• Lyons, D.B.
  Chemical Control Research Institute
  
• Buckner, C.H.
  Chemical Control Research Institute
  
• McLeod, B.B.
  Chemical Control Research Institute
  
• Sundaram, K.M.S.
  Chemical Control Research Institute
  

The toxicity of fenitrothion, Matacil, and Orthene to Rana clamitans larvae was investigated in the laboratory. The 24-h LC50 values obtained were 9.9, 247, 6433 ppm, respectively, for these three chemicals in static bioassays with approximately 2.9 l of test solution per gram of tadpole tissue; this ratio was satisfactory for static bioassays. Mortality of larger control animals and biphasic mortality of larger Matacil-treated test animals, however, suggested that 1.7 l/g was not a sufficient volume of test medium.

In both a laboratory aquarium and an aerially sprayed pond, Orthene, because of its high water solubility, was  not readily absorbed and subsequently absorbed in tadpole tissue. The result was that residual levels of Orthene in tadpole tissue seldom differed from ambient levels present in the water medium. Conversely fenitrothion is relatively lipophilic and was readily taken up by the larvae. The solubilities of these insecticides may thus affect their relative toxicity to aquatic organisms.

While fenitrothion rapidly disappeared from natural waters, at a rate approximating a negative exponential, Orthene was relatively persistent. Of the insecticides emitted from the aircraft, Orthene reached the temporary pond in a greater proportional deposit than did fenitrothion (21% and 15%, respectively). The Orthene concentration in the pond water suggested an even higher deposit than recorded from spray deposit sampling pans.

The behavioral repertoire of Rana clamitans tadpoles in the laboratory is discussed. The chemicals affected activity, swimming behavior, bubble-making, resting position, and color in these larvae. The possible effects of these chemicals on tadpole predators are discussed with special reference to predatory insects and Mallards.

Analysis of residual levels in pond water suggested that concentrations detrimental to anuran larvae would never be encountered under normal operational applications of these chemicals.