In 1982 an exotic mealybug of the genus Rastrococcus was first observed on mango trees in Ghana. In 1984 it was found only in a narrow coastal strip in Togo, but by early 1987 it had spread 500km north from the coast. The mealybug has become an increasingly serious pest of mango and citrus in Ghana, Togo, Benin, Nigeria, the Congo, and Côte d’Ivoire. In Benin the pest is so serious that some farmers destroyed infected trees to control the infestation.
The insect sucks sap from leaves which weakens the trees, and where the Rastrococcus occurs in large numbers, honeydew deposits encourage the growth of a sooty mold on the foliage which further reduces fruit production. Infestations of Rastrococcus present a serious threat to subsistence farmers in areas where mango and citrus are often the most available source of energy and vitamins in the diet. The threat of the Rastrococcus is felt as well by countries such as Ghana that wish to increase the export of fruit crops.
Chemical control was not an appropriate solution for this pest outbreak due to the wide range of host plants (at least 44 plants species belonging to 22 families) on which the Rastrococcus was found, the large areas affected.
In 1986 FAO’s Technical Cooperation Program contracted the Commonwealth Agricultural Bureaux International Institute of Biological Control (known as CIBC) to develop a biological control program for the mealybug in Togo. The researchers first determined the species of the pest was Rastrococcus invadens. Surveys for this pest’s natural enemies were conducted in India and Malaysia and the parasitoids were bred under quarantine in the U.K.. Colonies of R. invadens were maintained in the U.K. on potted mango seedlings in a greenhouse and on pumpkins in the laboratory. The parasitoids were bred on mealybugs maintained on pumpkins. The parasitoid which was selected for release in Togo was obtained from R. invadens in India, and was identified as a new species, Gyranusoidea tebygi.
G. tebygi, a wasp, is a member of the Encyrtidae family, known for its species which have been used as bio-control agents against other mealybug species. About 1 mm in length and yellow/brown in color, G. tebygi develops inside the mealybug. In 13 to 16 days the mealybug mummifies, and seven days later the adult G. tebygi emerge. The adults generally mate after emerging. Populations of G. tebygi have a marked female bias—about three females for every male. The females begin oviposition 48 hours after emergence, and during the 20 days which they live, can parasitize between 70 and 90 mealybugs. G. tebygi has a much shorter development time than R. invadens, and so can complete almost two generations for each one of the mealybugs.
Laboratory experiments revealed that G. tebygi are easily reared in R. invadens cultures on a range of plants. Other experiments showed that the parasitoid is specific to R. invadens.
In October of 1987, 250 G. tebygi were carried to Togo and a rearing system was established by personnel of the Plant Protection Service in Togo, with assistance from GTZ. The researchers used simple, low-cost technology for the rearing facilities. Two storage buildings were renovated to make them insect resistant and host plants were grown and infested with R. invadens in one of the buildings. The infested plants were transferred to the other building and exposed to G. tebygi in cages made from a wooden frame and cloth mesh sides. The G. tebygi produced from the R. invadens were collected with an aspirator and stored in glass vials closed with cloth treated with a smear of dilute honey. These vials were transported in a cool box to the release site.
G. tebygi were first released in Togo near Lomé in November of 1987. By May 1988 parasitism was recorded up to 15 km from the release sites and the mealybug was effectively controlled in the immediate area of the release. Parasitoids were also released and established in four other experimental sites which covered the main ecological zones of Togo. Initial results have been encouraging. Observers noted marked differences between mealybug numbers on plants inside and outside the area of influence of G. tebygi.
The CIBC supplied the International Institute of Tropical Agriculture (IITA) with G. tebygi early in 1988 for rearing and release in Ghana and Benin. Future collaborative work is planned with IITA for a regional program to cover all affected countries, and a UN Development Program-funded regional program for training in biological control is in progress as well.
It is possible that G. tebygi may not be effective in some environmental and ecological conditions. Further searches for additional parasitoids and predators are being conducted under the UNDP regional program, and CIBC is studying more natural agents in the event that the activity of G. tebygi may need supplementing under some conditions. While initial results suggest that these problems are unlikely to reduce control greatly, the spread of the mealybug needs to be assessed further to determine whether biological control is completely effective, and alternatives must be available if required. Unlike conventional chemical controls, the philosophy of bio-control is that no single agent will provide the solution to pest control problems. Ultimately the solution rests in a dynamic diversity of elements and techniques. v
For more information contact:
Dr. David Moore
CAB International Institute of Biological Control
Silwood Park, Buckhurst Road
Ascot, Berks SL5 7PY U.K.
Telex: 265871 MONREF G
Dr. David Moore