Rachid Hanna, email@example.com
Biological control is one of the oldest and most useful and ecofriendly approaches to pest management.
IITA has had an excellent tradition in biological control which has given it some of its greatest successes. These were captured by Peter Neuenschwander in his 2004 commentary in Nature.
The past and the present
The invasion of Africa by the cassava mealybug and the initial failure to find a solution to this devastating pest marked the start of a series of highly successful biological control efforts led by IITA in collaboration with partners from Africa and from around the globe. The mealybug invasion was shortly followed by the cassava green mite, also originating from South America. The duo devastated production throughout the cassava belt. Finding a natural enemyâ€”a tiny parasitic insect (renamed several times) Anagyrus (=Apoanagyrus =Epidinocarsis) lopeziâ€”for the control of the mealybug was relatively quick. Once it was released, the parasitoid quickly established. Aided by over 150 additional releases and within a span of 12 years the parasitoid covered nearly all of SSA, in the process leading up to 90% reduction in cassava losses and up to US$29 billion in estimated value of crop recovery.
The campaign for green mite biological control followed an approach similar to that of the mealybug, but it took much longer to find appropriate predators that would eventually provide the control needed for cassava recovery from the two South American invaders. Three promising predators (tiny phytoseiid mites) were established, one became extinct, another was restricted to humid regions, and a third species (Typhlodromalus aripo) became nearly the equivalent of A. lopezi (without the many name changes!).
Within a span of 11 years and over 450 additional releases, T. aripo has been established in at least 22 countries. Where it has been present for 2 or more years (and where cassava varieties have at least partly hairy apices that are favorable for T. aripo colonization and persistence), it brought down losses to the mite from an average of 35% to below 10% (after mealybug control), resulting also in billions of dollars in crop recovery.
The mealybug and green mite campaigns have been credited for bringing the capacity of biological control to SSA and spreading its science and practice as no other on any continent has done. Several successful projects followed, including that of the mango mealybug (an invader from India) and the spiraling whitefly (a polyphagous pest from the Caribbean), and extended to control of the water weeds. These successful efforts added another billion US$ in estimated savings.
These campaigns provided the stimulus and the capacity to build on successes with the continuation of biological control efforts at IITA. Presently, cowpea thrips and cowpea pod borer are being fought with the tiniest of known parasitoids (for thrips) and with a combination of a parasitic wasp and a virus (for cowpea pod borer). Programs are under way to tackle several other devastating pests: the coconut mite (from South America), the banana aphid (an invader from Southeast Asia and the only vector of the devastating banana bunchy top virus), the Sri Lanka fruit fly (now present in 24 countries and on more than 50 host fruits in Africa), and a variety of indigenous pests including the African root and tuber scale (in the Congo Basin), the sweet potato whitefly (vector of cassava mosaic viruses throughout Africa), cotton worms, several pests of cashew, and several fruit fly pests of fruits and vegetables.
However, biological control of indigenous pests is much more challenging than the control of exotic pests and tends to be insufficient alone. It often requires integration with one or more approaches, such as host plant resistance and cultural controls, among others. A prime example of this challenge is the control of the whitefly Bemisia tabaci, the vector of the viruses that cause the devastating cassava mosaic and cassava brown streak virus diseases in cassava fields in Africa (J. Legg, this issue). B. tabaci can be controlled with inundative releases of parasitoids and predators in protected and high-value agricultural systems. However, biological control alone has not been sufficient. Other complementary tactics, particularly host plant resistance, are necessary if we are to rein in this pest.
IITA in partnership with the Centre de cooperation internationale en recherche agronomique pour le developpement and Africa Rice has recently demonstrated the usefulness of the weaver ant in fighting off pests of mango and other tree crop pests. A sister species has been used in pest control by the Chinese for thousands of years. The article by Vayssieres et al. (this issue) shows that this native predator works principally by repelling the damaging fruit flies. The effectiveness of this ally has been demonstrated. Now its full potential is being exploited through a campaign to inform farmers how to manage this ant to promote its abundance and provide an additional weapon in the sustainable control of fruit fly pests.
Nearly parallel to the development of biological control with parasites and predators in Africa is IITA and its partners’ pioneering work on the development of microbial agents for the control of a variety of pests and pathogens.
The first pioneering success was the development of the biopesticide Green MuscleÂ®. Based on a naturally occurring fungus Metarhizium anisopliae var. acridum, this natural and now commercial product has proved to be the weapon of choice against a variety of devastating pests, such as the red and desert locusts.
The successes of biocontrol of the mealybug with parasitoids and green mite with predators opened the gate for numerous other programs that relied on arthropod biocontrol agents. The success of Green MuscleÂ® has also given substantial impetus to a variety of other efforts aimed at harnessing the power of naturally occurring microbes to fight harmful pests.
Microbial agents are being developed for pests that are very difficult to control, such as parasitic weeds. IITAâ€™s efforts to rein in the parasitic weed Striga hermonthica or witchweed is paying dividends. Striga attacks maize, sorghum, and millet, often the sole staple crops in many areas of the Sahel of West Africa. Biocontrol agentsâ€”host-specific strains of the common pathogen Fusarium oxysporum f. sp. strigaâ€”in combination with Striga-resistant/tolerant varieties are giving farmers in SSA hope for controlling a scourge that causes some $7 billion in annual crop losses.
Similarly, IITA and its partners have found strains of the fungus Beauveria bassiana that can serve as a full replacement to toxic pesticides used to control the diamondback moth, a pest that can cause complete loss of cabbage crops if unchecked. This pest has developed resistance to many pesticides and has forced farmers to rely on the excessive application of pesticides that are not even meant for use on food crops, such as cabbage. That treadmill can end with the new microbial agent.
More recently microbial control has taken another innovative and pioneering direction at several IITA locations in SSA. In a twist on the concept of using beneficial arthropods to fight off harmful arthropods, fungi that do not produce mycotoxins were identified and used to exclude toxogenic strains that contaminate stored grains. Similarly, endophytic fungi that grow harmlessly inside the banana plants have been used to impart resistance to pests such as weevils and nematodes that have, in some places, resulted in farmers abandoning banana and plantain production.
The list goes on. Last but not least, entomopathogenic nematodes are being explored for the control of an indigenous pestâ€”the African root and tuber scaleâ€”and an exotic pestâ€”the banana weevil.
For as long as there are problems and experienced scientists and staff to deal with emerging problems, IITA will continue to strive to be a leader in the field of biological pest control.
Key to successes
At the heart of these efforts and as the key to this success were fruitful partnerships and sustained donor support. The message: complementary capacityâ€”be it geographic, scientific, or politicalâ€”is essential for the development and implementation of successful technologies on a continental scale. (IITAâ€™s partners and donors can be found on our Web site at www.iita.org). IITAâ€™s partners are from all corners of the globe and run the spectrum of all research and development institutions. The core are CGIAR sister centers, universities, government (including NARS) and UN agencies, and (more recently) the private sector.
The sustainability of biological control has numerous requirements. Chief among them is donor and institutional commitment followed by the recruitment and retention of highly experienced scientists to lead the development of innovative pest control. All require long-term commitment, because the goal can take a long time to achieve but the payoff is often handsome.
While the mealybug and green mite programs brought biological control to Africa, training of African scientists has been the key to the successful dissemination of the capacity to do biological control. IITA has led this effort in Africa. The continuity and scope of such training will be necessary if we are to emulate past successes.
Of particular significance and another central element in the success has been the correct identification of pests and natural enemies; this is fundamental because it provides knowledge not only on identity but also about origin and distribution, biology, association with other organisms, evolutionary relationships, and other important topics. Without the proper naming and organization of organisms, biological control would be haphazard and chaotic, and can lead to failure and considerable waste in time and resources.
The scientific lessons learned from past and present efforts are numerous.
Anagyrus lopezi, the parasitoid responsible for the control of the cassava mealybug, is host specific and has an incredible capacity for dispersal (about 350 km per year) and for finding its host, which had been key to its ability to keep mealybug populations at low levels. The parasitoid originated from the LaPlata Valley (Paraguay) where conditions are not like those common in Africa. Nevertheless, A. lopezi proved to be highly adaptable. It quickly established and efficiently controlled the mealybug from the Sahel to the Congo Basin and to the East African Highlands. This example highlights some difficulties in predicting the outcome of biological control introductions that otherwise would eventually prove effective in controlling the target pests while relying on classical criteria for selecting natural enemy candidates.
A similar lesson was drawn from the green mite biocontrol program. Eleven species of predatory mites were introduced into Africa during the exploration phase of the program. T. aripo was the least efficient in terms of predation and efficiency in turning energy from consumed prey into the production of its own offspring. However, it turned out to be best at becoming established and dispersing (> 200 km per year), and in controlling green mite. The key was not its voracity but its ability to use the apical growing point of cassava as a shelter, to be so tuned to cues from its plant and prey to locate them, to use plant-based food for sustenance during periods of low prey abundance, and to have a phenomenal dispersal capacity. This is a landmark lesson: voracity and rate of population growth need not be always the top criteria in selecting effective natural enemies, as by these criteria, T. aripo would not have been selected for introduction into Africa.
Invasive weeds and arthropods continue to hamper development and ravage and disrupt agricultural and natural ecosystems, and doubtless many will continue to make it through porous borders. IITA has been a leader in developing options for controlling the invaders and restoring that balance. While classical biological control will likely continue to be the preferred way to deal with invaders, greater intensification of agricultural production, with its associated reliance on external inputs, necessitates an integrated approach. Biological control will be a component of a package that may include selective pesticides, varietal resistance, attractants and behavioral disruptants, as well as appropriate agronomic practices.
Hans Herrenâ€”former IITA scientist and recipient of the World Food Prizeâ€”in his foreword to the book on Biological control in IPM Systems in Africa (Neuenschwander et al. 2003, editors) wrote: â€œBiological control, however, cannot be a substitute for mismanaged plant production, in short, for bad farming. To have access to the full power and potential of biological control, the crop production system needs to be fully integrated in the larger agroecosystem, fulfilling the principles of agroecology. Under such a system, the powers of biological control can best be unleashed, and its synergistic effects with host plant tolerance/resistance, habitat management, and agronomic practices brought to bear maximum impact.â€
The old and timeless adageâ€”an ounce of prevention is worth a pound of cureâ€”captures the essence of the need and value of preventing the invasion of new species. Key to effective prevention is the development and maintenance of the capacity for quarantine, surveillance and readiness to develop appropriate options to deal with the invaders and limit their impact on African agriculture and the rest of the African ecosystems.
We also need more modeling for predicting the distribution and abundance of pests and their potential natural enemies to guide the development of experimental approaches to pest problems. Climate change can affect pests and their natural enemies in many ways. Modeling, along with long-term monitoring and demographic studies can be useful approaches to determining the effects of climate change on pests and their natural enemies.
Greater emphasis is needed on the use of the modern tools of molecular biology to trace the origin of invaders and use that knowledge to search for natural enemies where the invaders originated. Taxonomic services (and their associated collections) have been and will continue to be one of the essential tools in successful programs. In additionâ€”and among their many usesâ€”biodiversity collections can often serve as the first stop for foreign exploration against pests and weeds on other continents.
The targets will doubtless change. Highly trained and experienced staff will always be necessary to address the new targets. We also need to continue to strengthen existing links and develop new complementary partnerships. Training of local staff and African scientists is essential for them to become leaders in biological control in their countries, capable of running their own programs.
The younger generation is the one to find the solutions to future threats to the continentâ€™s food security and livelihoods of its inhabitants. Improvements in the health of ecosystems will go a long way in protecting and promoting biodiversity. Biological control, if carefully developed and implemented, is the greenest approach to saving farmlands, waterways, savannas, and forests from the ravages of pests.