Wilber Lwande

Intercropping increases parasitism of pests

As part of a programme for controlling lepidopteran stem-borers in cereal crops in Africa, we have investigated the effectiveness of combined cropping regimes of cultivatedand wild plants for reducing stem-borer damage. Intercropping with the non-host molasses grass, Melinis minutiflora, significantly decreased levels of infestation by stem-borers in the main crop and also increased larval parasitism of stem-borers by Cotesia sesamiae. Volatile agents produced by M. minutiflora repelled female stem-borers and attracted foraging female C. sesamiae. One of the volatile components released by intact M. minutiflora which attract parasitoids is also produced by herbivore-damaged plants and is implicated more widely as a cue for stimulating predation and parasitism.

Traditional use of mosquito-repellent plants in western Kenya and their evaluation in semi-field experimental huts against Anopheles gambiae: ethnobotanical studies and application by thermal expulsion and direct burning

Ethnobotanical survey in 2 communities in western Kenya revealed that the most commonly known repellent plants were Ocimum americanum L. (64.1%), Lantana camara L. (17.9%), Tagetes minuta L. (11.3%) and Azadirachta indica A. Juss (8.7%) on Rusinga Island, and Hyptis suaveolens Poit. (49.2%), L. camara (30.9%) and O. basilicum L. (30.4%) in Rambira. Direct burning of plants is the most common method of application for O. americanum (68.8%), L. camara (100%) and O. basilicum (58.8%). Placing branches or whole plants inside houses is most common for H. suaveolens (33.3 and 57.8% for the respective locations), A. indica (66.7 and 100%), and T. minuta (54.8 and 56.0%). The repellency of plants suggested by the ethnobotanical survey and other empirical information was evaluated against the malaria vector Anopheles gambiae s.s. Giles in experimental huts within a screenwalled greenhouse. Thermal expulsion and direct burning were tested as alternative application methods for the selected plants O. americanum, O. kilimandscharicum Guerke, O. suave Willd., L. camara, A. indica, H. suaveolens, Lippia uckambensis Spreng and Corymbia citriodora Hook. When thermally expelled, only H. suaveolens failed to repel mosquitoes, whereas the leaves of C. citriodora (74.5%, P < 0.0001), leaves and seeds of O. suave (53.1%, P < 0.0001) and O. kilimandscharicum (52.0%, P < 0.0001) were the most effective. Leaves of C. citriodora also exhibited the highest repellency (51.3%, P < 0.0001) by direct burning, followed by leaves of L. uckambensis (33.4%, P = 0.0004) and leaves and seeds of O. suave (28.0%, P = 0.0255). The combination of O. kilimandscharicum with L. uckambensis repelled 54.8% of mosquitoes (P < 0.0001) by thermal expulsion. No combination of plants increased repellency by either method. The semi-field system described appears a promising alternative to full-field trials for screening large numbers of candidate repellents without risk of malaria exposure.

Cleome monophylla essential oil and its constituents as tick (Rhipicephalus appendiculatus) and maize weevil (Sitophilus zeamais) repellents

The repellency of the essential oil of the shrub Cleome monophylla (Family: Capparidaceae) and identified constituents of the oil were evaluated against the livestock tick, Rhipicephalus appendiculatus and the maize weevil, Sitophillus zeamais. In a tick climbing repellency bioassay, the oil of C. monophylla exhibited repellency which, at the highest dose, was comparable to that of the commercial arthropod repellent N, N‐diethyl toluamide (DEET). In a Y‐tube olfactometer bioassay, C. monophylla oil showed higher or comparable repellency against S. zeamais relative to DEET at all the doses tested. 14 Compounds were identified in the C. monophylla oil by GC, GC‐MS and coinjection with authentic samples. Terpenolene was found to occur in largest quantity (14%) followed by 1‐α‐terpeneol (10%), pentacosane (9%), (α + β)‐humulene (8%), phytol (5%) and 2‐dodecanone (4%). The most repellent components against R. appendiculatus and S. zeamais were 1‐α‐terpeneol and 2‐dodecanone. The overall pattern of repellency activity of the C. monophylla constituents with respect to the two arthropods was, however, different. The potential of C. monophylla in tick and maize weevil control is discussed.

Gynandropsis gynandra Essential Oil and Its Constituents as Tick (Rhipicephalus appendiculatus ) Repellents

Abstract The repellency of the essential oil of the previously reported anti-tick pasture shrub Gynandropsis gynandra and identified constituents of the oil were evaluated against the livestock tick, Rhipicephalus appendiculatus. In a tick-climbing repellency bioassay, the oil of G. gynandra exhibited a repellency effect which at the highest treatment levels was higher than that of the commercial arthropod repellent N,N-diethyltoluamide. Twenty eight compounds were identified in the G. gynandra oil by GC, GC-MS and coinjection with authentic samples. Carvacrol was found to occur in largest quantity (29.2%), followed by trans-phytol (24.0%), linalool (13.3%), trans-2-methylcyclopentanol (7.2%) and β-caryophyllene (4.4%). m-Cymene, nonanal, 1-α-terpineol, β-cyclocitral, nerol, trans-geraniol, carvacrol, β-ionone, trans-geranylacetone, and nerolidol were the most repellent components against R. appendiculatus. Methyl isothiocyanate which occurred in the G. gynandra oil at a relative percentage of 2.1 and which was not tested in the bioassay due to its toxicity may also contribute significantly to the repellency of the oil. The repellency of the oil of G. gynandra supported earlier findings by other workers that G. gynandra repelled R. appendiculatus ticks.

Repellent Activities of Stereoisomers of p-Menthane-3,8-diols Against Anopheles gambiae (Diptera: Culicidae)

Abstract Four stereoisomers of p-menthane-3,8-diol, which make up the natural product obtained from Eucalyptus citriodora, were synthesized through stereoselective procedures. Repellency assays showed that all the four were equally active against Anopheles gambiae s.s. Racemic blends and the diastereoisomeric mixture of all the four isomers were also equally repellent. 1-α-terpeneol, with a single hydroxyl function at C-8 and unsaturation at C-8, and menthol, with a single hydroxyl function at C-3, were not repellent. The practical implication of these results is discussed.

Volatile resin exudate from stem bark of Commiphora rostrata: Potential role in plant defence

Abstract The volatile resin exuded from the stem bark of Commiphora rostrata has been examined and 22 oxygenated alkane components identified by GC and mass spectrometry. The potential value of this material in the defence of the plant against predators and fungal pathogens is discussed.

Analysis of airborne volatiles of cowpea

Abstract Airborne volatiles of cowpea ( Vigna unguiculata ) plants were analysed by drawing air over the plants in a glass chamber, trapping the airborne volatiles on Tenax TA adsorbent, and releasing the trapped volatiles into a GC-MS by heating the Tenax trap. Eleven volatile compounds; hexanal, 1-nonene, α-pinene, β-pinene, ( Z )-3-hexen-1-ol acetate, hexyl acetate, 1,8-cineole, limonene, ( E )-β-ocimene, nonanal and a-cedrene, were identified.

Cleome hirta essential oil as livestock tick (Rhipicephalus appendiculatus) and maize weevil (Sitophilus zeamais) repellent

The repellency of the essential oil of the shrub Cleome hirta and of three identified constituents (phytol, (+)-cedrol, n-octacosane) was evaluated against the livestock tick, Rhipicephalus appendiculatus and the maize weevil, Sitophillus zeamais. In a tick climbing repellency bioassay, the oil exhibited repellency which, at the highest dose, was comparable to that of the commercial arthropod repellent N,N-diethyltoluamide (DEET). In a Y-tube olfactometer bioassay, the oil showed higher or comparable repellency against S. zeamais relative to DEET at all the doses tested. The potential of C. hirta in livestock tick and maize weevil control is discussed.

Constituents of the Essential Oil of Helichrysum odoratissimum (L.) Less

ABSTRACT The essential oil of the flowers of Helichrysum odoratissimum (L.) Less. growing wild in Kenya was analyzed by GC and GC/MS. Nineteen compounds were identified, the major constituents being α-pinene (43.4%), (E, E)-farnesol (16.8%) and α-humulene (14.6%).

Constituents of the Essential Oils from Wild and Cultivated Lippia adoensis Hochst. ex Walp.

ABSTRACT The essential oils of wild and cultivated forms of Lippia adoensis Hochst. ex Walp. collected in Ethiopia were examined by GC and GC/MS. The oils from both forms were found to differ in their physical characteristics and chemical composition. Fourteen compounds representing 88–95% of the oil from the cultivated plants and sixteen compounds constituting 68–86% of the oil from the wild plants were identified. Linalool, which was absent in the oil of the wild form, is the major component (68.06–82.57%) in the oil of the cultivated form. Also limonene (3.44–32.73%), perillaldehyde (0.04–26.90%) and piperitenone (0.15–44.48%) are found in oils from the wild plants but not in oils from the cultivated plants. The uncommon monoterpene ketone, 2-methyl-6-methylene-2, 7-octadien-4-one (ipsdienone), was found in the oils of the cultivated plants (0.06–0.81%) and the wild plants (0.81–14.89%).