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The larvae feed on the roots of young and mature plants.
In carrots the young plants are attacked on the tap roots and may die, leaving gaps in the crop. Larger carrot plants are attacked on the base of the tap root and lower down, showing irregular brown channels under the surface, from which the creamy-white larva (maggot) can be extracted. Where damage to plants is severe, the leaves become reddish in colour and the plant may die, particularly in dry conditions.
In parsnips the damage is similar to that on carrots, but usually it is confined to the top 15 cm of the root.
In celery the larvae bore into the roots, crown and petioles, resulting in yellowing of the leaves and reduction in growth or death of young plants.
In parsley the larvae live in the surface of the tap root and in the lateral roots.
P. rosae causes economic damage over most of its range and some form of control is needed. Chemical control is probably still the most commonly used method. There are methods of forecasting the possible size of outbreaks using the accumulated day-degrees method.
Where crops of carrots, parsnips, celery or parsley are grown in short rotation close to earlier crops, population levels of the pest may become high. New plantings should be grown away from earlier crops. In Europe film covers over seedings have reduced the infestation by larvae.
In Sweden Ramert (1996) showed that intercropping with Medicago littoralis reduced the level of infestation with carrot fly but reduced the crop yield; onion has a similar effect.
Badly infested crops should be disposed of so that the larvae cannot complete their development. Adult flies require shelter and reduction of tall vegetation around fields will minimise the risk of infestation.
There are local methods of varying the time of planting of the crop to avoid coinciding with the emergence of the adult fly. These methods of control vary with different localities and the most similar climatic area should be used as the model.
Ellis et al. (1993) showed that the Libyan Daucus capillifolius had greater resistance to P. rosae than commercial carrot varieties and that crosses showed some increase in resistance. There is also variation in the natural resistance of carrot varieties. Cole (1985) showed that the resistance of different carrot varieties was related to the increased concentration of chlorogenic acid in the peel of more resistant varieties.
Attempts to use parasites to control carrot fly have not been highly successful. Maybee (1954) considered the introduction of Chorebus gracilis and Basalys tritoma into Canada but they failed to become established.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
On carrot P. rosae can be a severe pest, killing many seedlings early in the year or making the final crop unsaleable due to the level of larval mines, secondary rots and the uneven size of the roots. The first generation of larvae feed on the apices of the tap roots and may cause reddening of the foliage. It is the second generation which causes the most important damage to carrots. The surface mines render the carrots unsaleable, causing a reduction in quality rather than quantity.
Coppock et al. (1975) recorded that in England 60% of untreated carrots may be damaged if not harvested by early January. Toms (1972) estimated that an average attack resulted in 30% of carrots being rendered unsaleable.
On celery the second generation of the fly can also cause major losses by damaging the outer stalks and the base, and in some areas all crops are protected against infestation.