Philip C. Stevenson

Host-plant viral infection effects on arthropod-vector population growth, development and behaviour: management and epidemiological implications

Publisher Summary This chapter reviews the evidence for positive, negative and neutral effects on the population growth of several arthropod vector species when feeding on virus-infected compared with uninfected host plants. The emphasis is on whiteflies and aphids as these have received the greatest attention. Two important examples are the B. tabaci -borne cassava mosaic disease (CMD) pandemic that continues to devastate cassava production in large areas of sub-Saharan Africa (SSA); and the tomato leaf curl disease epidemics in the Indian subcontinent that have had equally serious implications for tomato production in this region. These problems have been considered in detail and experimental data have been included to show, for both pathosystems, that the B. tabaci colonizing virus-infected host plants have significantly higher-population growth rates compared to those colonizing virus-free hosts. This effect has been discussed in the context of the significantly higher densities of B. tabaci present on symptomatic cassava and the behavioral changes associated with this effect. Data has also been presented to show that the concentrations of four amino acids are significantly higher in the phloem sap of CMD-infected cassava plants. These interacting effects are considered in relation to probable mechanisms contributing to the rapid spread of these epidemics.

Phenolic Compounds on the Pod-Surface of Pigeonpea, Cajanus cajan, Mediate Feeding Behavior of Helicoverpa armigera Larvae

A methanol extract of the pod surfaces of Cajanus cajan, a feeding stimulant for fifth-instar Helicoverpa armigera, was shown to contain four main phenolic compounds. Three of these were identified as isoquercitrin, quercetin, and quercetin-3-methyl ether, by comparing UV spectra and HPLC retention times with authentic standards. The fourth compound was isolated by semipreparative HPLC and determined to be 3-hydroxy-4-prenyl-5- methoxystilbene-2-carboxylic acid (stilbene) by NMR spectroscopy and mass spectrometry. Quercetin, isoquercitrin, and quercetin-3-methyl did not affect the selection-behavior of fifth-instar H. armigera. However, larvae were deterred from feeding on glass-fiber disks impregnated with the stilbene. Furthermore, larvae exposed to quercetin-3-methyl ether consumed significant amounts of both disks. In a binary-choice bioassay, a combination of quercetin-3-methyl ether and the stilbene on one disk and pure quercetin-3-methyl ether on the other disk resulted in increased consumption of both glass-fiber disks by larvae. In contrast, consumption was reduced if the combination was presented to larvae on one disk with purified stilbene on the other disk. Cajanus cajan cultivars that varied in their susceptibility to H. armigera were surveyed for the presence of the four phenolic compounds. An absence of quercetin and higher concentrations of isoquercitrin than the cultivated variety characterized pod surface extracts of pod-borer-resistant cultivars. In addition, the ratio of the stilbene to quercetin-3-methyl ether was greater in the pod-borer-resistant cultivars. These findings are discussed in relation to the identification of chemical characters that can be used for crop improvement.

Effects of isoflavonoids from Cicer on larvae of Heliocoverpa armigera.

Four recently identified isoflavonoids, isolated from wild relatives of chickpea, Cicer arietinum, were shown to deter larval feeding by Heliocoverpa armigera at 100 ppm, judaicin and maackiain retained their antifeedant activity at 50 ppm and 10 ppm, respectively. The isoflavonoids were tested in combinations and with chlorogenic acid; the combinations containing judaicin and maackiain were most active, and chlorogenic acid enhanced the antifeedant activity of all four isoflavonoids. H. armigera was the only one of four noctuids to be deterred by all four isoflavonoids. Spodoptera littoralis was deterred by judaicin alone and S. frugiperda by maackiain alone. Heliothis virescens and S. exigua were not deterred from feeding by any of the isoflavonoids. When incorporated into a diet, isoflavonoids decreased the weight gain of early stadia larvae of H. armigera more than they did later stadia, and maackiain and judaicin were most potent. We conclude that the isoflavonoids, especially maackiain and judaicin, could play a role in decreasing the susceptibility of Cicer to attack by H. armigera.

Developmental inhibition ofSpodoptera litura (Fab.) larvae by a novel caffeoylquinic acid from the wild groundnut,Arachis paraguariensis (Chod et Hassl.).

A novel compound, 1-caffeoyl-4-deoxyquinic acid (1-CdQA) has been identified along with 3-caffeoylquinic acid (3-CQA) and 5-caffeoylquinic acid (5-CQA) (syn. chlorogenic acid) in the foliage ofArachis paraguariensis, a wild species of groundnut that is highly resistant to attack by the larvae ofSpodoptera litura. When neonate larvae were fed on diets treated with 3-CQA or 1-CdQA, their development was severely inhibited compared to larvae on untreated diets, and the effects were similar in nature and magnitude to those observed for larvae feeding on diets treated with 5-CQA, rutin, and quercetin. The effects of all the compounds were dose related, and their optimal concentration was approximately 3 mM, which corresponds to the total foliar concentration of both caffeoylquinic acids and quercetin diglycosides inA. paraguariensis. After 24 h, the development of third stadium larvae feeding on diets treated with 5-CQA, rutin and 1-CdQA was promoted compared to larvae on control diets, but after 96 hr larvae feeding on treated diets had gained significantly less weight than those on the control diets. 1-CdQA, 3-CQA, and quercetin dyglycosides (previously identified in the foliage ofA. paraguariensis) are considered to be valuable components in resistance of groundnuts toS. litura and may provide useful genetic markers in future breeding for resistance to this pest.

EFFECT OF VOLATILE CONSTITUENTS FROM Securidaca longepedunculata ON INSECT PESTS OF STORED GRAIN

Securidacalongepedunculata Fers (Polygalaceae) is commonly used as a traditional medicine in many parts of Africa as well as against a number of invertebrate pests, including insects infesting stored grain. The present study showed that S. longepedunculata root powder, its methanol extract, and the main volatile component, methyl salicylate, exhibit repellent and toxic properties to Sitophiluszeamais adults. Adult S. zeamais that were given a choice between untreated maize and maize treated with root powder, extract, or synthetic methyl salicylate in a four-way choice olfactometer significantly preferred the control maize. Methyl salicylate vapor also had a dose-dependant fumigant effect against S. zeamais, Rhyzoperthadominica, and Prostephanustruncates, with a LD100 achieved with a 60 μl dose in a 1-l container against all three insect species after 24 hr of exposure. Probit analyses estimated LD50 values between 34 and 36 μl (95% CI) for all insect species. Furthermore, prolonged exposure for 6 days showed that lower amounts (30 μl) of methyl salicylate vapor were able to induce 100% adult mortality of the three insect species. The implications are discussed in the context of improving stored product pest control by small-scale subsistence farmers in Africa.

Farmers' insect pest management practices and pesticidal plant use in the protection of stored maize and beans in Southern Africa

Storage losses due to pests threaten livelihoods of farmers across Africa. Synthetic pesticides provide effective control when used correctly but resource-poor farmers cannot afford them. A survey of farmer ethno-ecological knowledge of pests of stored maize and bean, and their pest management practices including pesticidal plant use, was conducted in eastern Zambia and northern Malawi. Almost all respondents reported serious pest damage, with bruchids (Callosobruchus maculatus) and grain weevils (Sitophilus spp.) being major pests in beans and maize, respectively. The larger grain borer (Prostephanus truncatus) was reported more widely in Malawi. In Zambia, 50% of farmers used synthetic pesticides during storage, while nearly all did so in Malawi. Despite differences in storage methods between Malawi and Zambia, farmers in both countries were familiar with pesticidal plants, where Tephrosia vogelii was the most frequently reported. Surprisingly few farmers actually used pesticidal plants, highlighting a promotion opportunity. Our results provide a foundation for optimizing the use of pesticidal plants and enhancing their value to resource-poor farmers, across Africa.

Schaftosides from rice phloem as feeding inhibitors and resistance factors to brown planthoppers, Nilaparvata lugens

The brown planthopper (BPH), Nilaparvata lugens, is the most important pest of rice world wide (Oercke, 1994). Natural resistance offers an inexpensive and environmentally benign form of pest management, but to use resistance most effectively it is necessary to understand the mechanism. Previous studies have shown that the feeding behaviour of BPH on different resistant rice varieties is not related to the wide variety of so-called resistance genes (Padgham et al., 1989). There appear to be at least two major components of resistance. The most common, including that in the variety ‘Ratthu Heenati’, is in the phloem (Padgham et al., 1989; Kimmins, 1989). In the two other resistant varieties studied in this paper, BG300 and BG379/2, the mechanism is derived from ‘Ratthu Heenati’ thus one might expect it to be the same. The second component, which is reported only for variety IR46 is increased surface activity of BPH attributed to leaf surface waxes (Woodhead & Padgham, 1988).

Applications of phytochemical and in vitro techniques for reducing over-harvesting of medicinal and pesticidal plants and generating income for the rural poor

Plants provide medicine and pest control resources for millions of poor people world-wide. Widespread harvesting of medicinal and pesticidal plants puts pressure on natural populations, thus severely compromising their contribution to the income and well-being of traders and consumers. The development of in vitro propagation techniques appropriate for developing countries will provide a robust platform for effective propagation and cultivation of endangered plants. This review focuses on advances in the application of phytochemical and in vitro tools to identify and rapidly propagate medicinal and pesticidal plants. Problems of over-harvesting can be alleviated and ex situ cultivation in agroforestry systems can be facilitated through improving seed germination, in vitro cloning and the use of mycorrhizal fungi. We also present a case for effective use of phytochemical analyses for the accurate identification of elite materials from wild stands and validation of the desired quality in order to counter loss of efficacy in the long run through selection, propagation or ex situ management in agroforestry systems. Future prospects are discussed in the context of medicinal activity screening, sustainable propagation, on-farm planting, management and utilization.

Chemical basis for resistance in sweetpotato Ipomoea batatas to the sweetpotato weevil Cylas puncticollis

The aim of this work was to determine the basis of resistance in a sub-Saharan sweetpotato variety, New Kawogo, to the African sweetpotato weevil Cylas puncticollis. This insect feeds on the roots, reducing quality and yield, and is the most important production constraint of sweetpotato in Africa. Laboratory bioassays were designed to determine how the performance of weevils differed on susceptible and resistant roots. Subsequently, liquid chromatography-mass spectrometry (LC-MS) analysis of the root surface and root latex identified quantitative and qualitative differences in the chemical profiles with higher levels of octadecyl and hexadecyl esters of hydroxycinnamic acids reported in the resistant variety. The compounds were synthesized to confirm their identity and incorporated into artificial diets for bioassays on C. puncticollis. High levels of mortality and developmental inhibition were recorded for larvae feeding on treated diets, and the effect was dose-dependent. Thus, in contrast to previous work on resistant African sweetpotato cultivars, resistance in New Kawogo is not only active, but is quantifiable and manageable for breeding. Work is underway to determine what effect these compounds have on the weevils at a molecular level. The inheritance of the root latex esters will be studied in new crosses and mapped in new populations using quantitative trait loci (QTLs) that are currently being developed.

Insect antifeedant furanocoumarins from Tetradium daniellii

The dried fruits of Tetradium daniellii yielded a new linear furanocoumarin, 5-(6-hydroxy-3,7-dimethylocta-2,7-dienyloxy)psoralen, together with six other structurally related furanocoumarins. A similar chemical profile was recorded by HPLC analysis of a fragment of T. daniellii fruit obtained from an historic herbarium voucher specimen collected in September 1917 during an expedition to Yunnan province, China. Four of the compounds identified caused a potent feeding deterrent effect towards larvae of Spodoptera littoralis and Heliothis virescens.