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Author: search.filters.author.Gitonga, LinusEnd date: 2009

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    Preventive control alternatives to routine foliar spray against Thrips tabaci in onions
    (2008-10) Waiganjo, M.; Sithanantham, S.; Gitonga, Linus; Mueke, J. M.
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    Scope for genetic enhancement of the parasitisation potential of four native strains of Trichogrammatoidea sp. nr. lutea Girault (Hymenoptera: Trichogrammatidae
    (Taylor & Francis, 2007-09) Baya, Joseph M.; Sithanantham, S.; Gitonga, Linus; Osir, Ellie
    In response to emerging interest in commercial mass production of Trichogramma for Helicoverpa armigera biocontrol in eastern Africa, laboratory experiments were undertaken to assess the scope for genetic enhancement of the parasitisation potential of native strains of the local common trichogrammatid species, Trichogrammatoidea sp. nr. lutea. Four promising strains (ex-Kilifi – Kilifi District, ex-Kwa Chai – Kibwezi District, ex-Rarieda – Bondo District and ex-Ebuhayi, Kakamega District) were tested for cross-mating in reciprocal combinations with focus on fecundity and progeny female ratio. While all the crosses resulted in F1 progeny of both sexes, significant differences were observed between homogamic and reciprocal heterogamic crosses in fecundity, progeny production, proportion of female progeny and adult longevity. Among all the crosses, the cross between ex-Rarieda strain females and ex-Kilifi strain males resulted in progeny that was significantly superior in fecundity and progeny female ratio. Conversely, Kilifi strain females crossed to males from ex-Rarieda strain gave rise to progeny with relatively low fecundity and female ratio. There were significant differences between homogamic crosses and most reciprocal heterogamic crosses in the major biological attributes. Genotypic and phenotypic variance-covariance matrices generated for six life-history traits showed high positive correlations for most traits in both inbred (P<0.05) and reciprocal heterogamic crosses (P<0.05 and P<0.001). Fecundity and number of female offspring were the most important factors in the heterogamic crosses. The results confirmed the scope for genetic enhancement through inter-strain crossing for improving the field impact potential of T. sp. nr. lutea being targeted for commercial mass production.
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    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.
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    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, Linus
    Diadegma 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.
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    SUSCEPTIBLE ONION GROWTH STAGES FOR SELECTIVE AND ECONOMIC PROTECTION FROM ONION THRIPS INFESTATION
    (Karatina University, 2008-03) Waiganjo, M.M; Mueke, J.M.; Gitonga, Linus
    The growth of the bulb onion is subject to significant stress and yield reduction caused by sap feeding onion thrips. In Kenya, the general control recommendation is to spray the crop with insecticides as soon as the pest appears and to continue thereafter, throughout the crop season. The practice is expensive and environmentally hazardous. Two field trials were conducted at the Kenya Agricultural Research Institute (KARI) Mwea-Tebere farm to identify the growth stages most susceptible to onion thrips infestation that would provide the highest economic benefits to the onion grower by managing thrips, with the aim of promoting need-based onion pesticide application. The trial design was randomized complete blocks of 16 treatments replicated three times. The treatments constituted insecticide treatments at different onion growth stages and frequencies. Thrips density/plant was estimated weekly through a destructive cutting and bagging method. Yield gains due to the application of the protection regimes were calculated and the marginal returns estimated as the income of yield gain divided by the cost of the control option. Thrips infestation was significantly (p=0.05) higher in the unprotected plots than all the protected plots in the first and second trial and resulted in significant yield losses of 59 and 29%, respectively. A significant onion yield response to thrips infestation was observed during the 2nd month (bulb-formation) and 3rd month (bulb-enlargement) after transplanting, but no significant response was observed in the 1st month (pre-bulbing) and 4th month (maturation) respectively. Onion protection only during the bulb-formation stage resulted in higher marginal returns than full season pesticide application. The studies suggested that control recommendations for onion thrips could be refined to optimize selective pesticide use in Kenya.