New methods are needed to combat
locusts. Researchers began to look at biocontrol agents and in particular
entomopathogens—microbes that infect insects and kill them.
Entomopathogenic fungi such as Beauveria and Metarhizium had
been identified years before, and natural epidemics of pathogens in locust
swarms caught under temperature inversions in cold, damp air were well
documented, if rare. The use of such biocontrol agents could provide an
environmentally friendly insecticide.
This idea led to an international project—the Biological Control of Locusts and Grasshoppers. The project is implemented by the International Institute of Biological Control (IIBC) (UK), The International Institute of Tropical Agriculture (IITA) (Nigeria) and AGRHYMET (Niger), an institute of CILSS (Comité Inter-Etats de Lutte contre la Séchéresse au Sahel).
The biocontrol project
From the outset the project was faced with the conflicting requirements for fungal spore germination and the arid realities of the typical locust control environment. The well-established high humidity requirements of fungal spores did not fit well with the high evaporation rates in arid and semi-arid environments. The use of a high-volume water-based spray for locust control on a typical Sahelian day seemed a laughable idea. However, previous work of the former project leader Dr Chris Prior on a cocoa project in Oceania provided the breakthrough required for the project to take off. He had shown that spores of Beauveria bassiana, a well known entomopathogen, had very lipophilic (oil and fat loving) cell walls. This meant that they were very well suited for suspension and high pathogenic activity in oil-based carrier liquids.
Phase 1 of the project then began in 1989 with a search for candidate entomopathogens for locusts and grasshoppers involving collection of diseased locusts and isolation of fungi from them. This soon revealed strains of Metarhizium flavoviride which were highly virulent to locusts. As predicted the fungal spores dispersed well in oil confirming the potential for use in ultra low volumes of oil by Controlled Droplet Application (CDA) spraying. Standard bioassays showed they also infected the desert locust Schistocerca gregaria in oil as well as water and in dry as well as damp environments.
How does it work?
As the project’s spray application specialist, Dr Roy Bateman, points out, the ambient relative humidity is not important to successful spore germination on and into the locust but the relative micro-humidity certainly is. When sprayed on locusts and grasshoppers the spores act as a contact mycoinsecticide, and grow by forming germ tubes (one per spore) producing a ‘pressure’ point which ‘cracks’ the insect’s cuticle through a combination of enzyme activity and some physical pressure. Once inside the locust the fungus invades the haemocoel (blood filled cavity) and produces blastospores which are distributed in the insect’s body fluids. The carrier oil helps by sticking the spore firmly to the insect’s body and possibly (depending on the nature of the oil) softening the waxy layer on the locust to allow water from the locust’s body to facilitate spore germination.
A planned series of progressive field tests spanning Niger, Mauritania and South Africa used whatever target pests were available. First off were ‘Arena’ tests against the Senegalese grasshopper Oedaleus senegalensis in Niger which in spite of its name behaves like a locust and causes more damage than tree locusts. These tests gave good results but appeared to paint an artificially rosy picture since the sprayed nymphs die much quicker in the ‘caged’ artificial environment of the arena than if they are left outside.
Initial field trials using the Micron Ulva+, a hand-held spinning disc (rotary atomiser) sprayer were followed up with much larger three-year investigations using Micron’s vehicle mounted rotary atomiser called the Ulvamast. This applicator for spraying chemical insecticides has been used with great success by Dr Bateman’s colleagues on the LUBILOSA Team (LUtte BIologique contre les LOcustes et SAuteriaux) in Niger to apply oil-based formulations of Metarhizium flavoviride conidia (reproductive body) just 100 g of mycoinsecticide in 2 litres of oil per ha. Very encouraging results have been achieved. In the latest Niger trials at Maine sora, supervised by Christiann Kooyman, against a high population of 40+ mixed grasshoppers per m2, results showed a 70% reduction in population after one month with a continuing decline. Trials have been extended into Mauritania where the project team are always likely to find desert locusts as the target pest and where good co-operation with the German Technical Assistance Agency (GTZ) has developed into formal links.
The commercial formulation of the mycoinsecticide is ‘Green Muscle 189’ (containing conidia the of the fungus Metarhizium flavoviride strain with about 50,000 billion conidia per kg). The action of Green Muscle is not immediate like some knock-down chemicals, as it takes 1-2 weeks to develop and spread—the cycle of spore germination, locust infection, spore production and healthy locust infection. As Dr Bateman acknowledges, this poses problems for farmers faced with swarms approaching high value crops. Other problems include the development of small scale local production and storage to make the product readily available, and the concerns of locust-affected states about releasing a fungal pathogen in their environment.
The strength of this biocontrol agent is its environmental friendliness, making it particularly suited for use in environmentally sensitive areas. Dr Bateman is both optimistic and realistic about the prospects of formulation for commercial application against locusts. He sees it as the first step towards preventative control of locusts allowing control organisations a stronger element in their management plans rather than just reacting to locust upsurges with hard pesticides.
Terry Mabbett is an independent
writer on crop protection issues.
[This article first appeared in Pesticides News No. 31,March 1996, page 15]