Highly diverse flower strips promote natural enemies more in annual field crops: A review and meta-analysis

Landscape fragmentation, loss of habitat and pesticide use have caused a loss of arthropod biodiversity and disrupted biological control. For annual field crops such as cereals, oil seed and vegetables, this issue is exacerbated by high landscape homogeneity and frequent soil disturbance. Pest control with insecticides has negatively affected biological control, and regulation of pesticides reduces growers’ access to chemical control of insect pests. Flower strips can be used to increase biodiversity in arable landscapes and augment populations of natural enemies and potentially improve biological control of crop pests. This review aims to synthesize the knowledge on the contribution by flower strips in annual field crops to natural enemy populations, and pest regulation, and assess the importance of flower strip composition. After screening literature for studies comparing flower strip margins to a control of grass margin, we found 24 studies, including 382 trials, which measured the natural enemy abundance near a flower strip and control, and 10 articles, including 79 trials, which assessed the effect of flower strip margin versus control margin on pest control. Overall, results found that flower strips showed no significant effect on natural enemy abundance, however when examining flower strips with more than two sown species, the effect was significant, corresponding to a 51 %, ±13.7 SE, (p=0.001). The effect of the flower strips increases significantly with increasing number of sown species at a rate of 3.5 %±0.65 for every flower species added to the mix. The effect of flower strips on pest populations was more variable and did not show a significant trend. For future studies on the contribution of diversification to biological control, we propose a protocol for data collection and reporting of study details such as the realized botanical composition of the flower strip and control.