Performance of Amaranth Accessions against Moisture Stress and Key Insect Pests and Their Indigenous Parasitoids in Arusha, Tanzania
Othim, Stephen Tarmogin Omburo
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Amaranths are African indigenous vegetables (AIVs) that have recently gained importance as a food source in Africa owing to the high nutritional value of their leaves and grains. Production of this crop is, however, limited by arthropod pests especially the lepidopteran leaf-webbers, leaf-worms and stem weevils. The use of insecticides for their management is uneconomical and also present health and environmental concerns. Host plant resistance (HPR) to insects is an effective, economical and environmentally friendly alternative that is poorly understood and unexploited among AIVs. The aim of this study was to assess Amaranthus accessions for resistance to leaf-webbers and stem weevils, their tolerance to water stress conditions and the performance of indigenous parasitoids on selected accessions. Field and laboratory experiments were conducted at the World Vegetable Centre (WorldVeg) in Arusha Tanzania with 36 accessions and lines of amaranth. Accessions VI036227, RVI00027, VI054569, VI033487, VI044432, VI048076, VI049639, VI049530 and VI049698 had high levels of pest resistance with significantly lower infestations (≤ 11.11±2.14%) and damage (≤ 68.06±3.90%) by leaf-webbers and leaf-worms. The accession VI036227 was found to be highly resistant against Spoladea recurvalis, exhibiting exemplary antibiosis by causing 100% larval mortality despite not being deterrent for oviposition. The accessions VI048076, VI056563 and VI047555-B demonstrated moderate resistance against the pest for specific parameters including low oviposition, moderate early stage larval mortality and reduced adult longevity. The relative growth rate (RGR) of accessions VI033479, VI049698 and VI056563 were not significantly affected by the three soil water levels (40%, 60% and 90% water holding capacity (WHC)). Apanteles hemara performed well on all the other moderately resistant accessions except VI056563 that recorded lower parasitism rates compared to the susceptible accession. The longevity of the parasitoid was significantly extended on the resistant accessions compared to the susceptible one. The functional response curve exhibited by A. hemara corresponded to type II functional response with an asymptote at the density of 30 larvae. Apanteles hemara parasitism was significantly higher in 1-2-day-old compared to 3-4-day-old larvae (P=0.04). Thus, accessions VI036227 and VI049698 were identified to be highly resistant to leaf-webbers in addition to 24 moderately resistant ones while VI047517-B, VI036227 and VI056563 had low levels of resistance against stem weevils. Accessions VI033479, VI049698 and VI056563 were also tolerant to moisture stress. The identified pest resistant and water stress-tolerant amaranth accessions from this study are recommended for multiplication and release to farmers to alleviate the effects of pests and drought. These can also be used in breeding programs to improve locally cultivated varieties. The identified parasitoids can also be reared and released into farmer fields to synergize host plant resistance.