Intercropping: a Strategy for Controlling Insect Pests in Oilseed Rape Crops

Oilseed rape (OSR) requires multiple plant-protection treatments. Growing this crop with companion plants is a promising approach for sustainable pest control.

Faced with the growing need to adopt more-environmentally-friendly agricultural practices, the European Union has banned numerous synthetic insecticides, particularly the neonicotinoids. Oilseed rape (OSR) crops are particularly affected as they require multiple treatments against weeds and insect pests. Furthermore, the growing resistance of insect pests to insecticides increases the need for alternative sustainable solutions.

The main pests

Among the main pests of OSR in Europe there are four Coleoptera species: the cabbage flea beetle (Phyllotreta spp.), the cabbage stem flea beetle (Psylliodes chysocephala), the rape stem weevil (Ceutorhynchus napi) and the pollen beetle (Brassicogethes aeneus). Each has a different life cycle and causes characteristic damage to different organs of the plant.

To ensure sustainable OSR production there is an urgent need to identify alternatives to plant-protection treatments for controlling these insect pests.

Intercropping

One alternative being studied is intercropping, which consists in growing OSR together with companion plants. These plants, such as vetch, faba beans or clover, are not grown for harvesting but rather to improve the environmental performances and production of the main crop. They can lessen weed competition, improve fertilisation and reduce the damage caused by certain insect pests.

A field trial was conducted over a two-year period on plots at Agroscope Changins to compare monocropped OSR with OSR intercropped with (frost-resistant) winter faba beans, (frost-sensitive) spring faba beans or artificial plants. The aim of the trial was to study the impact of companion plants on pests of oilseed rape as well as the mechanisms of action permitting a reduction of their incidence.

Mechanisms of action

Intercropping can reduce the incidence of insect pests via three different mechanisms of action – visual, physical and chemical – which can act separately or in combination.

Insects can rely on visual features of the field to locate their host plants. Reflection of the light by the companion plants or their shape can disorient the insects and impede their ability to identify and locate their host plants. This disruption is thus of a visual nature.

The presence of mechanical barriers due to the companion plants or a dilution effect rendering the host plants less accessible can hinder insect movement. This disruption is thus of a physical nature.

Lastly, volatile compounds interfere with the olfactory or gustatory systems of the insects, rendering the host plant less recognisable or less attractive. In this instance, the disruption is of a chemical nature.

The use of artificial plants in the Agroscope trial allowed the direct effects of companion plants associated with visual and physical disruptions to be uncoupled from the effects associated with chemical disruptions.

Effect of faba beans sown as companion plants on pests of oilseed rape 

In the trial, OSR was intercropped with faba beans. Although useful in autumn, frost-sensitive companion plants are often killed off by frosts in winter, which potentially limits their effect on spring pests. Hence, the benefit of studying the effect of frost-resistant winter faba beans versus (frost-sensitive) spring faba beans on the whole of the pest complex throughout the oilseed rape cycle, and of evaluating their impact on yield.

The study shows that intercropping OSR with spring or winter faba beans significantly reduces the incidence of the main insect pests. These companion planting systems lessen the damage caused by cabbage flea beetles and reduce the number of adult cabbage stem flea beetles in autumn. In spring, moreover, fewer rape stem weevil oviposition punctures and fewer adult pollen beetles are observed in the plots with faba beans. Intercropping OSR with (frost-sensitive) spring faba beans reduces the incidence of rape stem weevils and pollen beetles more markedly than companion planting with winter faba beans. This effect could perhaps be explained by the presence in spring of the lignified stems of the spring faba beans which are highly visible above the OSR, despite their frost in winter. 

Intercropping with artificial plants reduced the incidence of the four insect pests studied compared to monocropped OSR, but to a lesser extent than intercropping with the natural faba beans. The companion plants thus exercise a physical and/or visual disruption on the insect pests, reducing their incidence. As this point does not entirely explain the result observed for the faba beans, a chemical effect might also be involved.

There were no yield losses in the case of OSR intercropped with faba beans, with a tendency for better yields when OSR is intercropped with spring faba beans.

Over the next three years this study will be followed up with trials conducted by Swiss farmers as part of the CAPRI project.