Bionomics of the carrot fly Psila rosae F.; the infestation and sampling of carrot crops.

The headlands of a carrot field are generally more heavily attacked by carrot fly larvae than the remainder of the field. The methods of sampling a carrot crop are outlined: it has been found that, for fields up to 10 acres, samples of carrots taken in alternate twos and threes in the midfield, and in twos on the headlands, give a reliable measure of the infestation. For larger fields the number of carrots is doubled. Transects of headlands and whole fields have been made, and are useful in showing the actual amount of damage in a crop. Thus it is possible to represent each field as a three-dimensional body showing the amount and distribution of the attack. The deterioration of attacked carrot crops during autumn and winter has been followed. Deterioration is measured by (1) percentage carrots attacked, (2) number of mines per 100 carrots and (3) the percentage of carrots unsaleable or unfit for market. The relations between (1) mines per 100 carrots and time, (2) percentage attack and time, (3) mines per 100 carrots and percentage attack, and (4) mines per 100′carrots and percentage unsaleability are stated. Based on the above relationships, methods of prediction of deterioration have been worked out. The importance of shelter in determining the degree and position of attack is discussed and the relative effects of different types of shelter on infestation are compared.