Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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    Behavioural effects of fungal infection by Metarhizium anisopliae in adult malaria mosquitoes
    Ondiaka, S.N. - \ 2012
    Wageningen University. Promotor(en): Willem Takken; Marcel Dicke, co-promotor(en): W.R. Mukabana. - S.l. : s.n. - ISBN 9789461732934 - 179
    anopheles gambiae - vectoren, ziekten - malaria - vectorbestrijding - biologische bestrijding - metarhizium anisopliae - diergedrag - paringsgedrag - gedrag bij zoeken van een gastheer - voedingsgedrag - ovipositie - anopheles gambiae - disease vectors - malaria - vector control - biological control - metarhizium anisopliae - animal behaviour - mating behaviour - host-seeking behaviour - feeding behaviour - oviposition

    Malaria remains a major global health problem with the burden of disease greatest in Sub-Saharan Africa. The strategies for malaria control differ throughout the world according to levels of endemicity and the magnitude of disease but the focus remains either to control malaria parasites or vectors. A high degree of drug resistance and the absence of malaria vaccines are a major hindrance to control of the disease. In such circumstances, vector control becomes an alternative and has remained the most effective means to prevent malaria transmission. Contemporary adult mosquito control is almost exclusively based on indoor application of chemical insecticides in the form of indoor residual spraying (IRS) of walls and ceilings and insecticide-impregnated bed nets. However, sustainable use of chemicals is undermined by problems of insecticide resistance in mosquito populations, environmental contamination and risks to human health. Biological control based on fungal pathogens has shown potential to complement existing vector control methods. The entomopathogenic fungi (EPF) Metarhizium anisopliae and Beauveria bassiana have demonstrated ability to infect, kill and reduce the survival of malaria vectors. However, the effect of EPF on the behaviour of malaria vectors has not been fully addressed.

    This thesis was designed to provide baseline information on mosquito-fungus interaction focusing on the efficacy of entomopathogenic fungus M. anisopliae ICIPE 30 on the important life-history behaviours of the African malaria vector Anopheles gambiae Giles sensu stricto under laboratory and semi-field conditions. The information is important to facilitate the further development of malaria vector control based on biological control agents. Host-seeking, sugar-feeding, mating and oviposition were the behaviours investigated. Since mosquito-fungus contact is crucial for infection with EPF, a paper sheet (28.6 × 14.3 cm) lined inside a plastic cylinder (9-cm diameter and 15-cm height) was developed as a cost effective method of infection. Moreover, 0.1 g (approx. 1011 conidia/m2) of dry conidia and 6 hr exposure time sufficient for An. gambiae to pick up large numbers of conidia were established to cause high pathogenicity (Chapter 3). As the impact of EPF on insect behaviour was reported to occur at least three days post-exposure to fungal pathogen (Chapter 2), all experiments were conducted with a special focus on mosquitoes three days post-exposure to fungus. It is, however, important to mention that on average 50% of the mosquitoes died on the third day after fungal exposure (Chapter 3) and only those that survived were used for behavioural assays.

    The host-seeking capability of An. gambiae mosquitoes is an important parameter in the vectorial capacity equation. At short-range (1 m from host) assessment using a dual-choice olfactometer under semi-field conditions, infection with EPF strongly reduced the host-seeking response of mosquitoes, but did not impair their olfactory-based capability to discriminate between hosts (Chapter 4). At medium-range, using experimental cages (3 x 3 x 2 m) under laboratory conditions, fungal infection reduced the host-seeking response and feeding propensity of female An. gambiae mosquitoes (Chapter 7) whereas at long-range (7 m from host) inside a semi-field enclosure, infection with EPF sharply reduced the house-entry response and the hourly human-biting responses of host-seeking mosquitoes indoors and outdoors (Chapter 5). Plant sugar feeding is an important component in the biology of mosquitoes and is the main priority for both sexes at emergence. Infection with fungal pathogen strongly reduced the survival and sugar-feeding propensity of both sexes of the malaria vector An. gambiae but did not affect their potential to feed and digest meals (Chapter 6). Mating behaviour plays a key role in population growth. The activity takes place after sugar feeding and thereafter, the females search for their blood meal host. Infection with M. anisopliae strongly reduced multiple mating propensity and the mating performance of adult male An. gambiae mosquitoes in a large arena such as a screenhouse. Although this resulted in a reduction in the number of females inseminated, it facilitated the transfer of fungal conidia to conspecific healthy females during mating (Chapter 8). Finally, after blood meal intake, the females prepare to lay eggs. Infection with M. anisopliae reduced the oviposition propensity of female An. gambiae mosquitoes although the number of eggs laid remained unaffected (Chapter 7).

    In conclusion, these findings demonstrate that the entomopathogenic fungus M. anisopliae alters the major life history behaviours of An. gambiae mosquitoes. This is possible because the fungus strongly impairs flight performance of mosquitoes that makes the insect less able to fly and engage in host-seeking, sugar-feeding, mating and oviposition behaviours. The high mortalities observed in the early days of infection prior to conducting behavioural assays, mortalities observed while conducting behavioural assays and a reduction in behavioural response of M. anisopliae-infected mosquitoes collectively are likely to have a significant impact in suppressing a vector population. The susceptibility of male mosquitoes to fungal conidia opens a new strategy for mosquito vector control. Overall, this thesis has demonstrated that EPF may be a good complement to other mosquito vector control tools for the reduction of mosquito bites, and transmission of malaria and other mosquito-borne diseases.

    Beerling : 'GNO vergt een langduriger toepassing voor een goed effect' : interview
    Arkesteijn, M. ; Beerling, E.A.M. - \ 2009
    Onder Glas 6 (2009)3. - p. 27 - 29.
    kassen - pesticiden - resistentie tegen pesticiden - biopesticiden - metarhizium anisopliae - schimmelinsecticiden - gewasbescherming - bloementeelt - chrysanten - glastuinbouw - snijbloemen - greenhouses - pesticides - pesticide resistance - microbial pesticides - metarhizium anisopliae - fungal insecticides - plant protection - floriculture - chrysanthemums - greenhouse horticulture - cut flowers
    Het gebruik van gewasbeschermingsmiddelen van natuurlijke oorsprong (GNO's) wint terrein door het optreden van resistentie tegen chemische middelen en doordat de markt erom vraagt. Er zijn diverse goed werkende GNO's op de markt, maar een teler moet er anders mee omgaan dan met chemische middelen. Bovendien is nog onvoldoende bekend hoe je deze middelen optimaal moet toepassen. Lees daarom het etiket goed, volg de adviezen op en zit er met de neus bovenop door goed te scouten
    The entomopathogenic fungus Metarhizium anisopliae for mosquito control. Impact on the adult stage of the African malaria vector Anopheles gambiae and filariasis vector Culex quinquefasciatus
    Scholte, E.J. - \ 2004
    Wageningen University. Promotor(en): Joop van Lenteren, co-promotor(en): Willem Takken; B.G.J. Knols. - Wageningen : S.n. - ISBN 9789085041184 - 183
    culicidae - anopheles gambiae - culex quinquefasciatus - biologische bestrijding - metarhizium anisopliae - entomopathogene schimmels - anopheles gambiae - culex quinquefasciatus - culicidae - biological control - metarhizium anisopliae - entomogenous fungi
    Insect-pathogenie fungi for mosquito control (Chapters 1-3)Malaria and lymphatic tilariasis impose serious human health burdens in the tropics. Up to 500 million cases of malaria are reported annually, resulting in an estimated 1.5-2.7million deaths, of which 90% occur in sub-Saharan Africa. Malaria is caused by protozoa of the genus Plasmodium and is transmitted through bites of mosquitoes belonging to the genus Anopheles. Lymphatic filariasis is caused by helminths, the most widespread species being Wuchereria bancrofti, and is transmitted through bites of mainly Culex quinquefasciatus and certain Anopheles species. Worldwide, approximately 146 million people are infected with the disease.Mosquito vector control is an important way to tight these diseases. In Africa, vector control is almost exclusively based on chemical insecticides, used predominantly to impregnate bed nets and for indoor residual spraying. Growing concerns about their negative impact on human health, on the environment, and about insecticide resistance are the reasons for increasing interests in vector control methods that are not based on chemicais, such as biological contro!.veral biological control agents are known to be effective against mosquitoes such as predatory tish (e.g. Gambusia ajfinis and Poecilia reticulata), nematodes (e.g. Romanomermis culicivorax), microsporidia (e.g. Nosema algerae), bacteria (e.g. Bacil/us thuringiensis israelensis and B. sphaericus), and insect- pathogenie fungi (e.g. Lagenidium, Coelomomyces and Culicinomyces).All of these, however, target the larval stages of mosquitoes. To date, there is no biological control agent for use against the adult stage of mosquitoes. However, reduction in survival of adult mosquitoes is considered to have a much higher impact on transmission than a reduction in the number of mosquito larvae. The objective of this PhD thesis was therefore to search for a biological control agent for adult mosquitoes, and to develop a method to use such an agent in integrated vector management (IVM) in Africa. The primary targets for this research were the major malaria vector Anopheles gambiae s.l., and, to alesser degree, the lymphatic tilariasis vector Culex quinquefasciatus. In Chapter 2 the most important insect-pathogenic fungi for (mostly) larval mosquito control are reviewed. Of these, the Hyphomycetes possess a characteristic that gives them a major advantage over other biocontrol agents to be used for killing adult mosquitoes: Unlike with bacteria, nematodes or microsporidia, the infectious propagules of these fungi do not need to be ingested. Instead, contact with the cuticle is enough for the infective propagules (conidia) to infect the mosquito. A conidium penetrates the insect cuticle by secreting cuticle­degrading enzymes. Once inside, the fungus grows rapidly and secretes toxins, which kill the mosquito. Depending on temperature, fungal dosage, and susceptibility of the mosquito to the fungus, the process from inoculation to host death may take between approximately three and ten (or even more) days. After host death, and under favourable conditions of high humidity, the fungus will grow out of the cadaver and produce conidia asexually (sporulation).The strategy envisaged to infect and kill wild mosquitoes in sub-Saharan Africa is based both on the characteristic of Hyphomycetous fungi to infect insects through contact by penetrating the cuticle, and on the behavioural characteristic of An. gambiae mosquitoes to blood feed predominantly inside houses during the night, and remain indoors for at least several hours afterwards to rest and digest the blood mea!. If conidia are applied indoors on so-called 'mosquito resting targets' (see Chapter 9), mosquitoes are expected to acquire an infection ofthe fungus by resting on those targets.In Chapter 3, five different Hyphomycetous insect pathogenie fungi were tested on adult An. gambiae, including Beauveria bassiana, a Fusarium sp. and three isolates of Metarhizium anisopliae. Four of these fungi were isolated from insects in western Kenya, an area of endemie malaria. Isolate ICIPE30 of M. anisopliae proved to be highly virulent for the tested mosquito species, and it was decided to continue further studies with this isolate.The effect of the insect-pathogenie fungus Metarhizium anisopliae on African mosquito vectors (Chapters 4-7)As described in Chapter 4, M. anisopliae was tested both on An. gambiae as weil as on Cx. quinquefasciatus, and a standard contamination technique to infect adult mosquitoes was developed. Using this technique, the effect ofthe fungus on An. gambiae was quantified in more detail by a dose-response bioassay. This experiment showed that at a dose of 1.6 x 1010 conidia m-z, >83% were infected (i.e. mosquito cadavers with sporulating fungus), with a mean LT50 value of 5.6::1: 0.4 days. Later experiments (Chapters 6 and 8) showed that the fungus could be even more effective at that same dose, with infection levels up to 96.4%, and all mosquitoes dead by day 6, whereas uninfected female An. gambiae lived much longer with L T 50 values > 18 days.Apart from the principal effect of the fungus, causing mosquito death by direct contact with conidia, infection with M. anisopliae also caused at least two secondary effects (Chapter 5). One ofthose secondary effects is a reduction in feeding propensity. In one ofthe experiments of Chapter 5, individual female An. gambiae s.s. were offered a total of 8 blood meals. It was found that mosquitoes, inoculated with a moderately high dose of fungal conidia (1.6 x 109 conidia mOz), exhibited reduced appetite upon increasing effects of fungal growth. Of the fungus-infected females, the proportion of mosquitoes taking a second blood meal was reduced with 51 %. This was further reduced to 35.3% for the 4th blood meal. The other observed secondary effect was that infected females took smaller blood meals, resulting in fewer eggs per gonotrophic cycle.In order to achieve the highest possible impact on mosquito populations, it is desirabIe that other contamination pathways besides the primary mode of contamination are utilized to spread the fungus through the population, such as horizontal transmission. The results of experiments described in Chapter 6 showed that, under laboratory conditions, conidia can be transferred from an inoculated female to a 'clean' male during the process of mating, with mean male infection rates between 10.7::1: 302% and 33.3 ::I: 3.8%.Since the mosquito inoculation method described above is based on mosquitoes that rest on conidia-impregnated sheets, it is desirabIe that mosquitoes are not repelled by conidia. To test this, behavioural effects of female An. gambiae in close vicinity of the fungus were investigated (Chapter 7). The results showed that dry conidia have a significant repellent effect (p<0.05). However, when conidia were applied in a suspension of 8% adjuvant vegetable-oil formulation and impregnated on paper, this effect ceased (p=0.205). The results suggest that if the fungus is to be applied as a biological control agent against Afrotropical mosquitoes, conidia should be impregnated on e.g. cotton sheets in an oil-based formulation to avoid repellency effects.Practical approach to mosquito vector control in Africa using M. anisopliae (Chapters 8-10).From a practical and economic point of view, the interval between applications ofthe control agent should ideally be as long as possible, without the agent losing too much efficacy. In the case of commonly used chemical residual insecticides such as permethrin this is about 6 months. Laboratory experiments (Chapter 8) showed that M. anisopliae conidia impregnated on paper and on netting material remained virulent to An. gambiae up to one month after impregnation. Experiments on conidial shelf life under different conditions showed that conidia kept in 8% vegetable oil remained viabie up to at least I month. Conidia stored in 0.05% Tween 80 exhibited only slightly reduced viability after 3 months at 27° and after 6 months at 4°C. Dry conidia stored with silica gel retained viability for at least 6 months. The results suggest that, if applied in the field, re-impregnation should be carrPractical approach to mosquito vector control in Africa using M. anisopliae (Chapters 8-10).From a practical and economic point of view, the interval between applications ofthe control agent should ideally be as long as possible, without the agent losing too much efficacy. In the case of commonly used chemical residual insecticides such as permethrin this is about 6 months. Laboratory experiments (Chapter 8) showed that M. anisopliae conidia impregnated on paper and on netting material remained virulent to An. gambiae up to one month after impregnation. Experiments on conidial shelf life under different conditions showed that condidia kept in 8% vegetable oil remained viable up to at least 1 month. Condidia stored in 0.05% Tween 80 exhibited only slightly reduced viability for a least 6 months. The results suggest that, if applied in the field, re-impregnation should be carried out monthly, but dry conidia can be stored for at least 6 months under conditions of very low relative humidity. Chapter 9 of this thesis describes a field study of domestic application of M. anisopliae in houses in south east Tanzania, a region holoendemic for malaria and lymphatic filariasis. The fungus was applied on black cotton sheets, attached to ceilings as indoor mosquito resting targets. Indoor resting catches of mosquitoes were carried out daily and collected mosquitoes were kept alive in small containers as long as possible to determine survival. Almost 90% of all collected mosquitoes were An. gambiae s.l. (of which 94.7% were An. gambiae s.s. and 5.3% An. arabiensis). In total, 181 wild An. gambiae s.l. and 6 wild Cx. quinquefasciatus were infected with the fungus. Infected mosquitoes died significantly sooner than uninfected mosquitoes, with an average daily survival rate of 0.722 for infected female An. gambiae, against 0.869 for uninfected females. Calculated from the total number of An. gambiae s.l. and Cx. quinquefasciatus that were caught from the fungus­impregnated resting targets, respectively 33.6 and 10.0% had acquired fungal infection. Of the total number of 580 female An. gambiae collected from the houses containing fungus­impregnated sheets, 132 were infected, which is an effective coverage of22.8%. Ifthis same coverage level is assumed at village level, and, together with the reduced daily survival rate, is introduced into a malaria transmission model, the total number of infectious bites per person per year (Entomological Inoculation Rate; EIR) drops from 262 to 14 (Chapter 10). Although the field experiment was on a relatively small scale and of short duration, the predictions of the malaria transmission model strongly indicate that application of M. anisopliae, aimed at the adult stage of African mosquito vectors can have a high impact on transmission intensity. It is argued that large-scale application of this method, implemented as part of an integrated vector management (IVM) strategy including larval control using biological control agents, the use of repellent plants and of unimpregnated bednets, malaria can effectively be controlled without the use of chemical insecticides. This thesis may form a first step towards such a strategy. Further research is necessary, especially in 1) searching for a fungal isolate that has even higher virulence against the targeted mosquito species, 2) testing of non-target effects and safety of the most effective fungal strain for registration, 3) searching for the most optimal formulation and application method to increase infection percentages.
    Recherches sur la maladie, due à Metarrhizium anisopliae chez le criquet pèlerin
    Veen, K.H. - \ 1968
    Wageningen : Veenman (Mededelingen / Landbouwhogeschool Wageningen 68-5) - 77
    insectenplagen - acrididae - sprinkhanen - schimmelinsecticiden - biologische bestrijding - schimmels - organismen ingezet bij biologische bestrijding - dierziekten - dierpathologie - metarhizium anisopliae - insect pests - acrididae - locusts - fungal insecticides - biological control - fungi - biological control agents - animal diseases - animal pathology - metarhizium anisopliae
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