22 TH Eo AtUeD*UFB ON 7B sO ae aa 
In recent years the development of insect strains resistant to certain 
pesticides has aggravated the problem. Yet, paradoxically, the recent 
discovery that certain vertebrates develop similar resistance may necessitate 
a reexamination of criticisms of pesticides and their effects on wildlife. 
Although a number of vertebrate groups are being investigated for 
possible insecticide resistance, the known cases are restricted to D.D.T. 
and aldrin resistance in two species of cricket frogs (Acris) 1, 2, 4, and 
resistance or tolerance to 9 chlorinated hydrocarbon compounds and methyl 
parathion (an organic phosphorous compound) in mosquito fish (Gambusia) 
3,5. Approximately 300 times as much as Strobane is required to produce 
equivalent mortalities in resistant populations of mosquito fish as com- 
pared to non-resistant ones. 
Vertebrate pesticides resistance research has made certain facts evident. 
(1) Resistant populations develop in heavily treated agricultural areas. 
Cotton producing regions of the South are ideal. (2) Resistance is a genetic, 
and therefore, heritable result of natural selection from periodic exposure 
to lethal quantities of pesticides. The development of resistant strains 
through artificial selection has been demonstrated in the laboratory. (3) 
Less than 20 years are required to produce a resistant population under 
field conditions. (4) Resistance to one pesticide may confer protection 
against related chemical compounds (cross-resistance). 
The ultimate value of vertebrate resistance is still a matter for specula- 
tion. Levels of resistance observed in mosquito fish appear sufficient to 
protect them from normal contamination in the field. If an effort were 
made to select pesticides to which insect pests were highly susceptible and 
vertebrates were least susceptible, the margin of safety might be increased. 
The most probable mechanism of resistance depends upon chemical 
degradation of the toxicant to a non-toxic or less toxic metabolite. Since 
the capacity for resistance is genetic, and since it may involve complicated 
physiological adaptations, it is quite likely that some animals will be in- 
capable of developing resistance. This is more apt to be true of species 
that have restricted distribution and low populations. Species with wide 
distribution over a variety of habitats are most likely to develop resistance. 
Hence pesticide use must be accompanied by good judgment and discrimi- 
nation. Also, since resistance is the result of selective. mortality, the 
possibility offers little solace to man. On the contrary, we must be content 
with the advantages bestowed upon the lower vertebrates and invertebrates. 
However, these advantages, accompaniied by self-protection against agri- 
cultural chemicals, should ultimately grant us the time required to devise 
solutions to this difficult problem. 
Department of Zoology, Mississippi State University, State College, Mississippi 
717 North Elm St., Sandwich, I!linois 
LITERATURE CITED 
1. VINSON, S. B., C. E. BOYD, & D. E. FERGUSON, 1963. Aldrin Texicity and Possible 
Cross-Resistance in Cricket Frogs. Herpetologica 19 (2): 77-80, fig. 1. 
2. BOYD, C. E., S. B. VINSON, & D. E. FERGUSON. 1963. Possible D.D.T. Resistance in 
Two Species of Frogs. Copeia (2): 426-429, tig. 1, tab> * 1-2: 
3. FERGUSON, D. E. 1963. Mississippi Delta Wildlife Developing Resistance to Pesticides. 
Agricultural Chemicals, Sept. 3 pp. 
4. VINSON, S. B., C. E. BOYD, & D. E. FERGUSON. 1963. Resistance to D.D.T. in the Mos- 
quito Fish, Gambusia affinis. Science, 139 (3551): 217-218, VANS Fe 4s 
5. BOYD, C. E. & D. E. FERGUSON. 1963. Apparent Resistance to Methyl Parathion in the 
Mosquito Fish. Nature (In Press). 
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