ON THE REARING OF TRICHOGRAMMA EVANESCENS WESTW. AND ITS UTILIZATION FOR THE CONTROL OF SUGAR CANE BORERS

PU CHIH-IUNG & TANG TE-HAI South-China Agricultural College South-China Agricultural Research Institute

Sugar cane borers infest the cane and cause a considerable injury to the plant in China. Four species of sugar-cane borers are commonly found in Kwangtung Province, viz. Diatraea venosata Wk., Chilo infuscatellus Snellen, Eucosma schistaceana Snellen and Sesamia inferens Wk.Trichogramma evanescens Westw. occurs in nature as an egg parasite of the sugar-cane borers.The present experiment on breeding Trichogramma centres around the ways of increasing its viability.The quality and the quantity of the nutrient for the larvae and adults of Trichogramma play an important role in relation to their viability. The contents of the egg of Sitotroga cerealella Oliv. which has been commonly used as a host of the Trichogramma are of poor nutrient quality and quantity for the larvae of this parasite. The eggs of Angoumois grain moth are so small that only one adult Trichogramma emerges from each egg. Such adults are usually smaller in size, less active and with lower reproductive potentiality as compared with those that emerge from the field hosts; besides, the female sex ratio gradually decreases in the successive generations.From the results of an experiment on Trichogramma breeding, it is found that the eggs of Attacus cynthia ricini Boisd. and of Dendrolimus sp. are the most suitable hosts for the propagation of Trichogramma evanescens. The adults of the parasites emerging from such eggs are generally large in size, very active and possess high reproductive potentiality; besides, the female sex ratio remains unchanged in the successive generations. Evidently, the quality and the quantity of the egg contents of these two species of insects fulfil the nutritional requirements for the development of the Trichogramma larvae and they are recommended as hosts for Trichogramma propagationThe average number of adult Trichogramma emerging from a single egg of Dendrolimus sp. and that of Attacus cynthia ricini is 27.3 and 28 respectively with the respective maximum reaching 52 and 59 in the present record. If too many adults emerge from a single egg, both their size and reproductive potentiality would decrease and, moreover, the number of the male individuals would increase and the adult life would shorten. Undoubtedly, all these features are unfavourable for the utilization of the parasites for control of the insect pest as far as the effectiveness of the parasites is concerned. From the standpoint of increasing viability of the parasite, regulation of the number of parasites in the egg during the procedure of propagation of Trichogramma is necessary. Regulation of the number of parasites in the egg means regulation of the quantity of the nutrient for the parasites. The method of checking the occurrence of superparasitism is to regulate the ratio of the number of female Trichogramma and host eggs to 1:1 through the procedure of propagation; the period of oviposition should not exceed one day.The quality of nutrient for adult Trichogramma greatly affects its longivity and reproductive potentiality. The result of a nutritional experiment shows that honey is the most suitable nutrient for it. The adults fed with honey have the longivity increased 8.6 times and the number of the offspring 14.7 times as compared with those fed with distilled water.Cold storage effectively reduces the speed of development of the immature stage of Trichogramma. The mature larvae about to pupate within the host eggs maintain their life for 57 days under 4—7℃ and all of them will come out as adults at room tempeature. The fresh eggs of Dendrolimus sp. and of Attacus cynthia ricini kept in cold storage of 0— 4℃ for 97 days and 61 days respectively are still usable for rearing Trichogramma.In order to increase the adaptability of Trichogramma to the environment of the sugarcane field, the rearing procedure is suggested to proceed in the field or in an indoor environment with fluctuating temperature and humidity. The continuous artificial rearing should not exceed five generations before the liberation of the parasites.The distance of dispersion of Trichogramma eva