Differential Reactions of Two Biotypes of the Corn Leaf Aphid to Resistant and Susceptible Varieties, Hybrids and Selections of Sorghums1

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Abstract
The discovery of 2 biological races (KS 1 and KS 2) in the corn leaf aphid, Rhopalosiphum maidis occurred during the present studies on the resistance of sorghums to this insect. The range of distribution of these races in nature is not yet known. Their biology and their behavior were analyzed quantitatively during preference and antibiosis tests in the presence of, or on susceptible, intermediate, and resistant sorghums. Biotype KS 2 was able to reproduce itself on a resistant host while the other only occasionally, barely subsisted. Biotype KS 2 always produced heavier adults than biotype KS 1. The resistance of sorghums was analyzed by measuring separately the mechanisms of preference and antibiosis. Tolerance may be an important factor but so far has been difficult to study. In 50 varieties, hybrids and selections of sorghums, involving 5000 plants, preference was a part of the inherited qualities which remained constant under varying intensities of infestation. Non-preference, as observed in the F1 hybrid of a cross between White Martin (preferred) and Piper 428-1 (non-preferred) appeared to be transmitted as a dominant character. The F2 generation was intermediate between the parents. There was in White Martin (preferred) a factor which, within a test period of 1 hour, stimulated the females to give birth to nymphs. Conversely, there was in Piper 428-1 (non-preferred) a factor which, within a test period of 1 hour, inhibited almost totally the stimulus to give birth to nymphs. The expression of antibiosis in reducing the fecundity of biotype KS 1 behaved as a completely dominant factor in Piper 428-1, partially dominant in the F1, and recessive in White Martin; in reducing the weight of the adults, the character behaved as dominant in Piper 428-1, partially dominant in the F1, and recessive in White Martin. The expression of antibiosis in reducing the fecundity of biotype KS 2 appeared to be partially dominant in Piper 428-1 and recessive in the F1 and White Martin; while in reducing the weight of the adults, the expression appeared to be partially dominant in Piper 428-1 and the F1 and recessive in White Martin. The antibiosis of Piper 428-1 affected biotype KS 1 by reducing the weight of the females as soon as they fed on that host and preventing the maturing of their progeny, if any. Biotype KS 2 was affected to a lesser degree. In the progeny of Piper 428-1, the near immunity against biotype KS 1 has been lost probably through segregation, and both biotypes were equally affected. The antibiosis of the original selection must have been the result of a very particular genic composition. As observed from the progenies of single viviparous females caged on F2 plants, the resistant or the susceptible genotypes reduced or increased the fecundity of the females and the weight of the adults in the progenies as the original parents did. Aphids reared separately on the heads o''f White Martin were heavier and their rate of reproduction was doubled and tripled compared to others reared on lower leaves of the same plants. Antibiosis totally or partially inhibited the mechanisms of wing production in the progenies of apterous females. Six mechanisms of either preference or antibiosis were found responsible for resistance. These mechanisms appeared to be either dominant or partially dominant in their expression because the biotypes showed differential reactions to the same host. The non-discrete segregation in the F2 plants and the F3 lines suggests a multiple factor hypothesis.

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