It is generally agreed that inoculations by individuals of a group of vectors are separate and independent events, and that virus transmission efficiency by groups of insects will follow that predicted by probabilities generated by expansion of the binomial theorem (Watson, 1936); (Storey, 1938); (Watson and Roberts, 1939); (Posnette and Robertson, 1950); (Storey and Ryland, 1955); (Sylvester, 1955, 1956). However, (Kirkpatrick and Ross (1952)) reported that in the transmission of potato leafroll virus by Myzus persicae (Sulz.) to Physalis angulata the rate of successful inoculation by groups was lower than expected on the basis of this hypothesis, and that the efficiency of individuals in a group decreased as the size of the group was increased. They considered that this could be due to one or more of the following: (1) the presence of immune or highly resistant plants in the test plant population which would not become systemically infected in spite of the presence of relatively large numbers of vectors per plant and relatively long inoculation access periods; (2) interference of aphids with one another which could prevent feeding by any one of them for a period long enough for transmission of this virus; and (3) aphids feeding for relatively long periods tended to cause the test plants to become more resistant to systemic infection by this virus. A fourth hypothesis (4) might be added, viz., that such observations are due to chance variation. In the following work, tests were designed in an attempt to specifically test each of these four hypotheses.

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