ADAS was commissioned by the Home-Grown Cereals Authority (HGCA) to develop more reliable thresholds for pollen beetles based on scientific evidence which takes crop tolerance into account. In particular there was a need to quantify the threshold number of pollen beetle pests in an oilseed rape crop, above which, yield losses may be expected and control measures required.
Current pollen beetle thresholds are of unknown provenance and farmers have little confidence in them, often resulting in an unnecessary use of insecticides to control the pest. Overuse of insecticides can result in the development of pollen beetle resistance to certain insecticides. It is known that oilseed rape crops can tolerate the loss of some flower buds before yield losses are incurred. However, current thresholds do not take account of different levels of crop tolerance.
Controlled environment studies were used to quantify how many flower buds each individual pollen beetle can destroy. Field experiments were used to quantify how many flower buds different types of oilseed rape crop could lose without incurring a yield penalty. Pollen beetle surveys were used to develop practical methods for assessing pest numbers in the field. Statistical analysis and modelling was then used to combine all the information into a practical scheme that farmers can use to assess whether pollen beetle numbers are high enough to justify control measures.
The project showed that an individual pollen beetle could destroy up to nine oilseed rape buds. An individual plant could lose up to 300 buds without incurring yield losses. Crops with a low plant population density have a greater pollen beetle threshold per plant. There was little difference in the pollen beetle distribution between the edge and centre of the field. The pollen beetle threshold scheme that was developed showed that most crops can tolerate a surprisingly high number of pollen beetles without losing yield. The new pollen beetle threshold will give farmers more confidence about when to treat their crops with insecticides.
Pest threshold development requires expertise in entomology to understand the biology of the pest and expertise in crop physiology to understand how the crop can tolerate pest damage. ADAS has practical entomologists and crop physiologists with a proven track record in collaborating together in distilling research findings into practical messages for the farming industry. ADAS also has a proven track record in carrying out high quality field experiments and insect studies. HGCA have used the project findings to produce a farmer friendly information sheet to help farmers make sound crop management decisions.