Obscure wasp species could help farmers protect OSR

A parasitic wasp discovery could offer chemical-free pest control for oilseed rape growers
A parasitic wasp discovery could offer chemical-free pest control for oilseed rape growers

An obscure species of parasitic wasp discovered by chance could provide farmers with a chemical-free way of controlling cabbage stem flea beetles.

The discovery was made when the wasps appeared mysteriously in colonies of cabbage stem flea beetles (CSFB) across three sites in Norfolk.

Researchers, who were studying to test feeding preferences on oilseed rape, said 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 the sites had been infected by a parasitic wasp that lays eggs within the beetle’s body.

Genetic sequencing and enquiries by the Natural History Museum identified the wasp as a rare species called Microctonus brassicae, which was first reported in 2008 with no further identifications until now.

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 John Innes Centre study raises the possibility of employing Microctonus Brassicae as a biocontrol to protect OSR from CSFB attacks.

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

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

The ban on these and other systemic pesticides followed research linking their use to decline in pollinators. Further legislation in 2019 upgraded the ban to include other broad-spectrum pesticides.

The study raises the possibility of employing the wasp as a biocontrol to protect oilseed rape
The study raises the possibility of employing the wasp as a biocontrol to protect oilseed rape

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

Figures for 2016 valued the damage at £18.4m with an approximate loss of 5% of the national crop area of winter oilseed rape to CSFB.

The estimated best-case crop production for 2020/21 is 1.26 million tonnes, a year on year decline of 489,000 tonnes, putting the future of the valuable UK rapeseed crop in doubt.

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

Lead author Dr Rachel Wells, of the John Innes Centre, said: “Something that was initially 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 of oilseed rape against cabbage stem flea beetle as part of an integrated pest management approach”.

Using beneficial insects for biocontrol has been investigated in the past with five species of parasitoid wasps found to target CSFB.

But these have demonstrated limited effectiveness and 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.

Further research has been performed by Rothamsted Research to look at wasp presence and parasitism levels across the UK.