Summary
The synthetic insecticides have been used in steadily increasing quantities since the Second World War and are non‐specific poisons, and so pose the possibility of sublethal effects on wild populations of animals.
The organochlorine insecticides in general, and some of the cyclodienes in particular, are usually much more persistent than the organophosphorus and carbamate insecticides, both in the environment and within individual insects.
Analyses of samples suggest that traces of the organochlorines are widely dispersed over the entire globe.
Although the evidence is incomplete, all of these synthetic insecticides appear to exert their primary toxic lesion by interference with the conduction of impulses in the nervous system.
The complete pattern of events that ends in death includes the release of secondary toxins into the blood. These are probably physiologically active compounds, normally present, but liberated in excessive amounts by excessive nervous stimulation.
A condition similar to the late, paralytic, stage of insecticidal poisoning can also be produced, in Periplaneta americana, by excessive physical stimulation and there are similarities in the physiological changes caused by both types of stimuli. This suggests that a generalized stress syndrome develops after excessive nervous stimulation. However, this similarity has so far only been demonstrated for this one insect species.
Sublethal effects have so far only been looked for cursorily. Interpretation of sublethal effects is sometimes difficult; apparent differences between dosed and control individuals may sometimes be caused by selective mortality of the dosed individuals. Four main types of effect have been found.
Latent toxicity, which is evident as a reduction of longevity or of death at particular developmental stages, has been found only with organochlorines, appears to be linked with fat metabolism, and shows clearly that sublethal effects may be produced a long time after acquiring a dose of a persistent insecticide.
All types of synthetic insecticide can affect reproductive potential, and may increase or decrease the number of eggs produced. Egg fertility or development of offspring may also be affected. Reductions are sometimes caused directly by inhibition or distortion of ovary development, sometimes indirectly by reduced feeding.
The few studies on behaviour have shown that activity may increase, feeding decrease, and the acceptance threshold of sucrose be lowered.
DDT induces the formation of more detoxifying microsomal enzyme in Triatoma infestans.
I suggest that latent toxicity and some behavioural changes are probably caused by direct effects on the nervous system, but that some of the other results may be secondary effects associated with the stress syndrome. There is also the possibility of direct insecticidal action on other systems.
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