Effects of Plant Diets on Selected Aspects of the Biology of some Species of Phlebotomine Sandflies (Diptera: Pyschodidae) and the Infectivity of Leishmania major yakimoff and Schokhor 1914 (Kinetoplastida: Trypanosomatidae)

Abstract

Laboratory investigations were carried out to assess phytophagy and its effects on the biology of phlebotomine sandflies and their capacity to transmit Leishmania parasites. Preliminary trials to determine the optimum working standards in terms of anthrone concentration, exposure period and duration, effect of sandfly species and/or sex and effects of diets on the feeding rate, led to the choice of 1.25% as the concentration (w /v) of anthrone reagent to detect the presence of sugar in the gut of sandflies exposed overnight to different test plants an and controls (sucrose and water). During the trials experiments neither sandfly species and/or sex nor diet type showed a significant influence in relation with exposure period or duration. Anthrone tests with Sergentomyia ingrami and Phlebotomus duboscqi fed on seven plant species and controls showed a high selectivity in each sandfly species towards different test plants. S. ingrami had the highest feeding rate on Rumex usambarensis, the lowest among the plants being on Tagetes minuta while no presence of sugar was detected in 288 flies dissected. Phlebotomus duboscqi had the highest preference for Melia azedarach , the lowest being for T. minuta , with 1 out of 288 flies exposed to water showing a positive reaction for sugar. In both sandflies species the two most preferred plants were R. usambarensis and M. azedarach which displayed along with sucrose (positive control) a feeding rate beyond 50 percent. Plant parts and age were found to significantly affect the feeding response due to physical and/or physiological state of plant materials. As for the feeding performance with reference to carbohydrates expressed by the blue colour intensity of the reaction between anthrone and sugar in the sandfly gut, the number of the flies exhibiting a low level sugar content (faint blue) was higher than the sum of those with intermediate level (medium blue stain intensity) and high level (deep blue stain intensity). And the distribution of different levels was significantly different according to individual diets. For instance S. ingrami was found with more sugar from R. usambarensis and sucrose while in P. duboscqi, the highest level was more important with M. azedarach, sucrose and R. usambarensis. In general there was a correlation between the feeding rate and the feeding performance. The diversity in the feeding response by the test flies to the different test plants raised the question as whether there is any correlation between the feeding rate of the flies and the sugar content in the different plants. The results of anthrone test and spectrophotometry used to estimate the concentration of sugars in each plant did not reveal any correlation. It was rather noted that plants with a mild (Azadirachta indica) or poorest feeding rate (T. minuta) could be associated with the highest sugar concentration. In contrast bioassay tests effected to determine the behavioural response by P. duboscqi to olfactory stimuli from some test plants indicated a close relationship between the feeding and behavioural responses: plants with the higher feeding rate (R. usambarensis, M. azedarach) attracted much more flies than those with lower feeding rate (Ocimum suave, T. minuta). After a blood meal to induce oogenesis, S. ingrami and P. duboscqi females were maintained on R. usambarensis, S. incanum, sucrose and water, respectively in order to evaluate the effect of these different diets on the fecundity and reproductivity of the test flies on one hand and the developmental duration and survival of larval and pupal stage of their progeny on the other hand. In both sandfly species, R. usambarensis and sucrose were associated with more or less significantly higher fecundity and reproductivity as compared with Solanum incanum and water. But, life cycle and survival in immature stages of the progeny were not significantly affected by the difference in mother's diet. Longevity in S. ingrami and P. duboscqi maintained on R. usambarensis, S. incanum, sucrose and water (without a previous blood meal for females) as established by checking daily mortality in each batch and for each diet until the last fly died. In two sandfly species, the longest life span was recorded in the batches maintained on R. usambarensis or sucrose S. incanum and water were associated with the lowest longevity. Development and quality of L. major (ICIPE 226) parasites were assessed in P. duboscqi maintained on R. usambarensis, S. incanum, sucrose and water after an infective blood meal from an infected BALB/c mouse. From the 4th day (incubation period) after the infective meals flies were dissected, infection recorded and location and motility of promastigotes observed in the different regions of the sandfly midgut. Higher infection rate, location in the foregut and movement of promastigotes towards the mouthparts were observed more frequently in those flies maintained on R. usambarensis or sucrose as compared to those maintained on S. incanum or water.