The variations in susceptibility of different varieties of peas to attack by the Pea Moth was investigated and an attempt made to determine and measure the factors concerned. Six varieties of peas differing widely in haulm length and earliness of maturation were used in each of two trials. In the first trial (sown 29th March) the early maturing varieties came into flower before the moths were recorded on the crops and suffered the lowest attacks. The later varieties were exposed to attack over a much longer period and suffered the heaviest infestations. In the second trial (sown 3rd May) the attack was more uniform over all varieties with the early varieties more heavily affected than in the first trial; they were exposed to attack from the beginning of flowering until harvesting.

An estimate of the changes in the active moth population during the flight period was obtained and the varieties were compared in relation to the proportion of this population to which each had been exposed. There was a strong positive correlation between the degree of exposure and the incidence of attack on the different varieties.

The infestation of the varieties was also found to be influenced by the amount of cover which each provided; those with the most dense cover suffered the heaviest attacks.

Statistical analyses showed that the two factors, exposure and plant cover, were closely associated and exerted a joint influence on subsequent attack.

Data from other trials corroborated these findings and showed that strains of peas bred to mature early suffered substantially lower pea moth attack than did the later maturing types from which these had been bred.