Department of Biological Sciences
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Item Functional response of Orius albidipennis (Hemiptera: Anthocoridae) to Megalurothrips sjostedti (Thysanoptera: Thripidae)(Karatina University, 2002-05-02) Gitonga (a), Linus M.; Löhrb (b), B.; Magambo (a), J. K.; Mueke (c), J. M.A locally available anthocorid predator, Orius albidipennis Reuter, was evaluated as a candidate biocontrol agent for Megalurothrips sjostedti Trybom, a major pest of French beans, Phaseolus vulgaris (L), in Kenya. The functional response of adult O. albidipennis to larval and adult M. sjostedti was studied at 15, 25, and 28 °C and densities of 5, 10, 20, and 30 larval and adult M. sjostedti per cage, over 24 h. More larvae and adult M. sjostedti were killed at the higher densities and with an increase in temperature. The data provided good fit to both type I and II functional response models. Attack rates increased with temperature for both the second instar larva and adult M. sjostedti. Handling times for the larvae decreased with an increase in temperature while those for the adults increased with increase in temperature. The implications of these results are discussed.Item Temperature-dependent development of Megalurothripssjostedti and Frankliniellaoccidentalis (Thysanoptera: Thripidae)(2002-09) Gitonga, Linus; Lohr, B.; Overholt, Bill; Magambo, J. K.Megalurothrips sjostedti (Trybom) and Frankliniella occidentalis (Pergande) (Thysanciptera: Thripidae) are among the most serious pests of French beans, Phaseolus vulgaris var. Monel in Kenya and occur in all major growing areas. The former seems to be prevalent during cold and rainy periods while the latter is mainly a problem during hot and dry conditions. The development of the two species was studied at temperatures ranging from 15 degreesC to 30 degreesC. The average developmental times of each stage are given. For both species, the developmental times decreased with an increase in temperature. The degree-days required to complete development from egg to adult were 212 and 256 above the estimated threshold temperatures of 9.6 degreesC and 9.0 degreesC for M. sjostedti and F.occidoitalis, respectively. Between 19 and 23 generations of M. sjostedti and between 17 and 20 generations of F. occidentalis; could possibly develop under Kenyan field conditions in one year. Overall mortality was lowest at 25 degreesC. Mortality rates for both species decreased from larval instars to adult and were higher for M. sjostedti than for F. occidentalis. Since the developmental times were not different, the differential pest status of M. sjostedti and F. occidentalis must be a result of other factors.Item Antimicrobial acitivity of extracts from Tephrosiavogelii Hook F.(Jomo Kenyatta University of Agriculture & Technology (JKUAT), Kenya, 2006) Wanga, B. N.; Akenga, T.; Imbuga, M.; Gitonga, L.; Olubayo, F.; Namungu, P.The dichloromethane extracts of the root and leaf of Tephrosia vogelii Hook F. (Family: Papilionaceae) grown in Kenya, were subjected to antimicrobial assays against Staphylococcus aureus, Escherichia coli and Fusarium phoseolida. The minimum inhibition concentrations (MIC) ranged between 0.25 to 6.4 μg/ml for all the microorganisms tested. The crude root and leaf extracts were also toxic to the brine shrimps, organisms used as indicators of toxicity (LC50: 0.960; 0.958μg/ml,respectively). Chromatographic separation led to the identification of (6aß, 12aß, 5'ß)-6a, 12a, 4',5'-tetra hydro-2,3-dimethoxy 5'-ß- isopropenyl–furano (3',2': 8,9)-6H-rotoxen-12-one, (rotenone) in 9.75% yield, (roots), and 11.75% (leaves). Characterisation was achieved by use of spectroscopic techniques and comparison with rotenone standard and literature data. Novel HPLC conditions for purification of rotenoids were established. The MIC of rotenone against the tested microbes was found to be 5.2 and 1.0 μg/ml for Staphylococcus aureus and Escherichia coli respectively, while the LC50 against the brine shrimps was found to be 3.20 μg/ml. The findings indicate that the extracts of T. vogelii contain metabolites that have antimicrobial activity comparable to other medicinal plants.Item Biological impact of the exotic parasitoid, Diadegma semiclausum (Hellen), of diamondback moth, Plutella xylostella L., in Kenya(Karatina University, 2006-08) Momanyi, Caleb; Loehr, Bernhard; Gitonga, LinusDiadegma semiclausum (Hellen) was introduced to Kenya from Taiwan for the control of diamondback moth (DBM), Plutella xylostella L., and released for the first time in July 2002. Seven and twelve months after release, field mortality studies using the cage exclusion method were conducted at two release sites (Werugha/Taita Hills and Tharuni/Central Kenya). Four treatments were applied: fully closed and partially open cages, both with and without glue applied to the base of the plants for the exclusion of crawling predators. During this time interval, parasitism of exposed larvae rose from 15 to 60% at Werugha and from 8 to 18% at Tharuni, with a concomitant decrease of parasitism by indigenous parasitoids. Crawling predators did not seem to have an effect on mortality. Unexplained mortality increased by half between the first and second experiment at both sites, indicating additional mortality due to the parasitoid. A subsequent field simulation with different parasitoid populations showed that in the presence of parasitoids more larvae died because of abandoning the plant than due to parasitism. Dissections of larvae on the ground showed most were parasitized. Thus, it appears that mortality due to D. semiclausum is more than twice the measured rate of parasitism. Additional information on parasitism was collected from concurrently executed DBM yield loss trials. Parasitism by D. semiclausum was in the same range as in the cage exclusion method. Seven months after release, local parasitoids were present at Werugha and dominant at Tharuni, after one year, they were almost completely displaced at both locations.Item Functional response of Orius albidipennis (Hemiptera: Anthocoridae) to Megalurothrips sjostedti (Thysanoptera: Thripidae)(Karatina University, 2002-05-02) Gitonga, Linus M.; overhalt, W.A.; Löhr, B.; Magambo, J. K.; Mueke, J. M.A locally available anthocorid predator, Orius albidipennis Reuter, was evaluated as a candidate biocontrol agent for Megalurothrips sjostedti Trybom, a major pest of French beans, Phaseolus vulgaris (L), in Kenya. The functional response of adult O. albidipennis to larval and adult M. sjostedti was studied at 15, 25, and 28 °C and densities of 5, 10, 20, and 30 larval and adult M. sjostedti per cage, over 24 h. More larvae and adult M. sjostedti were killed at the higher densities and with an increase in temperature. The data provided good fit to both type I and II functional response models. Attack rates increased with temperature for both the second instar larva and adult M. sjostedti. Handling times for the larvae decreased with an increase in temperature while those for the adults increased with increase in temperature. The implications of these results are discussed.Item Relationship between virulence and repellency of entomopathogenic isolates of Metarhizium anisopliae and Beauveria bassiana to the termite Macrotermes michaelseni(Journal of Insect Physiology, 2009) Mburu, D.M.; Ochola, L.; Maniania, N.K; Njagi, P.G.N.; Gitonga, L.M.; Ndung'u, M.W.; Wanjoya, A.K.; Hassanali, A.Termites encounter a diverse array of potentially useful and harmful fungi in their subterranean habitats. These vary from symbiotic to harmful species with varying levels of virulence. How these hemiedaphic insects survive in habitats with infective fungi is not well understood. Possible mediation of olfactory signals in avoiding contact with entomopathogenic fungi has been explored by a number of workers. In the present study, we initially found that Macrotermes michaelseni detected a virulent isolate of Metarhizium anisopliae from some distance and avoided direct physical contact. We hypothesized that there may be a relationship between virulence and repellency of different isolates of M. anisopliae and Beauveria bassiana to the termite. We compared these for selected isolates of the two fungi. Positive correlations between the two parameters for both sets of isolates of the fungi were obtained. The results show an interesting co-evolutionary phenomenon in which the termite’s response to either M. anisopliae or B. bassiana is directly related to potential harm these fungi can inflict on the insect and that the virulent strains are more likely to be recognized from some distance and avoided.