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Background  The African malaria vector,  Anopheles gambiae  s.s., is known to feed selectively on certain plants for sugar sources. However, the adaptive significance of this behaviour especially on how the extracts of such plants impact on the fitness of this vector has not been explored. This study determined the toxicity and larvicidal activity on this vector of extracts from six selected plants found in Kenya and two compounds identified from  Ricinus communis : 3-carbonitrile-4-methoxy-N-methyl-2-pyridone (ricinine), and its carboxylic acid derivative 3-carboxy-4-methoxy-N-methyl-2-pyridone, the latter compound being reported for the first time from this plant.    Methods  Feeding assays tested for toxic effects of extracts from the plants  Artemisia afra  Jacq. ex Willd,  Bidens pilosa  L.,  Parthenium hysterophorus  L. , Ricinus coummunis  L.,  Senna didymobotrya  Fresen. and  Tithonia diversifolia  Hemsl. on adult females and larvicidal activity was tested against third-instar larvae of  Anopheles gambiae  s.s. Mortality of larvae and adult females was monitored for three and eight days, respectively; Probit analysis was used to calculate LC 50 . Survival was analysed with Kaplan-Meier Model. LC-MS was used to identify the pure compounds.    Results  Of the six plants screened, extracts from  T. diversifolia  and  R. communis  were the most toxic against adult female mosquitoes after 7 days of feeding, with LC 50  of 1.52 and 2.56 mg/mL respectively. Larvicidal activity of all the extracts increased with the exposure time with the highest mortality recorded for the extract from  R. communis  after 72 h of exposure (LC 50  0.18 mg/mL). Mosquitoes fed on solutions of the pure compounds, 3-carboxy-4-methoxy-N-methyl-2-pyridone and ricinine survived almost as long as those fed on the  R. communis  extract with mean survival of 4.93 ± 0.07, 4.85 ± 0.07 and 4.50 ± 0.05 days respectively.    Conclusions  Overall, these findings demonstrate that extracts from the six plant species exhibit varying bioactivity against the larvae and adult females of  An. gambiae  s.s.  T. diversifolia  and  R. communis  showed highest bioactivity against adult females  An. gambiae  and larvae while longevity of female  An. gambiae  s.s. decreased with exposure time to the two pure compounds.

Toxicity of six plant extracts and two pyridone alkaloids from Ricinus communis against the malaria vector Anopheles gambiae