Sowing oilseed rape with other plants reduces damage caused by adult flea beetle, according to a study conducted at Agroscope. This intercropping approach could limit reliance on insecticides and the emergence of resistant pests.
From 2014 to 2017, Agroscope conducted a study on companion planting in sowing oilseed rape (OSR) as an integrated approach to controlling a major pest of OSR, the flea beetle. These beetles can be divided into two groups: the small crucifer flea beetles, and the cabbage-stem flea beetles. At the adult stage, both groups damage seedlings by feeding on the young shoots. At the larval stage, only the cabbage-stem flea beetle, which overwinters in the stalks, has the potential to cause major yield loss. Researchers tested whether intercropping OSR with two companion-plant mixtures, the first consisting of berseem clover and niger, the second additionally containing a mixture of buckwheat, lentil, vetch, grass pea and broad bean, impacts the insect populations.
The number of plants attacked by adult flea beetles was lower in OSR grown with companion plants – regardless of the mixture – than in pure-stand OSR. This result could be explained by the companion plants causing olfactory and/or visual confusion, thereby luring the insects away from the OSR.
Conversely, intercropping did not reduce the number of larvae per plant. As most of the OSR yield is typically lost to larval damage, new strategies for limiting their numbers still need to be developed.
New trials are underway to test whether the addition of other companion plants in the mixture could limit damage caused by the larvae. Frost-sensitive crucifers (mustard, fodder radish or turnip rape) could act as dead-end trap plants, their high levels of glucosinolates – compounds which are prized by the beetles – distracting pests from the OSR crop.
The authors also suggest testing the effect of flower strips and broad beans. These plants could favour the presence of beneficials, which predate on or parasitise flea beetles at the adult or larval stage. Lastly, they suggest testing less frost-sensitive plants to extend the confusion effect resulting from intercropping until the end of the year, thereby reducing cabbage-stem flea-beetle larvae numbers.
Reducing Pesticide Use
The study conducted in Changins demonstrates the potential of intercropping as a more sustainable weapon in the arsenal of pest control. This approach is therefore congruent with the aims of the National Action Plan for risk reduction and sustainable use of plant-protection products (NAP 2017). Innovative strategies based on the management of interactions between species, the landscape and farming methods are more essential than ever.
Previously, flea-beetle were controlled with OSR seeds coated with neonicotinoid-based products – insecticides deregulated in Switzerland in 2013, and banned in 2018. Since then, the use of pyrethrinoids (the only insecticides approved for control of this pest in Switzerland) is increasing, but instances of resistance have already been observed in Europe.
Agroscope is conducting various trials to discover sustainable alternatives.
- In light of these initial results, it is essential that we continue to improve the choice of plants for intercropping with OSR. Agroscope is working towards this aim using a variety of approaches:
- The push-pull (i.e. repulsion-attraction) effect of certain plants, to attract predators and parasitoids of the flea beetle to the OSR fields, or distract the flea beetles from their target by luring them to trap plants such as radishes or mustards.
- The use of slightly more-frost-tolerant plants in the mixtures. This could extend the confusion effect until the adults have finished laying their eggs, and potentially reduce the number of cabbage-stem flea-beetle larvae.