Hi-tech optical sensors in fields could provide an effective means of monitoring beetle numbers arriving in oilseed rape fields, according to a new study.

Results from an optically sensed field were compared to those baited with standard water traps and in-field counting by hand.

The optical sensors recorded an increase in pollen beetles two days ahead of water traps and four days ahead of plant counts.

In terms of early detection and numbers of beetles recorded, this was clearly the most efficient pollen beetle monitoring method.

“Understanding the dynamics of pest immigration into a crop field helps us find effective and timely management strategies,” confirmed Prof. Emily Bick of the University of Wisconsin-Madison who led the study, alongside scientists from Rothamsted Research, Fauna Photonics A/S and the University of Copenhagen.

“The pollen beetle is a significant pest of oilseed rape at the budding stage,” she continued.

“Many farmers in western Europe are now abandoning oilseed rape as a crop because damage is so severe from pollen beetles, additional insect pests, and reduced availability of approved synthetic insecticides.

“Better surveillance of in-field pests could help us identify a way round the problem.”

Optical sensors

Automated near-infrared optical sensors recorded the signal of light backscattered by insects flying through a detector beam.

Researchers were able to record insects actually in flight and detect each insect’s wing beat frequency, which often differs from species to species.

For instance, pollen beetles have a wing beat frequency of 120 Hz, so insects within the range of 100–140 Hz were considered pollen beetles.

Sensors were run continuously in fields in Denmark, alongside parallel experiments on the spatial distribution of pollen beetles in oilseed rape crops at Rothamsted in the UK.

In all fields across the study, pollen beetles were found in significant numbers and were shown to aggregate.

Beetle density was related to plant growth stage, with more beetles occurring on plants after the budding stage than before inflorescence development.

Rothamsted’s Dr. Sam Cook, the senior author of the study added: “Our study suggests potential for precision agriculture to reduce insecticide use through targeting of pollen beetle aggregations – in other words treating only areas of the crop where pollen beetle density is high.

“Optical sensing of pollen beetles gives us more efficient monitoring in both time and space, so it is a promising tool for early warning of insect pest immigration.

“The aggregation pattern of pollen beetles post-immigration could be used to precisely target control measures in oilseed rape crops.”

The aggregation of pollen beetles usually occurs first on the downwind edge of fields and then expands to the centre.

Early detection via sampling and monitoring should correspond with this migration pattern, allowing for more effective control, according to the research.