Management of Tuta absoluta with introduced and native biocontrol agents

Abstract

Tomato, Solanum lycopersicum L. (Solanaceae) is severely damaged by the South American tomato moth, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae). Surveys among producers confirmed that the use of synthetic insecticides increased since the invasion of T. absoluta. The risks associated with the residual effect of insecticides in edible food and the negative effects it may have on ecosystem services,also increased. The aim of this project was to investigate the management of T. absoluta with introduced and native biocontrol agents in Kenya. These included a parasitoid introduced from South America,Dolichogenidea gelechiidivoris Marsh (Hymenoptera: Braconidae), native parasitoids, and the entomopathogenic fungus, Metarhizium anisopliae ICIPE 20. Release of D. gelechiidivoris was preceded by studies of its functional responses and the tritrophic interactions between the parasitoid and native parasitoids. For the functional responses, densities of T. absoluta larvae (20, 50, 100, 150, and 200) were tested with two densities of parasitoid (1 and 3 females). For the interaction with the native parasitoids, short term interspecific competition assays between D. gelechiidivoris and Stenomesius sp.near japonicus (Ashmead) (Hymenoptera: Eulophidae) have been conducted. Bioassays were also performed to investigate long term interaction between D. gelechiidivoris and a native parasitoid,Bracon nigricans Szépligeti (Hymenoptera: Braconidae). The susceptibility of D. gelechiidivoris to M.anisopliae ICIPE 20 was also assessed, followed by testing of the parasitism performance of D.gelechiidivoris and M. anisopliae individually as well as in combination under greenhouse conditions. The exotic parasitoid was also released in open-field and its dispersal was evaluated.Parasitism by D. gelechiidivoris was positively correlated with density of the larval host, regardless of whether the wasp occurred individually or in groups. The highest emergence of these wasps from parasitized host larvae was at a high larval pest density of 100 larvae. Two native endogenous parasitoids species viz. Bracon nigricans, and Stenomesius sp. near japonicus, were identified and successfully maintained on T. absoluta larvae. Using an ecological niche prediction, high to very high suitability was shown for occurrence of B. nigricans in major parts of Africa. In a short-term interaction between D. gelechiidivoris and Stenomesius sp. near japonicus, the exotic parasitoid performed much better than the native species. However, in a long-term interaction study between D. gelechiidivoris and B. nigricans, the native parasitoid negatively affected population growth of the exotic D. gelechiidivoris. The same level of pest control was achieved where both species co-occurred as well as where each species of parasitoid was individually present. The investigation of the potential use of M. anisopliae ICIPE 20 with D. gelechiidivoris showed the fungus to negatively affect the longevity of the adult parasitoids as well as the survival of parasitised larvae. However, the percentage parasitism by fungus- infected female wasps, remained high (> 70%). Fungus infection of parasitized larvae at different ages,reduced parasitoid emergence by 35% and 23% for infection at one and five days post-parasitisation.Exposure of healthy-D. gelechiidivoris adults to a plant-sprayed with fungus did not affect their longevity, and no discriminatory host selection was found. The parasitoid did also not differentiate between sprayed and non-sprayed host plants. Significantly fewer T. absoluta moths eclosed from host plants sprayed with M. anisopliae ICIPE 20, prior to being exposed to D. gelechiidivoris compared to infested plants sprayed with the fungus only or exposed to the parasitoid only. These results were confirmed in a greenhouse trial with fewer fruits infested with T. absoluta when both agents were combined for control of the pest. Establishment of D. gelechiidivoris after its release was confirmed with recovery from the area of release, one year after its introduction. The findings of this thesis offer promising tactics including the use of the exotic parasitoid D. gelechiidivoris, native parasitoids, and an entomopathogenic fungus to develop sustainable IPM methods to control T. absoluta