Biological control of rice stem borers: a feasibility study.
AbstractThis is a discussion, based on a review of literature and visits to several Asian countries, of the feasibility of initiating programmes of biological control against stem borers of rice, especially in the Philippines. The principal rice borers in Asia are Chito suppressalis (Wlk.), Tryporyza incertulas[Scirpophaga incertulas] (Wlk.), Sesamia inferens (Wlk.), T. innotata (Wlk.) and C. polychrysa (Meyr.). The first three are found throughout Asia, including the Philippines [cf. R.A.E., A 53 638], and the first two are much the most important; T. innotata is the most injurious species in Indonesia and is also reported from India, Pakistan and the Philippine island of Mindanao, and C. polychrysa is a major pest in Malaya. The two species of Tryporyza are generally considered specific to rice, and C. suppressalis is practically confined to it but has also been recorded from grasses, maize and millet; S. inferens attacks many graminaceous crops. In the Americas, the main rice borer is Diatraea saccharalis (R), and in Africa the principal species are Maliarpha separatella Rag., (L 2700) 9/66 [A] 33 Parerupa africana (Auriv.), Adelpherupa sp., S. calamistis Hmps., Saluria sp., Scirpophaga sp., C. partettus (Swinh.) and Sesamia nonagrioides botanephaga Tams & Bowden.
The author reviews successful and unsuccessful introductions of insect enemies of pests in various countries (appending lists of cases of complete or partial control), draws from it certain general principles for effective biological control and applies them to conditions in the Philippines. Favourable factors are the overlapping generations of the borers and the tropical island nature of the Philippines, where areas of continuous rice culture exist and many fields are not treated with chemicals; unfavourable ones comprise the seasonal extremes of rainfall in some regions, the increasing use of insecticides, the economic damage often caused by even such small numbers of borers as would be required for maintaining an introduced natural-enemy population, and the long establishment of the borers in the Philippines, so that their country of origin is unknown.
Artificial means of biological control, such as sterile males, light-traps and chemical attractants, and natural control agents, such as pathogens (viruses, bacteria, fungi, Protozoa and nematodes) and entomophagpus insects, are enumerated and discussed. Since the borers remain within the rice stems throughout larval life, pathogens requiring ingestión are unsuitable. Several species of fungi causing disease in borers of rice and other graminaceous crops are known (and a list of them is given), but, although the warm damp conditions in rice-fields are theoretically favourable to their development, no species giving effective control has been found. Very little is known about predators. Parasitic insects therefore appear the best choice, but it is suggested that exotic species are more likely to give lasting results than parasites already found in the control area; in some other Asian countries, the augmentation of natural populations of Trichogramma spp. had only a temporary effect, since the existing parasite numbers were already the highest that could be maintained under conditions obtaining. The native para-sites observed and the exotic ones introduced on rice and other graminaceous crops, mostly in Asia and Africa, are reviewed, and lists of these species (with the host stage attacked) and a summary of the world distribution of rice borers and their native parasites, are appended. T. australicum Gir., T. japonicum Ashm. and T. minutum Ril. have been introduced into the Philippines from Formosa, Japan and the United States, respectively, against various pests, and the last two species have been recovered from the eggs of C. suppressalis and the second from those of Tryporyza incertulas. Lixophaga diatraeae (Tns.) has also been intro-duced and liberated. Some species thought to be primary parasites of rice borers have proved to be hyperparasites, and a list of these is appended. The possibility of integrated control in the Philippines is discussed.
Finally, the author makes recommendations as to the procedure to be followed and the facilities required before, during and after the execution of biological control programmes. Especially important are thorough field surveys of the borers and their natural control factors, ecological studies of the proposed parasite in its native land, and careful evaluation of results after release. In selecting a parasite for introduction, it is necessary to consider the crop, climate and country in which it originated, the host stage attacked, and the number of species to be released simultaneously; against rice borers in the Philippines, larval parasites, introduced a few species at a time from one Philippine island to another or from tropical Africa, appear to have the best chance of success.