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 kn own to be sensitive to the mosquitocidal bacterium Bacillus thuringiensis v ar. israelensis (B.t.i.). In this study, bioassays and electron microscopic investigations were carried out with third- and fourth-instar larvae of Ch ironomus thummi thummi Kieffer (subfamily Chironominae) and Psectrocladius psilopterus Kieffer (subfamily Orthocladiinae) in order to study the sensit ivity of species belonging to different chironomid subfamilies. Both specie s showed susceptibility to increased B.t.i. concentrations, with LC50 value s (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. Ul trastructural investigations of the anterior midgut showed cellular alterat ions in larvae exposed to a high B.t.i. concentration (2.8 mg/L, about 50-f old the LC50 for A aegypti), such as swelling of mitochondria, dilatation o f intercellular spaces and basal labyrinth, fenestration or disorganization of the Golgi complex, concentric arrangement of rough endoplasmic reticulu m, 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 th ummi. The alterations coincide with those known from target organisms (Culi cidae, Simuliidae). This study shows that there is a difference in sensitiv ity to B.t.i. between chironomid species from different subfamilies and tha t the susceptibility of chironomid larvae to the bacterial toxins is due to damage of the midgut epithelium as it is in target organisms. (C) 1999 Aca demic Press.