The role of host plants, temperature and natural enemies in the development, survival and reproduction of edible grasshopper Ruspolia differens (Orthoptera: Tettigoniidae)
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The edible long-horned grasshopper (Ruspolia differens Serville) is a very important source of food and income in East Africa. However, the insect is currently obtained from seasonally swarming wild populations which are erratic and declining; and there has been a confusion on the identity of the species of grasshoppers consumed in Uganda. Development of effective protocols for artificial mass rearing of these grasshoppers is required for sustainable supply. Among the factors which are limiting mass production of long-horned grasshoppers involve the confusion over the identity of the grasshopper, paucity of information on the natural diets, diseases, and the optimal temperatures. Using molecular and morphological tools, the current study identified edible long-horned grasshoppers collected from different locations in Uganda as only R. differens. The study further identified host plants of R. differens, using molecular characterization of gut content of wild collected insects; and later investigated the performance of the grasshopper on identified host plants mixed with important food components. Temperature requirements for R. differens development, longevity, fecundity, oviposition and distribution were determined using Insect Life Cycle Modelling software based on linear and non-linear models, and used to predict suitable habitats for the insect under current and future climatic conditions. Natural enemies of R. differens were isolated and identified through molecular tools. The host plants of R. differens were identified as Ageratum conyzoides (L.), Citrus depressa Hayata, Cynodon dactylon (L.), Digitaria gayana (Kunth), Eragrostis Mexicana Hornem, Eucalyptus saligna SM., Indigofera arrecta Hochst. ex A. Rich., Persicaria nepalensis (L.), and Sorghum halepense (L.). The optimum temperatures for incubation of R. differens eggs ranged between 30 and 32°C, while the optimum temperatures for nymphs and adults ranged from 28 to 30°C. The potential areas for R. differens distribution as of 2000 climatic scenario were predicted as East, Central, West, Southern and the Horn of Africa; and in 2050 climatic scenario only East and Central Africa will be well suited for the distribution of the insect whereas the suitability of West, Southern and the Horn of Africa will reduce. Major pathogenic fungi identified in R. differens were Fusarium equiseti (Corda) Sacc., Mucor fragilis (Fresen), Clonostachys rosea (Link) Schroers, Aspergillus tamari Kita, Aspergillus niger (van Tieghem), Trichoderma koningii Oudem, Clavispora lusitaniae (Rodrigues de Miranda), Lichtheimia corymbifera (Cohn) Vuill, and Epicoccum sorghinum (Sacc.) Aveskamp. Entomopathogenic bacteria isolated from R. differens included Serratia marcescens (Bizio), Bacillus thuringiensis (Berliner), Enterobacter cloacae (Jordan), Enterococcus faecalis (Andrewes and Horder), Staphylococcus sciuri (Kloos et al.), Proteus vulgaris (Hauser), Klebsiella pneumoniae (Schroeter) and Proteus penneri (Hickman et al.). Glaurocara flava Thomson was the only identified parasitoid of R. differens. Survival of R. differens in all diets mixed with individual host plants exceeded 50% unlike the control with survival rate less than 30%. Artificial diets with Cynodon dactylon (L.) and Panicum maximum (Jasq) were the best for rearing R. differens as compared to the other diets. The number of eggs laid by R. differens raised on the diet mixed with P. maximum was significantly higher than for those raised on control diet and D. gayana. However, inclusion of host plants in the diets had no influence on R. differens adult weight. Information generated from this study can help in development of protocols for effective mass rearing of R. differens.