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ABSTRACT 
 
Toxicology of Pesticides on the Larvae of Two Tropical Amphibians, 
Physalaemus pustulosus and Bufo marinus. 
 
 
Nicole Joanna Sookoo 
 
The optimal rearing conditions in the laboratory for Physalaemus pustulosus and 
Bufo marinus were determined through investigation of several parameters of 
which the following where found to have a pronounced effect. These included 
amount of feed, absence and presence of soil, frequency of water change and 
crowding levels. Tadpoles were found to thrive best when fed as much as they 
could consume, with soil as a substrate, water changes every three days and with 
no more than ten (10) tadpoles per liter of water. This finding was further 
confirmed in the density experiments which revealed the mitigation of the 
crowding effect at densities of 20 – 30 tadpoles per liter of water when soil 
substrate was utilized. The finding of these two experiments were crucial to the 
toxicity experiments as they revealed the ideal conditions in which P. pustulosus 
tadpoles could be maintained and a suitable method for the laboratory rearing of 
P. pustulosus tadpoles for possible use in ecotoxicology studies.  
 
Pesticides can cause significant mortalities in non-target organisms such as 
amphibians. The toxicity  of three insecticides (Fenobucarb, Revelo 350CS, 
Evisect) and three herbicides (Karmex, Roundup and Gramoxone) were examined 
in P. pustulosus and B. marinus species in water alone (single phase) and with the 
presence of soil (dual phase). The Spearman-Karber 96-hr median lethal 
concentration [LC50] values were determined for each pesticide, with and without 
the presence of soil per species. The LC50 average values at 96 h for P. 
pustulosus in the single phase were Fenobucarb 2.1 mg L-1, Revelo 350CS 563.0 
mg L-1, Evisect 3.3 mg L-1, Karmex 0.3 mg L-1, Roundup 7.9 mg L-1, and 
Gramoxone 1.0 mg L-1 and in the dual phase were Fenobucarb 3.0 mg L-1, Revelo 
350CS 237.2 mg L-1, Evisect 0.8 mg L-1, Karmex 0.3 mg L-1, Roundup 4.6 mg L-
1, and Gramoxone 1.1 mg L-1. The LC50 average values at 96 h for B. marinus in 
the single phase were Fenobucarb 29.5 mg L-1, Revelo 350CS 187 mg L-1, Evisect 
0.8 mg L-1, Karmex 1.4 mg L-1, Roundup 9.6 mg L-1, and Gramoxone 278.8  mg 
L-1 and in the dual phase were Fenobucarb 26.5 mg L-1, Revelo 350CS 621.5 mg 
L-1, Evisect 2.4 mg L-1, Karmex 1.4 mg L-1, Roundup 60.1 mg L-1, and 
Gramoxone 461.7 mg L-1. 
 
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The results revealed upon comparison of the LC50 values that the feeding 
behavior of P. pustulosus (benthic feeder) significantly enhanced its sensitivity to 
the dual phase toxicity runs while the presence of soil decreased the overall 
toxicity to B. marinus (primarily a surface feeder). P. pustulosus was found to be 
the more sensitive to the pesticides even in the single phase when compared to B. 
marinus. This finding is important as it reveals a shortfall in the typical acute 
toxicity tests which do not include the use of natural substrate as a factor in the 
evaluation of toxicity.  
 
Keywords: Pesticides, Physalaemus pustulosus, Bufo marinus, Median Lethal 
Concentration [LC50].