Sugarcane production in Malawi : pest, pesticides and potential for biological control

Abstract

Sugarcane is an importance source of energy and livelihoods worldwide. The production of sugarcane is significantly affected by several insects, weeds and pathogens commonly referred to as pests. In addition, climate scientists predict that climate change or variability will affect sugarcane production and its associated pests. Chemicals called pesticides, beneficial pathogens and insects called natural enemies or biological control agents are used to control these pests. Little is known about the diversity and richness of both pest and natural enemy species nor the properties of the pesticides used against them in Malawi Few studies indicate that insects such as stemborers and aphids, and weeds are the most common pests; and that their control is heavily dependence on pesticides in Malawi. However, most pesticides are harmful to the environment and improper use may lead to human poisoning. Knowing the main pests and using pesticides that are least harmful to the environment and natural enemies coupled with good crop management practices may contribute to solving this problem.

To document pest composition and how they were controlled, a review of literature, questionnaire and farm surveys were conducted in the major sugarcane growing areas of Malawi. The questionnaire survey was administered to 55 farmers and 7 representatives of 1474 farmers. We collected 221 insect samples from 48 sugarcane fields and isolated beneficial fungi from 12 soil and 60 plant samples collected from 12 sugarcane fields in southern Malawi, respectively. The best way to inoculate sugarcane was determined) in a potted experiment conducted using a commercially available formulation of beneficial fungi (Beauveria bassiana strain GHA). We identified the fungi and insects samples to genus and/or species level largely using morphological characteristics. Molecular characterization based on partial sequencing of Bloc gene region in 50 fungal samples and cytochrome oxidase subunit I (COI) gene region in 65 insect samples, respectively, were conducted to support morphological identifications. Separate DNA polymorphism and phylogenetic analyses were performed for the insect and fungal samples. Environmental and human health risks associated with pesticides in use were determined using the environmental impact quotient (EIQ) and World Health Organization (WHO) Classification of Pesticides by hazard. We also explored the likely impact that climate change or variability will have on the type and amount of pesticides used in sugarcane production using Malawi as a case study.

The results indicated that weeds and insects infest sugarcane in Malawi. The main insect pests were stemborers (Chilo partellus and Busseola fusca), soil-dwelling insects’ pests (Heteronychus licas and H. arator, Anomala spp.), sugarcane thrips (Fulmekiola serrata), red spider mites (Tetranychus urticae), aphids (Sipha flava) and the fall army worm (Spodoptera frugiperda sp. 1). DNA polymorphism analysis revealed low genetic differentiation among C. partellus and B. fusca populations in Malawi. A total of 16 pesticides were used to manage the pests. These are slightly to moderately hazardous to humans, 50 % are highly toxic to bees and 70% can contaminate the environment. Individuals who sprayed these pesticides had minimal protective wear. As a result, at least 65% had experience skin irritation, headache, coughing and running nose as a result of being exposed to these pesticides. The predicted rise in temperature will reduce the efficacy of less toxic pesticides especially cypermethrin. Leaching of sorbed pesticides through high rainfall intensity and increased frequency of floods.

Beneficial fungi in three genera namely Beauveria, Metarhizium and Isaria were identified from soil and sugarcane samples collected from southern Malawi. More isolates (81.7%) were collected from soil than from plants (36.7%). The majority of these isolates (72%) were Beauveria species. Molecular identification and phylogenetic analysis identified the Beauveria isolates as B. bassiana were closely related to B. bassiana AFNEO_1 clade isolated from the coffee berry borer, Hypothenemus hampei in coffee fields of South America and in Africa. However, the Malawian B. bassiana clearly clustered in a separate clade. This is the first report of B. bassiana occurring as an endophytes of sugarcane; and B. bassiana, Metarhizium and Isaria species occurrence in agricultural fields in Malawi.

Results from the sugarcane inoculation experiment showed that B. bassiana could be effectively inoculated in sugarcane using foliar and soil sprays, and stem injections. Stem injections were highly effective (75%) compared to foliar sprays (43%) and soil sprays (25%) plants inoculated, respectively. The inoculated B. bassiana was recovered in both old and new leaves and stem tissue, even though the recovery rate decreased with time. However, plants that had got stem injections were much shorter that plants that had foliar and soil inoculation, and control plants.

The results especially those on natural occurrence of beneficial fungi particularly B. bassiana and Metarhizium spp. will be useful in the control of not only of pests in sugarcane but also in several crops mainly vegetables.

Sukkerrør er en viktig kilde til energi og som levebrød over hele verden. Produksjonen av sukkerrør er betydelig påvirket av insekter, ugras og plantesykdommer ofte betegnet som skadegjørere. I tillegg forutsetter klimaforskere at klimaendringer eller variasjon i klima vil påvirke sukkerrørsproduksjonen og tilhørende skadegjørere. Kjemiske plantevernmidler og biologiske kontrollmetoder brukes til å kontrollere disse skadegjørerne. I Malawi kjenner vi lite til forekomst og diversitet av skade- og nytteorganismer i sukkerrørproduksjonen eller til egenskapene til plantevernmidlene som brukes. Tidligere studier tyder på at ulike sommerfugllarver, bladlus og ulike ugrasarter er blant de vanligste skadeorganismene og at kontroll er sterkt avhengighet av plantevernmidler. Plantevernmidler kan imidlertid være skadelige for helse og miljø. Å kjenne de viktigste skadegjørerne og bruke plantevernmidler som er minst mulig skadelige for miljøet og nytteorganismer kombinert med god agronomi, kan bidra til å løse dette problemet.