Parasitic wasp discovery offers chemical-free pest control for OSR

A species of parasitic wasp discovered by chance could provide farmers with a chemical-free way of controlling a major pest.

Researchers made the discovery when the wasps appeared mysteriously in colonies of cabbage stem flea beetles (CSFB) they were studying to test feeding preferences on oilseed rape.

The wasps appeared even though the beetles were confined to potted oilseed rape plants inside micro-perforated bags.

Further exploration revealed that the colonies of around 3,000 beetles collected from three sites around Norfolk had been infected by a parasitic wasp that lays eggs within the beetle’s body.

Genetic sequencing and enquiries by the Natural History Museum, UK, and the Swedish Museum of Natural History identified the wasp as an obscure species called Microctonus brassicae. The species was first reported in 2008 with no further identifications until now.

The study carried out by researchers at the John Innes Centre is the first English published description of this parasitoid of the adult CSFB.

Experiments showed that within controlled conditions, the presence of wasps in sufficient numbers led to the collapse of CSFB colonies.

Beetle hosts are rendered sterile and die after the wasp larvae emerge from the body after passing through its digestive system.

The short generation time of 43.5 days from egg to adult means it would be possible to rapidly rear multiple generations in controlled conditions.

The study raises the possibility of employing Microctonus brassicae and other species of genetically-similar parasitic wasps as a biocontrol to protect oilseed rape and a range of commercially-important crops prone to attack by CSFB.

The beetle is a major threat to oilseed rape, particularly the winter crop, throughout the UK and Europe. It causes characteristic damage known as “shot-holing” to leaves often resulting in crop failure or poor crop establishment.

The beetle has become a prominent pest in the UK, particularly in East Anglia, and surrounding counties following the European ban on neonicotinoid seed treatment use in flowering crops.

With the removal of seed treatments for oilseed rape the numbers of CSFB and the damage they cause have increased.

Using beneficial insects for biocontrol has been investigated in the past with five species of parasitoid wasps found to target CSFB. But these were found to be economically unnecessary while the now-banned pesticides were in use.

Under captivity in this study M. brassicae the rate of parasitism was greater than 44%. The research suggests that the wasp may have the potential to deliver positive effects under field conditions.

Lead author Dr. Rachel Wells of the John Innes Centre said: “Something that was initially very annoying leading to the collapse of our research colonies has turned out to be fortunate.

“It offers the possibility of using parasitoid wasps as bio-controls and more sustainable pest management practices for farmers and growers of oilseed rape and brassica vegetables against cabbage stem flea beetle as part of an integrated pest management approach.”