The increase in demand for pesticide-free produce, and the reduction in the numbers of insecticides available to control the major pests of field vegetable crops have led growers and researchers to consider other control options. This article considers ways of using various ‘barriers’ to prevent pest insects colonising field vegetable crops.

Crop covers

A variety of fleeces and mesh covers are now available to growers and, apart from their uses for crop advancement, they offer considerable opportunities for pest control. Some years ago, Defra commissioned a project to investigate the use of crop covers for controlling pest insects in field vegetable crops. The aim of the project was to reduce pesticide usage and exclude pests that were difficult to manage using conventional methods of control. The experimental work was done at HRI Kirton, HRI Stockbridge House and ADAS Arthur Rickwood. The project covered a range of crops including cauliflower, Chinese cabbage, swede, carrot, lettuce and leek. The intention was to establish general principles about the use of crop covers for pest control, rather than compare the performance of a range of covering materials.

The studies showed that crop covers offer considerable opportunities for pest control in situations where conventional methods are unsuitable or ineffective. To achieve effective control it is important that:

• Crops are covered before pests are likely to colonise them.
• Covers are not used on crops where pests are already on the plants or in the soil.
• Covers are sealed well.
• Any damage to covers is repaired promptly.

Brassica crops

In the Defra-funded trials on cauliflower, replicated plots were covered with fleece and compared with uncovered plots in which conventional insecticide treatments were applied. The covers excluded cabbage root fly, cabbage aphid and caterpillars effectively and these pests caused little or no damage to plants, provided the covers remained intact and were anchored firmly. The covers did not appear to affect crop quality adversely. However, they did alter the time of crop maturity. In some cases early crops under covers were advanced by as much as 10 days, whereas, in contrast, late crops were delayed by up to 7 days. This is because curd initiation in cauliflower is triggered by the accumulation of units of relative cold. The covered crops were not able to accumulate cold units as rapidly as those that were uncovered.

The cabbage root fly is notoriously difficult to control on root brassicas, where even small amounts of feeding damage can reduce crop quality. Since Approval for the use of chlorfenvinphos (Birlane 24) in the UK was withdrawn, no effective alternative insecticide treatments have been available. Hence crop covers are the most reliable way of achieving a swede crop free from cabbage root fly damage. However, the final yield may be lower than that from an uncovered crop, depending on the type of cover used.

Carrot

Crop covers can also be extremely effective for carrot fly control and are used by some organic growers to good effect. Once again, there may be a yield penalty, but this will depend on the type of cover used and whether the crop is covered for its entire life.

Lettuce

On lettuce, covers were used to exclude the lettuce root aphid. Winged lettuce root aphids fly from poplar, their overwintering host, and colonise lettuce crops during a period of 4-5 weeks in June/July. The start of the aphid migration from poplar to lettuce occurs when about 672 air day-degrees (above a base temperature of 4.4°C) have been accumulated from 1 February. Hence, it is quite easy to identify the crops that are at risk from colonisation. In the Defra-funded trials, crops covered with fleece for one or more weeks were compared with crops that were left uncovered. The results showed that lettuce root aphids were excluded effectively, provided the covers 1) were in place before winged aphids could enter the crop, and 2) stayed intact. However, when lettuces were covered for more than about four weeks, they suffered serious damage due to leaf scorch, rots and misshapen heads. Obviously, a different type of cover, for example a mesh cover rather than fleece, might have reduced such damage.

Leek

Thrips are a major pest of Allium crops and are difficult to control with the insecticides approved currently. Trials at Stockbridge House indicated that fleece covers could also be used to exclude thrips from leeks. This appears to be an effective method of control, provided the mesh size is sufficiently small and the covers are applied in time.

References

COLLIER, R.H. (2001). Crop covers offer considerable opportunities for pest control. The Vegetable Farmer, December 2001, 18-20.

COLLIER, R.H. (2002). Using crop covers and mulches as an aid to pest control. The Vegetable Farmer, December 2002, 16-19.