“Landscape mosaics” of long-term grasslands coupled with crop diversification are the key to naturally managing pest and weed pressure on arable farms, according to a 7-year study in France.
Researchers analysed 974 arable fields on 450 different farms in central western France between 2013 and 2019. They used a range of sampling methods on randomly selected squares in the fields to estimate the number of pests present in the fields and the rate of their predation, and calculated the crop diversity at different points during the study, and the age of meadows.
Clear links between habitat and pest control
They found that grasslands had a stronger effect on pest predation than woody habitat or crop diversity. When grassland increased from 0-40% over a given year, predation rates of ground-level pests increased by 23%. Grasslands also had a greater impact than temporary hay meadows, and this effect was more pronounced the longer the meadow had been in place.
While significant effects on ground-level pests were seen, this was not the case in crop canopies, except from in the case of oilseed rape. Such habitat did not have the same impact on predation of weed seeds that it did on pest predation, except in the case of oilseed rape fields where there was a significant effect. Grassland also decreased the abundance of weeds in maize.
The study further revealed that crop diversity increased seed predation in cereal, wheat and sunflower fields, but not in those growing oilseed rape.
Reassuringly for growers considering changes tom practises, the scientists made a firm connection between predation and the abundance of ground-level pests and weeds. They found that with a 20% increase in predation, pests decreased by 19.5% and weeds by 5.6%.
“Our study provides strong evidence that two facets of landscape heterogeneity are simultaneously needed to support both pest and weed control services in agricultural landscapes. On the one hand, long-term grasslands support the bulk of insect pest control services, while on the other hand, crop diversity increases weed seed control,” the researchers wrote in the journal Proceedings of the National Academy of Sciences.
Policy changes needed?
The effects of meadows on pests are largely due to them being undisturbed habitat, providing stable environments for beneficial invertebrates, they explained, but may also be partly due to meadows diluting the amount of resource available to pests. Meanwhile, species which are noted for their predation on weed seeds, such as ground beetles, are known to respond well to crop diversity.
Landscape alterations to encourage predation and parasitism of pests are considered a critical part of integrated pest management strategies. The scientists were keen to stress that their study, with its high sampling effort over a number of years, does not support the idea that crop diversity could compensate for a lack of semi-natural habitat, and that a rethink is needed to ensure that food production can be reconciled with conserving biodiversity.
“These findings are critically important in regard to agricultural policies of most industrialized countries…which has invariably advised farmers to increase crop diversity at the expense of long-term grasslands which are weakly protected and globally highly degraded,” they added.
