The Control of the Carrot Fly ( Psila rosae , Fab.) (Diptera) with DDT.

(1) The proprietary emulsion “Guesapon ”, containing 5 per cent. DDT, gave an unsatisfactory control of carrot fly damage on a plot scale. (2) Treatment of the carrot foliage with this emulsion was more effective than treatment of the soil. (3) A I per cent. emulsion of DDT applied to carrot foliage in the field gave 100 per cent. mortality of carrot flies in the laboratory for 26 days after treatment and showed marked toxicity after a further 38 days. (4) The DDT spray deposits were very, resistant to treatment with water and remained active after death of the foliage. They were also very toxic and lasting on potato foliage. (5) Deposits given by ½ per cent. and I per cent. DDT emulsion were similar in toxicity and persistence. The ½ per cent. is, however, slower in action. (6) From a treated surface the carrot fly is able to acquire a lethal dose of DDT in a very short time, probably by absorption through the feet. (7) Emulsions containing ½ per cent. and I per cent. DDT applied to carrot foliage under cages in the field both gave 95 per cent. kill of carrot flies for a month after application. The treatments also gave a high kill of other insects. (8) Soil samples showed that the larval population and the damage was much lower in the treated than in untreated cages. (9) In a field trial 2⅓ acres of carrots were treated with ½ per cent. DDT emulsion at about 50 gals. an acre. (10) Sweeping showed a rapid decline in abundance of flies after treatment and high mortalities were observed. These features did not occur on neighbouring untreated carrots. (11) These effects lasted for some 21 days after treatment and a marked reduction of larval population and damage resulted.