The susceptibility of the third, fourth, and fifth instars of the Egyptian cotton leafworm, Prodentia litura F., to 4 insecticides was determined. Acetone solutions of the insecticides were opically applied to the larvae using glass micropipets. The most suitable weight for testing within each instar was determined so as to achieve consistent results. As the larvae increased in age and weight, their susceptibility to insecticides decreased. However, the susceptibility of the unit body weight remained about constant. Observations in the field have always shown that timing of insecticide application is one of the important factors that affect the outcome of spray programs. Much research was conducted to find out the reason for this phenomenon.
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Eldefrawi et al. (1964) studied this question.