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Is my carrot soil contaminated?

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Maria Del Carmen Casarrubios Maria Del Carmen Casarrubios | 09:56 UK time, Wednesday, 20 October 2010

Maria del Carmen Casarrubios, who's a tutor and practical instructor at Capel Manor College, answers your questions about growing veg.

DI Hughes from Carmarthenshire asks: For many years I have successfully grown carrots. During the last two seasons carrots have been totally destroyed by carrot fly. Is it possible that my soil is now contaminated? Would a dose of  Jeyes fluid this winter help? Is an application of Jeyes fluid good for the garden?

Answer: Jeyes fluid is a disinfectant. Very useful around the home, patio and glasshouses, but not to be used on the soil!

The carrot fly breeding cycle will produce two generations within a year as follows:

The first generation is produced from the pupae and larvae from the previous year, which will overwinter in the soil and in any affected roots left in the ground. The shiny and black adults emerge from these pupae/larvae in May to June. When they emerge, they lay eggs in small white clusters near their intended food source, hatch into larvae a week later, and feed for about a month before pupating in the soil.

The second generation emerges in August/September and feed into the winter before pupating again in the following May/June.

Some things to try are:

  • Clean and dig over the plot to expose the pupae and larvae to predators such as birds.
  • In May/June, try creating a physical barrier with clear polythene sheeting, gardening fleece, or fine netting, all of which should be tucked into the soil leaving no gaps.
  • Sow carrot seeds in early May to avoid the first attack.
  • Sow the seeds thinly to avoid thinning them out. If they do have to be thinned, dispose or eat the thinnings immediately.
  • Do not leave the mature carrots in the ground any longer than is necessary.
  • Try growing resistant cultivars such as 'Sultan' or 'Flyaway'.
  • A soil drench insecticide can be used to limit the damage in August/September.

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