During its Annual General Meeting held in Addis Ababa, Ethiopia, in November 2018, the icipe Governing Council, that consists of globally reknowned scientists, honoured six outstanding postgraduate scholars, currently undertaking their research at the Centre.

Best published science paper by an icipe scholar

Teresiah N. Njihia (PhD Scholar)

icipe Mentor – Dr Régis Babin

Paper: Njihia T.N., Torto B., Murungi L.K., Irungu J., Mwenda D.M. and Babin R. (2018) Ripe coffee berry volatiles repel second instar nymphs of Antestia bugs (Heteroptera: Pentatomidae: Antestiopsis thunbergii). Chemoecology 28, 91–100. https://doi.org/10.1007/s00049-018-0259-3. IF 1.642

Contribution to Science: A major threat facing coffee production in Africa is the Antestia bug Antestiopsis thunbergii (Gmelin), whose attack on both the vegetative and reproductive parts of Arabica coffee results in high yield and quality losses. Besides, A. thunbergii infested coffee berries after processing has unpleasant smell and taste known as ‘potato taste defect’.  Recently, the incidence of potato taste defect in coffee from the region has escalated reaching up to 50%, causing loss of market and export earnings for a region whose economy is very dependent on them. The existing control options for A. thunbergii are limited to conventional methods and use of pesticides that have shown various limitations on coffee as for other crops. In this study, we explored a management strategy completely new for this species: the use of semiochemicals as a component of integrated management of the pest. 

We employed chemo-ecologically approaches including behavioral, electrophysiology and chemical analysis to identify semiochemicals involved in the pest host food selection and discrimination processes. In a prior publication, we reported the volatiles responsible for the attractiveness of the unripe berries to A. thunbergii (https://doi.org/10.1007/s00049-017-0248-y). In the present study, we identified an additional 5-component blend from the volatiles of ripe coffee berries, which repelled A. thunbergii. This 5-component blend comprising two ketones (3-hydroxy-2-butanone and 2-heptanone), two pyrazines (2-isopropyl-3-methoxypyrazine (IPMP) and 2-isobutyl-3-methoxypyrazine (IBMP) and the sesquiterpene (E)-β-caryophyllene) when combined with an attractant of A. thunbergii inhibited the preferential orientation of Antestia bugs to the attractant indicating that the 5-component blend could be used both as repellent and to mask unripe berries (preferred host food).

Although, repellents have a wide range of application in suppressing pests through development of resistant plant varieties, repellence, host masking and in synergy with attractants in “push-pull” systems, very few studies have explored this aspect as compared to the vast number of attractants that are available for use in pest control. This study highlights techniques that can be used to identify repellents for other insect pests especially pentatomids and our promising findings emphasize the need for scientists to research more on this neglected field of science.

Funding: The scholar was supported by a German Academic Exchange Service (DAAD) In-Region Postgraduate Scholarship. Further financial support was obtained from the following organizations and agencies: Centre de Coopération Internationale en Recherche Agronomique pour le Développement (CIRAD), Montpellier, France ; CHIESA project (Climate Change Impacts on Ecosystem Services and Food Security in Eastern Africa) and -icipe funded by the Ministry of Foreign Affairs of Finland; UK Aid from the UK Government ; Swedish International Development Cooperation Agency (SIDA); the Swiss Agency for Development and Cooperation (SDC) and the Governments of Kenya and Ethiopia. The views expressed herein do not necessarily reflect the official opinion of the donors.

University: The scholar is registered at Jomo Kenyatta University of Agriculture and Technology, Kenya.

Beatrice T. Nganso (PhD Scholar)

icipe Mentor – Prof. Baldwyn Torto

Paper:  Nganso B.T., Fombong A.T., Yusuf A.A., Pirk C.W.W., Stuhl C. and Torto B. (2018) Low fertility, fecundity and numbers of mated female offspring explain the lower reproductive success of the parasitic mite Varroa destructor in African honeybees. Parasitology, 1–7. https://doi.org/10.1017/S0031182018000616. IF 2.511

Contribution to science:  Although the ecto-parasitic mite Varroa destructor is the most serious threat to the honeybee, Apis mellifera L., some honeybee populations such as Apis mellifera scutellata in Kenya can survive mite infestations without requiring miticide treatment. Previously, we identified grooming behavior as a potential tolerant mechanism that enables this specific A. m. scutellata populations in Kenya to remove more mites from their colonies and inflict significantly more damage categories to the mites than the susceptible A. mellifera hybrids of European origin found in the USA. However, this behavior could not explain the lower mite infestation levels (~ three-fold lower) recorded in A. m. scutellata colonies than their European counterparts; suggesting that other resistant mechanisms that affect the reproductive fitness of the parasite might be involved in suppressing the mite population growth in colonies of A. m. scutellata. In this study, we identified reduced mite reproductive success as a key adaptive resistant mechanism that explains the slow rate of mite population growth in colonies of the savannah honeybee. This lower reproductive output was mainly due to the high mite infertility rates (26-27%) and percentage of unmated daughter mites (39-58%) as well as low mite fecundity (1.7-2.2, average offspring produced) recorded in infested worker brood cells of this subspecies. Our findings suggest that the ability of the savannah honeybee to survive mite infestation is dependent on host factors rather than parasite virulence. Additionally, our findings suggest that the A. m. scutellata populations studied herein have evolved natural behavioral defense mechanisms to counteract the mite’s attack without any human intervention. Hence, currently, beekeepers in Kenya do not have to treat the mite infestations with miticide in honeybee colonies.

Funding: We gratefully acknowledge the financial support for this research by the following organizations and agencies: The United States Department of Agriculture (USDA)/ARS- grant #58-6615-3-011-f; UK Aid from the UK Government; Swedish International Development Cooperation Agency (SIDA); the Swiss Agency for Development and Cooperation (SDC) and the Governments of Kenya and Ethiopia. The views expressed herein do not necessarily reflect the official opinion of the donors.

University: The scholar is registered at University of Pretoria, South Africa.

Nelson L. Mwando (MSc. Scholar)

icipe Mentor - Dr. Subramanian Sevgan

Paper: Mwando N.L., Tamiru A., Nyasani J.O., Obonyo M.A.O., Caulfield J.C., Bruce T.J.A. and Subramanian S. (2018) Maize Chlorotic Mottle virus induces changes in host plant volatiles that attract vector thrips species. Journal of Chemical Ecology 44, 681–689. https://doi.org/10.1007/s10886-018-0973-x. IF 2.419

Contribution to science:  This study investigates the underlying mechanism mediating thrips vectors- maize chlorotic mottle virus (MCMV)-host plant (maize) interactions to better understand the vector ecology and disease epidemiology. We have demonstrated that MCMV pathogen elicits changes in volatile profiles of the host maize (Zea mays) plants. Moreover, we characterized the MCMV-induced volatile semiochemicals mediating attraction of thrips vectors to virus infected maize plants. The behavioral assay showed that both sexes of maize thrips (Frankliniella williamsi) and male onion thrips (Thrips tabaci) are significantly attracted to volatiles from maize plants infected with MCMV compared to healthy plants and solvent controls. Though, previous studies have shown that plant viruses could induce changes in plant volatile profiles, little information is available on the effect of MCMV infection or other viruses belonging to the family Tombusviridae. The significant attraction of thrips vectors to MCMV-infected maize plants in the current study represents evidence of pathogen manipulation of host chemistry and vector behavior. We hypothesize vectors attraction to MCMV- infected host plants could enhance the virus spread if thrips subsequently feed on the infected plants for sufficient time to acquire the pathogen prior to dispersal. MCMV is a causative agent of maize lethal necrosis disease, a devastating disease of maize, which causes yield losses up to 90% in Sub-Saharan Africa undermining food security in the region. This information could serve as useful input for designing an effective and ecologically sustainable integrated vector (thrips) and disease (maize lethal necrosis) control strategies, which is an integral part of icipe’s mandate.

Funding: We gratefully acknowledge the financial support by the following organizations and agencies: The German Federal Ministry for Economic Cooperation and Development – Grant 81141840 for the Thrips Project, the European Union Grant Contract No. DCI-FOOD/2014/346-739; Biovision Foundation; UK Aid from the UK Government; Swedish International Development Cooperation Agency (SIDA); the Swiss Agency for Development and Cooperation (SDC) and the Governments of Kenya and Ethiopia. The views expressed herein do not necessarily reflect the official opinion of the donors.

University: The scholar was registered at Egerton University, Kenya.

Best science poster by an icipe scholar

Juliet Ochola (MSc Scholar)

Poster Title:  ‘Wrap and Plant’ technology: Elucidating the mechanisms of banana fibre paper in the management of potato cyst nematodes

Supervisors: Laura Cortada³, Margaret Ng’ang’a², Ahmed Hassanali², Danny Coyne³ and Baldwyn Torto¹

¹International Centre of Insect Physiology and Ecology, P.O. Box 30772-00100, Nairobi, Kenya; ² Kenyatta University P.O. Box 43844- 00100, Nairobi, Kenya; ³International Institute of Tropical Agriculture, P.O. Box 30772-00100, Nairobi, Kenya.

Brief Summary: Potato (Solanum tuberosum, L) is the second most important food crop in Kenya. In the recent past, there has been a major decline in potato production in the country. This could be attributed to a number of factors, among them, potato cyst nematode (PCN) which was first reported in the country in 2015. Potato cyst nematode (PCN) is a quarantine pest with a yield reduction potential of up to 80% under high infestation. The prolonged diapause and quiescence of PCN cysts hinders most management strategies. Recent attempts to assess suitable PCN management options in Kenya demonstrated the effectiveness of the ‘Wrap & Plant’ (W&P) technology. This is an innovation developed in North Carolina State University that uses a field deployable biodegradable matrix produced from banana fibres (banana paper) and impregnated with a nematicide (abamectin) to control plant parasitic nematodes. Field trials with the W&P technology found that the wrapped potato seeds had a 4-fold increase in yield compared to farmers’ practices (control), and 83% decrease in PCN’s cyst population in the soil. The untreated banana fiber paper also increased potato yields by 3-fold and reduced the nematode population by 43%. The present study sought to elucidate the underlying mechanisms by which banana paper (treated and untreated) is able to control PCN. Using behavioural experiments, we investigated the response of PCN to potato root exudates in the presence of banana paper. Our results showed that untreated and treated banana paper reduced PCN hatching by ~37% and 35% respectively in the first 8 weeks of exposure to potato root exudates (PRE) as compared to the PRE alone. Dual choice chemotaxis assays also showed that in the presence of the banana paper (treated and untreated), PCN juveniles were not attracted to PRE (stimulus) suggesting that the paper could be masking the attraction cues from the exudates interfering with PCN host location and also acting as a physical barrier preventing the migration of these juveniles. Chemical analysis by liquid chromatography−quadrupole time-of flight−mass spectrometry (LC-QTOF-MS) of the potato root exudate, exposed and unexposed to banana paper, showed that the paper adsorbed the PCN hatching factors solanoeclepin A, α-chaconine, α-solanine, among other compounds present in PRE. This was confirmed by recovery studies (extraction and analysis) of the adsorbed banana paper. We conclude that a combination of all the above mechanisms could likely result in late invasion and development of PCN, enabling a wrapped potato to act as a ‘trap crop’ for PCN.

Funding: The study was conducted with funding from the Bill & Melinda Gates Foundation and the International Institute of Tropical Agriculture through icipe’s Dissertation Research Internship Programme (DRIP).  We also gratefully acknowledge the contribution from icipe core funding provided by UK Aid from the UK Government, Swedish International Development Cooperation Agency (Sida), the Swiss Agency for Development and Cooperation (SDC), Government of the Federal Democratic Republic of Ethiopia, and the Kenyan Government. The views expressed herein do not necessarily reflect the official opinion of the donors.

University: The scholar is registered at Kenyatta University, Kenya.

Olabimpe Yewande Olaide (PhD Scholar)

Poster Title: Zebras as potential sources of potent repellents for savannah tsetse fly control

Supervisors: David P. Tchouassi, Abdullahi A. Yusuf*, Christian W. W. Pirk*, Daniel K. Masiga, Rajinder K. Saini, Baldwyn Torto,

International Centre of Insect Physiology and Ecology (icipe), Kenya; *University of Pretoria, South Africa

Summary: Tsetse flies are exclusive blood feeders and vectors of Human and Animal African trypanosomiasis (HAT and AAT), a severe wasting disease which poses serious economic consequences. Despite the recent success in the control of HAT, AAT continues to threaten sustainable agriculture and livestock production and calls for innovative control efforts. A recent advancement in vector control employs repellents which can be identified from non-preferred hosts of tsetse flies such as waterbuck, wildebeest and zebra to protect livestock from tsetse bite and break the disease transmission cycle. A four-component repellent blend previously identified from waterbuck is now in use as innovative repellent collars for cattle. However, other non-preferred hosts of tsetse flies remain to be exploited for cheaper and, possibly, more potent tsetse repellents. We tested the hypothesis that zebra skin odour, much like the waterbuck, contribute to its avoidance by tsetse flies and, therefore, contain repellents for these flies. We collected crude skin odours from zebra and evaluated their effects on catches of wild savannah tsetse flies (Glossina pallidipes). Further, we identified the electrophysiologically-active compounds in the crude skin odours of zebra and evaluated these compounds for repellency in extensive field trials. We found that crude skin odours of zebra significantly reduced field catches of G. pallidipes and the key components responsible were the three ketones 6-methyl-5-hepten-2-one, acetophenone and geranylacetone, which was made into a three-component repellent blend. Both crude zebra odour and the identified three-component repellent blend performed competitively with the existing waterbuck repellent compounds (WRC). Our study provides an evidence for the contribution of allomones in zebra skin odour to its avoidance by tsetse flies and identify a three-component tsetse repellent blend which represents a new effective tool in the integrated control of tsetse flies and Animal African trypanosomiasis.

Funding: We gratefully acknowledge the financial support for this research by the following organisations and agencies: European Union; icipe core funding provided by UK Aid from the UK Government, Swedish International Development Cooperation Agency (Sida), the Swiss Agency for Development and Cooperation (SDC), Government of the Federal Democratic Republic of Ethiopia, and the Kenyan Government. The scholar is supported by a German Academic Exchange Service (DAAD) in-region postgraduate scholarship. The views expressed herein do not necessarily reflect the official opinion of the donors.

University: The scholar is registered at University of Pretoria, South Africa.

Hillary K. Kirwa (MSc. Scholar)

Poster Title: The Plant Hormone Zeatin Identified in Tomato Root Exudate Elicits Attraction in the Root Knot Nematode, Meloidogyne incognita

Supervisors: Lucy K. Murungi² and Baldwyn Torto¹

¹International Centre of Insect Physiology and Ecology (icipe), P.O. Box 30772-00100 Nairobi, Kenya

²Jomo Kenyatta University of Agriculture and Technology, P.O. Box 62000-00200 Nairobi, Kenya

Brief summary:  Tomato (Solanum lycopersicon) is a high value vegetable crop grown globally. Production constraints include above and below ground pests and diseases such as root knot nematodes (RKN; Meloidogyne spp.). The RKNs are a group of plant parasitic nematodes of high economic importance in horticulture. They invade a wide range of horticultural crops causing malformation of the roots, hampering the processes of nutrients and water uptake leading to reduce crop yields. The RKN host location and invasion of plants is caused by infective second stage juveniles (J2s) and involves a complex process mediated by chemical signals released by plant roots. Volatile organic compounds act as long-range cues that signal the presence of a host plant, whereas non-volatiles are short range cues associated with site of root invasion. In this study, we investigated the responses of second stage juveniles (J2s) of Meloidogyne incognita to the root exudate of tomato, a natural host plant. Using bioassay-directed fractionation, combined with dose-response assays, we identified specific non-volatile compounds in the root exudate that elicited differential stylet and chemotaxis responses in J2s. These specific compounds included a phytohormone, flavonoids and the alkaloids. The phytohormone zeatin elicited the strongest attraction in J2s suggesting that its presence in the root exudate informs J2s of a healthy growing plant and an opportunity for it to invade the host root and the subsequent penetration. This finding brings new insights in RKN-plant interactions which might have potential application in RKN management.

Funding: This work was conducted with funding from the following organizations and agencies: USDA−ARS Project 58-6615-3-011-F, icipe core funding provided by UK Aid from the UK Government, Swedish International Development Cooperation Agency (Sida), the Swiss Agency for Development and Cooperation (SDC), Government of the Federal Democratic Republic of Ethiopia, and the Kenyan Government.  The views expressed herein do not necessarily reflect the official opinion of the donors.

University: The scholar is registered at Jomo Kenyatta University of Agriculture and Technology (JKUAT).