Pest insects in carrot and Apiaceous crops: turnip moth

Turnip moth caterpillars, also known as cutworm, can infest carrot and other Apiaceous crops. Find out about the cutworm life cycle, the damage it can cause to crops and how to monitor populations. 

'Cutworm' is the name given to caterpillars of certain Noctuid moths, particularly those of the turnip moth cutworm (Agrotis segetum). The adult moths lay eggs on plants or on pieces of litter and debris in the soil, usually from the end of May or early June. These hatch in around 8–24 days, depending on temperature. The young caterpillars feed on the aerial parts of plants.

In a further 10–20 days, again depending on temperature, the caterpillars go through their second moult, becoming ‘third instar’ caterpillars. At this point, they adopt the cutworm habit, becoming subterranean. 

Unhatched turnip moth eggs and the older, subterranean cutworms can’t usually be killed off by wet weather or insecticides. The two early caterpillar instars differ. However, if there is substantial rainfall* while these caterpillars are feeding above ground, this causes high mortality, and irrigation can be used to control cutworm if applied at this stage. 

*Defined as 10 mm or more of rain falling in showers of moderate intensity over a 24-hour period. 

Turnip moth caterpillars are most vulnerable to insecticides while feeding on the foliage (before the third larval instar), and output from the cutworm model in the Pest bulletin can be used to inform treatment timings. 

There is a second generation of turnip moths towards the south of the UK, but the caterpillars from this generation are not considered a threat to crops. 

The name ‘cutworm’ comes from the habit of the older caterpillars feeding underground, damaging plant roots and stems, sometimes so badly that the plant collapses. Crops differ in their susceptibility to cutworm damage, with crops like carrot, celery and parsnip being moderately susceptible.

Pheromone traps can be used to monitor the numbers of male turnip moths. When male moths are captured in the traps, it’s assumed that female moths are laying eggs at the same time. Data from trap captures in 2005–2007 at Wellesbourne were used to estimate a day-degree (D°) sum for the start of flight activity. This was 340D° above a base of 7°C from 1 January. The cutworm model is a computer program that uses weather data to predict the rate of development of turnip moth eggs and caterpillars. It also predicts the level of rain-induced mortality among the early-instar caterpillars. The cutworm model published by Bowden et al. (1983) has been programmed into the MORPH decision-support software and can be used to produce cutworm forecasts, with the weather data used to produce the carrot fly forecasts.  Consult the AHDB Pest Bulletin for further details. 

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.