Swede midge control methods

Find out why effective and consistent management of swede midge depends on an integrated pest management (IPM) approach.

In North America, swede midge control depends mainly on crop rotation and the use of synthetic chemical insecticides. Monitoring of swede midge numbers and the use of thresholds can be used to inform spray decisions.

Due to the low damage threshold of certain crops, mating disruption using pheromones is a promising novel approach for the control of swede midge. However, its cost and delivery in the field need to be optimised.

Similarly, strategies that prevent host location and subsequent egg-laying by female adults, such as volatile lures or repellents, may have the potential for IPM programmes in the future. Details are given below with regard to some of the cultural, physical, chemical and biological control methods for swede midge.

Crop rotation

Crop rotation is often the single most effective method to reduce swede midge damage. This is because swede midge cannot disperse very far because the adults are relatively weak fliers and have a short lifespan.

You can reduce damage by making sure crops are not grown in the same field or near to where there have been infestations over the last two to three years. Try to have a distance of at least one mile between a new crop and previous infestation.

Crop rotation disrupts the life cycle, preventing adults from finding suitable plant hosts when they emerge from overwintering in the following summer.

Time of planting

Larger plants are less susceptible to attack, so planting crops earlier in the season may help to reduce damage.

By planting brassica crops after the majority of the first generation has emerged in May–June, there will be fewer suitable hosts. This will help to reduce damage to crops.

Remove host weeds and avoid using brassica cover crops in infested areas.

Intercropping

Intercropping brassicas with several non-brassica vegetable, herb and cover crop species has not been shown to offer reasonable control.

Host plant susceptibility and resistance

The host range of swede midge includes almost all lines of cultivated brassica species.

High levels of crop damage have been reported for crops such as broccoli, Brussels sprouts, cauliflower and Chinese cabbage.

However, some varieties of broccoli have been shown to be less susceptible to swede midge damage.

Physical exclusion techniques, such as insect netting, can be used to reduce or prevent crop damage by swede midge in fields that have been free from infestations for two to three years.

In addition, mesh fencing, at the height of 1.4 m, can reduce damage as swede midge is a relatively low-flying insect. However, this may not be effective if adults are carried over fencing by the wind.

Chemical insecticides with contact action do not offer reasonable control for swede midge. This is because larvae regularly feed on parts of the plant which are inaccessible to sprays.

Adult midges are generally easier to target with systemic treatments. However, timing is crucial for insecticide application because of their short lifespan.

It is vital to control the first generation of midges to reduce the number of individuals that go on to produce subsequent generations. Managing swede midge infestations with insecticides is most effective when they are first colonising host plants, as population numbers are lower.

Timing insecticide application

The Contapre model, developed in Europe and validated under irrigated field conditions in Germany, didn’t work under non-irrigated conditions in Canada. In Canada, where an alternative forecasting system (MidgEmerge) is used.

You can time insecticide applications based on when you capture adult male swede midges in pheromone traps.

There can be a significant variation in numbers of adult midges during spring and summer. It is, therefore, better to time your insecticide application using action thresholds based on numbers of midges captured, rather than calendar-based spray programmes.

This approach has been shown to successfully reduce swede midge damage in cabbage. The action threshold is based on capturing four to ten males in each pheromone trap per day, with a minimum interval of a week before the insecticide is reapplied. You will need to monitor traps at least two to three times a week to see increases in the number of captures.

After the cabbage head has formed, there is less need to apply insecticides, even when the threshold is exceeded. This is because the marketable part of the cabbage is no longer at risk of damage.

Further research is needed to find effective thresholds in other brassica crops.

In North America, there are growing concerns about resistance, where insecticides are applied more widely.

Biopesticides

The timing of the biopesticide application is essential.

Biopesticides will be less effective if you apply treatments too early before egg-laying. This is because they will degrade in the field because of temperature, UV light and moisture.

For this reason, you should apply treatments when enough adults are first detected by monitoring with pheromone traps.

Overall, pyrethrin and spinosad offer the most significant potential as biological control options.

Essential oils

Lab studies have shown that using garlic and eucalyptus lemon oils can significantly reduce numbers of swede midge larvae. They affect the host-finding behaviour and subsequent egg-laying. There is potential to test these essential oils as swede midge repellents in the field.

Pheromone mating disruption

Releasing large quantities of the female sex pheromone in the field may provide swede midge control by preventing adult males from locating and mating with females. However, swede midge sex pheromone is complex and expensive to make, so this control method is unlikely to be commercially viable.

Mating occurs mostly during the first five hours after dawn, so to reduce costs, pheromone dispensers could be turned off at other times.

Natural enemies

Currently, no specialist predators, parasitoids or other natural enemies have been identified for swede midge in Europe.

Note: This swede midge information is based on a review of worldwide literature commissioned by AHDB and compiled by the University of Warwick in 2019.