Assessment by Performance Attributes of Indigenous Egg Parasitoids (Hymenoptera: Trichogrammatidae for Biocontrol of African Bollworm (Helicoverpa Armigera) in Kenya

Abstract

Trichogrammatid egg parasitoids are used extensively for the control of several lepidopteran pests in many areas of the world. They ar~he most promising biological control agents for inundative releases against the African bollworm Helicoverpa armigera, an injurious pest of several agricultural crops in Kenya and elsewhere in Africa. To enhance their potential impact in Kenya as well as eastern Africa, collections of indigenous species/strains (Trichogramma sp. nr. mwanzai and Trichogrammatoidea sp. nr. lutea from low altitude, Trichogramma sp. nr. mwanzai and Trichogrammatoidea sp. nr. lutea from medium altitude and Trichogramma bruni and Trichogrammatoidea sp. nr. lutea from high altitude) were made from different altitudes in Kenya and were evaluated following different criteria to select potential candidates for use against H. armigera. The major focus was their adaptation to a range of temperature and humidity regimes. The criteria included functional response, lifetime parasitism and development and population growth characteristics at six temperatures (10, 15, 20, 25, 30 and 35°C) and two humidity regimes (40-50 and 70-80%). Preference of parasitoids for target pest and relative suitability for development among five lepidopteran hosts was also investigated, in addition to testing their capacity to attack H. armigera eggs occurring on two host plants (tomato and okra) in laboratory and field experiments. Functional response studies revealed that temperature affected parasitisation rates of the strains significantly, while relative humidity did not. Trichogrammatoidea sp.nr. lutea from high altitude, Trichogramma sp. nr. mwanzai from low altitude and T. sp. nr. mwanzai from medium altitude showed higher parasitism across the widest temperature range. There was no relationship between source altitude/climate and performance of the strains at the temperatures tested. Temperature and humidity interactions affected lifetime fecundity and progeny production. The highest parasitism at the two humidity levels was at 30°C for all the strains. The interaction of the two factors also affected adult longevity, which was longer at the lower than higher humidity. Survival followed a type l-survivorship curve at lower temperatures and a type III survivorship curve at the higher temperatures. These studies revealed that T sp. nr. mwanzai from both low and medium altitudes and T sp. nr. lutea from medium altitude appear as promising candidates for augmentative biocontrol of H. armigera. Temperature and humidity interactions also affected the population growth and development of parasitoids although temperature appeared to be more important. Developmental time was inversely related to temperature. The intrinsic rate of increase was found to increase with increasing temperature up to 30°C. The finite rate of increase also followed the same trend. The net reproduction rate also increased with increasing temperature up to a maximum at 30°C. Both net reproduction and intrinsic rate of increase were higher at the lower humidity. Temperature negatively affected generation time ofparasitoids regardless of the humidity level. Host acceptability studies showed that all the six species/strains did accept all the tested host species, although acceptance levels varied among strains and hosts. Most species/strains showed greater preference for noctuids over pyralids and yponomeutids. Suitability of hosts for progeny development varied from one species/strain to another. The different species/strains did not show significant differences in parasitising eggs of H. armigera when placed on two host plants, namely tomato and okra. From these studies, T. sp. nr. mwanzai CM), T. sp. nr. mwanzai CL) and T. sp. nr. lutea (M) offer great potential as candidates for augmentative biocontrol of H. armigera in Kenya.