Optical sensors warn of beetle infestations

BY BÁRBARA PINHO | IMAGE: SHUTTERSTOCK

Optical sensors could help farmers better monitor beetle populations in oilseed rape fields.

In a recent experiment in agricultural fields in the UK and Denmark, scientists found that optical sensors detected pollen beetles, a common European pest of oilseed rape crops, faster and with higher sensitivity than any of the other methods studied (Pest Management Science, DOI: 10.1002/ps.7538).

‘Optical sensors detected pollen beetles immigrating into an oilseed rape crop four days before they were [manually] detected on plants,’ says Samantha Cook of Rothamsted Research in the UK.

Oilseed rape is the main oilseed crop grown in Europe, with 23.7m t produced in 2020. It’s mainly grown for food-grade cooking oil and the meal is used for animal feed. The pollen beetle enters these crops during flower bud development and causes damage by feeding on young buds, reducing yields.

To control the pest, farmers across the continent spray oilseed rape fields with insecticides. Around 90% of fields are sprayed with products, many of which are applied repeatedly throughout the growing season.

How much farmers spray should depend on the number of insects per plant (or the amount of damage per plant). However, Cook explains that monitoring methods to determine whether thresholds have been exceeded are usually burdensome for farmers and crop consultants, so many crops are treated ‘just in case’. This potential overuse leads to problems with insecticide resistance and damages the environment.

Previous studies have shown that the pollen beetle moves in certain patterns around oilseed rape crops. Knowing these movement patterns could help farmers apply pesticides only where beetles are expected to be, reducing the amount of insecticide needed.

So Cook and her colleagues set out to understand how pollen beetles move around oilseed rape crops. They did this by tracking their arrival and distribution using three methods: manual counts, water traps and optical sensors spread across some of the trial plots.

The team compared the number of pollen beetles detected by each method and found that the distribution of beetles across the field was always not uniform. They saw that the insects aggregated in different areas of the field depending on the stage of development of the plant. The researchers also saw that of the three methods, the sensors appeared to be the most effective.

The sensors recorded about 18 times and 6.5 times the number of pollen beetles compared with plant counts and water traps, respectively. In addition, the optical sensors recorded an increase in pollen beetles two days before water traps and four days before plant counts.

This makes the method a potential monitoring tool for farmers, helping them to better track where and when beetles are found and to apply insecticide accordingly.

‘This technology will enable future precision agriculture, meaning that insecticide sprays are used only when and where necessary,’ says Cook.

Cook adds that the next steps in this research include focusing on where to place sensors in the field to get the best results, and how many are needed for the most cost-effective spatial coverage.

Although the results are promising, Teja Tscharntke from the University of Göttingen in Germany, who was not involved in the study, reiterates that more research is needed. ‘These results are based on a few fields and the farmer’s costs of monitoring pollen beetles with in-field optical sensors still need to be calculated. Even the traditional methods (water traps, counting) mean more work and costs for the farmer, which is the reason why monitoring is not regularly applied,’ he explains.

Cook is aware that cost is a potential barrier for farmers at this stage. However, she argues that these findings can already help those working in the fields. ‘Farmers can immediately use the knowledge generated about immigration patterns of pollen beetles – that is, where to focus initial monitoring and other preventive measures.’