A review of invertebrate pest thresholds

Current best practice on the need for pest control relies on the use of thresholds. For these to be of value to farmers/agronomists they must be based on sound scientific research and evolve with developments in crop production and physiology. Although some thresholds are based on experimental evidence others are based on much less robust information. Also many thresholds were developed at least 20 years ago and so may not be relevant to current varieties and agronomic practices. The aim of this project was to: 1) assess and score the robustness of existing thresholds; 2) identify how recent advances in the understanding of yield formation can be used to develop new thresholds; and 3) to highlight knowledge gaps for future research.

Of 22 pest species that attack oilseed rape and cereals, thresholds have been developed for 16 and the origin of eight of these is unknown. Of the eight thresholds of known origin two are more than thirty years old. Most farmers are aware of thresholds and consider them an important component of pest control. However, pesticide usage survey data suggests a lot of insecticides are applied unnecessarily. In oilseed rape there is an excessive level of insecticide use compared with the proportion of crops in which pests exceed the thresholds, and in most years all of the British rape area is sprayed at least once for unspecified reasons. Lack of confidence in existing thresholds, time consuming and complex pest assessment methods and inexpensive insecticides are all potential reasons why farmers/agronomists may not use thresholds to assess pest risk.

The physiological mechanisms of yield formation are reviewed for wheat, barley and oilseed rape. Improved physiological understanding of how yield develops has determined some of the minimum crop parameters required to achieve potential yield. Using a physiological approach, along with an understanding of pest biology, will help to develop more quantitative/mechanistic thresholds. It is clear that crops have a very wide range of tolerance to pest damage which depends on variation in crop growth caused by weather and crop management. The tolerance of autumn sown crops may have been increased by warmer temperatures increasing over-winter growth, although the trend towards lower seed rates for wheat may make this crop less tolerant of shoot loss.