Introduced fungus, Lagenidium giganteum, for controlling populations of mosquito larvae; laboratory investigations

Abstract

Lagenidium giganteum, ATCC 36942, was tested for its potential for infection of mosquito larvae in the laboratory. To studies on fungal propagation were done by culturing the fungus in Aedes albopictus cell lines (C6/36). It was found that the number of mosquito cells were reduced and appeared degenerate. No zoospores were found. After studying the target organs of larvae by histopathology, it was found that the fungus spread over both the inside and outside of the body of the infected larva. Many zoospores and oospores occurred in the head, thorax and digestive tract. Lagendium giganteum was tested against Anopheles dirus, Aedes aegyoti, and Culex quinquefasciatus, vectors of human disease in Thailand. It seemed that L. giganteum plays a more important role in the infection of culicine mosquitoes than of anophelines. When the fungus was tested for infection of An. dirus in various depths of water, i.e., 1.8 cm., 5.4 cm., and 9.0 cm., the results showed that the infection rate at the 1.8 cm. depth was better than at higher depths, but was still less than the infection rate obtained in culicines. Five kinds of media were tested for fungal infection, i.e., PYG, HS, WGYG, Z, and SFE. The best medium was Sunflower seed extract (SFE). L. giganteum cultured on Sunflower seed extract produced 6.0 x 10⁶ zoospores/ml, at a fungal rate of 23.4 cm² area of SFE agar culture for 5 days. This fungal inoculum and Ae. Aegypti were used in most test. Mosquito larvae were infected most easily when 1 – 2 days old. The effectiveness of infectivity of L. giganteum varied considerably according to environmental conditions. The optimal conditions for infecting larvae by L. giganteum were; pH of water between 6.0-7.5 and a range of temperatures 20 – 28 ℃. High organic polluted water limited the infection rate of L. giganteum. When treating the larvae in different types of water, high tide water from Chao Praya river at Sathupradid, (pH of 6.82, BOD of 3.43 mg/l and COD of 19.920 mg/l) and rain water from a home reservoir (pH of 6.42, BOD of 2.403 mg/l, and COD of 12.943 mg/l) were good for mosquito control, with percentages of mortality of 84.0% and 60.5% respectively.