In addition, the cultivars “Green Challenger” and vi I “Thousand Headed” had the highest (4263 ± 312.3 nm) and lowest (1560 ± 140.6 nm) spaces between their wax crystals respectively. Conversely, the former had the least density of wax crystals. Thus the pest resistance attributes between cultivars tested display only subtle differences, which can, nevertheless, be harnessed in integrated pest control schemes against diamondback moth.