Abstract:
In an attempt to address the insect pest menace in Jatropha (Jatropha curcas L.), a study was conducted to document the major insect pests of Jatropha in Kenya and evaluate the bioactivityof aqueous T. vogelii and L. camara extracts against adult golden flea beetle (GFB). A stratified simple random sampling procedure was used to gather information on the major insect pests of Jatropha in Bondo and Kibwezi districts. In the laboratory and field studies, aqueous crude extracts, at four rates (0, 2.5, 5.0 and 10.0% w/v) and a synthetic insecticide, Karate (2.5% w/v), were evaluated for bioactivity against adult GFB. The laboratory experiments were laid out in completely randomized design (CRD) with 4-5 replicates per treatment. In the field experiments, treatments were laid out in a randomized complete block design (RCBD) with three replicates per site. Data collected on the major insect pests were analyzed using GLM of SPSS version 12. Data on percent mortality, pest reduction, feeding deterrence and repellence were to analysis of variance and means separated by Tukey‘s HSD test. Contact toxicity data were subjected to probit regression analysis using EPA Probit Analysis Program version 1.4 and LC50 values obtained. Field survey results showed that GFB was the most important insect pest of Jatropha. In the laboratory bioassays, the results showed that aqueous crude extracts of T.vogelii and L. camara had significant (P<0.0001) inter-plant-, dose-, and contact duration –dependent toxic, anti-feedant and repellent effects against GFB adults. At the dose range tested and 8 days of contact, L. camara and T. vogelii extracts caused 18–56% and 50.0–62% insect mortality T. vogelii extract produced the highest deterrence coefficient of 100%, at 5-10% w/v and 168 h after treatment, compared to 28-36% for L. camara extract. Irrespective of concentration and exposure time, L. camara and T. vogelii produced the same percent repellence values except the end-point repellence after 24 h in which L. camara was more repellent than T. Vogelii. Results from field bioassays have showed that percent pest reduction, leaf damage per plant, number of feeding holes and chlorophyll content per leaf were significantly (P<0.0001) influenced by interplant, concentration applied, exposure time and site. Irrespective of the plant assayed, exposure time, and site, T. vogelii (29-50%) caused higher percent pest reduction than L. camara (11-22%) at the concentration range tested. The results of this study show that the test botanical plants, Tephrosia vogelii and Lantana camara, have moderate to strong bioactivity (toxicity, repellence and anti-feeding) against the golden flea beetle and hold good promise as eco-friendly and cost effective alternatives to synthetic pesticides for field insect pest control.
