Impact of bacillus thuringiensis var. israelensis On larvae of chironomus thummi thummi and psectrocladius psilopterus (Diptera: chironomidae).

Chironomid larvae, especially species of the subfamily Chironominae, are known to be sensitive to the mosquitocidal bacterium Bacillus thuringiensis var. israelensis (B.t.i.). In this study, bioassays and electron microscopic investigations were carried out with third- and fourth-instar larvae of Chironomus thummi thummi Kieffer (subfamily Chironominae) and Psectrocladius psilopterus Kieffer (subfamily Orthocladiinae) in order to study the sensitivity of species belonging to different chironomid subfamilies. Both species showed susceptibility to increased B.t.i. concentrations, with LC50 values (24 h) ranging from about 40- to 60-fold the LC50 for Aedes aegypti (LC50 (24 h) is 0.77 mg/L for C. thummi thummi and 1.17 mg/L for P. psilopterus). C. thummi thummi was shown to be twice as sensitive as P. psilopterus. Ultrastructural investigations of the anterior midgut showed cellular alterations in larvae exposed to a high B.t.i. concentration (2.8 mg/L, about 50-fold the LC50 for A. aegypti), such as swelling of mitochondria, dilatation of intercellular spaces and basal labyrinth, fenestration or disorganization of the Golgi complex, concentric arrangement of rough endoplasmic reticulum, and an increase of lysosomes and myelin figures. Electron-lucent regions within the cell, cell protrusion, and, in some cases, swelling or lysis of cells were further effects observed in treated animals. Most effects were found in both species, though they seemed to be more severe in C. thummi thummi. The alterations coincide with those known from target organisms (Culicidae, Simuliidae). This study shows that there is a difference in sensitivity to B.t.i. between chironomid species from different subfamilies and that the susceptibility of chironomid larvae to the bacterial toxins is due to damage of the midgut epithelium as it is in target organisms. Copyright 1999 Academic Press.