More than 81% of questions about carrot pests asked through RHS Gardening Advice are about carrot fly. Barrier fencing is widely recommended to stop the pests reaching the crop and laying eggs. To ensure the advice we give is realisble I decided to test three different barriers at the RHS field research facility.
We covered the harvest and then graded roots on our crop for damage - both in the sweltering heat of July and a wet miserable November. After some careful data analysis, the results are in!
Carrot fly prevention
We found that the damage from carrot fly to carrots sown in June and harvested in November (late sowing) was much greater than that observed on crops sown in the same plots during May and harvested in July (early sowing).
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This was probably due to second generation carrot fly emerging from within plots and attacking carrots - as well as the pests from outside that were able to overcome the various barrier protections. This underlines the need for gardeners to practise crop rotation whatever type of barrier or cover protection they are using.
Carrot fly netting: what's best?
Secondly, and perhaps unsurprisingly, we found that carrots that were completely covered in insect-proof netting suffered the least damage from carrot fly. Although some damage was still observed. This damage may have resulted from some sneaky carrot fly creeping in whilst we were weeding or thinning under the netting or maybe even carrot fly larvae travelling underneath the soil from surrounding plots (they have been recorded as travelling over 60cm underground). Despite this small amount of damage, completely covering the carrot plots was definitely the best option.
Completely covered is best
In the first harvest, when damage from carrot fly was low, all barriers gave some protection and slightly improved the harvest when compared to leaving the plots uncovered. In our experiment however, there was little damage early in the season and even our unprotected plots gave an average yield of 80% undamaged carrots. This means that in our case the small increase in undamaged carrots - from average of 80% to 90-100% - when protected by barriers, may not have been worth the trouble of putting up the barriers.
The second sowing / harvesting of carrots generally had higher damage levels. The usefulness of the barriers when compared to completely covering the crop was limited and for the November harvest, the completely covered plots provided significantly more protection than any of the barriers. The cumulative pest pressure from two generations of carrot fly was enough to overcome much of the protection afforded by the barriers.
Advice for gardeners from this study:
- Garden crop rotation practices must still be used, even when completely covering the carrots, as even the covers did not give complete protection. Flies that managed to enter the coverings during the first generation then laid eggs underneath the barriers leading to greater damage levels later in the season. This could be prevented if carrots are grown in new areas.
- If you are willing to accept some carrot fly damage and plan to harvest the carrots early or grow them in an area where carrot fly hasn’t previously troubled you then using barriers, rather than covers, may be an option. The 90cm and 60cm barriers with an overhang may offer almost as much protection as a complete cover whilst being both easier to install and garden within.
- Despite being previously recommended, the amount of damage seen in plots protected by a 60cm barrier was not significantly different to those with no protection at all suggesting that these short barriers may not be worth the effort.
Carrot B5: where is it now?
For any of you waiting anxiously to know the fate of the famous 800g giant carrot, affectionately known as B5, it was made into a wonderful carrot cake to reward everyone who helped on the project.