Pest insects in carrot and Apiaceous crops: reducing the impact of carrot fly

You can use several methods to manage carrot fly infestations, such as insecticides, crop covers or fencing. 

Information about specific insecticides is not given here as authorisations change so quickly. 

The AHDB Pest Bulletin Carrot Fly Forecast should be used to decide what time to spray, depending on which part of the life cycle is affected by the spray. For instance, some sprays are effective only against adult carrot fly and ineffective against carrot fly larvae in the soil. In this case, research has shown that sprays applied one week before the forecasted 10% (first) egg laying gave better carrot fly control than when the first insecticide spray was triggered by the first capture of flies on sticky traps. 

Research has also shown that the length of time insecticide residues remain effective on carrot foliage depends on temperature. 

It is important to remember that once a spray regime is introduced, few, if any, flies should be caught on sticky traps. If they are caught, then this should raise an alert. If carrot flies are in the crop when it is sprayed with insecticides, they will be killed. Females have a diurnal pattern of activity, usually leaving the shelter of field boundaries to lay their eggs. To maximise the ‘knockdown’ effect, sprays should be applied between 4:00pm and 6:00pm on warm days, as this is when most female flies are in the crop. 

Partial levels of resistance to carrot fly are claimed for carrot varieties such as Flyaway and Resistafly. However, these varieties are not currently grown commercially for other reasons. The most resistant varieties support about 50% of the insects and have around 50% of the damage compared with a very susceptible variety. These resistance levels are too low to provide adequate carrot fly control on their own but can be an important part of an IPM strategy. 

When carrots are first grown on land away from areas where they have been grown before, carrot fly is unlikely to cause a problem until numbers have built up. Unlike many pest insects, adult carrot flies move relatively short distances, which means you can develop a management strategy based on the isolation of new crops from sources of carrot fly. Numbers can be kept low, particularly if early and late crops are well separated to break the sequence of host plants in the pest’s life cycle. 

Research at Wellesbourne showed that relatively few carrot flies moved more than 1 km from a previously infested crop. Numbers also declined steadily (and predictably) with increasing distance from the ‘source’ of carrot fly.

Susceptibility to attack varies with sowing date of the crop. Crops that emerge before mid-May are available to first-generation adults for egg laying. This may increase the level of second-generation damage since adults emerging from the first generation are likely to remain near the crop. 

Woven (e.g. fine mesh netting) or non-woven covers (fleeces) can be used to prevent carrot fly attack. The covers can be applied at sowing or later but must be applied before the flies start to lay eggs. If they are applied at the right time, the mesh size is sufficiently small, and they stay intact, crop covers successfully exclude all carrot fly.  

If they are not applied until the start of the second generation, then first-generation carrot fly must be controlled very effectively with insecticides. Otherwise, the use of covers may make the infestation worse. The downside of using crop covers is their management, cost and potential effects on disease and weed control. 

Early lifting of the entire crop cuts short development of second-generation carrot fly damage and prevents carryover of large numbers of carrot fly larvae from one year to the next. The carrot fly forecast model, available in the AHDB Pest Bulletin, has been adapted to predict when damage symptoms will appear in carrot crops. 

The use of vertical fences (up to 2 m high) to prevent colonisation by carrot fly has been investigated at Wellesbourne and elsewhere. However, although these reduced the size of carrot fly infestations, they did not exclude all flies.  

If you would like to order a hard copy of the Carrot & Parsnip Crop Walkers' Guide, please contact publications@ahdb.org.uk or call 0247 799 0069.

Content for this web page was originally authored by Rosemary Collier, University of Warwick.