There is an increased interest in “conservation biological control,” which involves managing the agricultural landscape to promote natural enemies and help suppress pest infestations – see OMAFRA Agronomy Guide for Field Crops for descriptions of natural enemies. Though much research is still needed in this area, there is evidence of some successful practices that can increase natural enemy abundance. It is well known that monoculture cropping systems tend to decrease natural enemy diversity and therefore increase the frequency of pests. Perennial crops like mixed forages tend to support a diverse community of natural enemies compared to annual crop species. By increasing plant biodiversity across the agricultural landscape, particularly with perennial species like trees and shrubs along field boundaries, natural enemy abundance increases. These buffer strips, or natural habitats, help to provide predators and parasitoids with shelter, pollen and nectar sources and some protection from the pesticide applications taking place in adjacent fields. However, plant selection is important to not encourage pest populations that may be equally attracted to these plant species. Also, these buffer strips should not be encouraged in a food-grade cropping system where plant viruses carried by these bordering plants can be vectored to the crop by aphids and other pests, and can impact quality. Inter-cropping/strip cropping has also shown some potential but requires a thorough understanding of the pest history of each field before pursuing, to ensure pest problems do not increase because of the companion crops planted.
Tillage can have a negative impact on natural enemies, as many species use the crop residue as shelter and overwintering habitats. Moving towards a no-till or reduced tillage system needs to be well thought out, as it can increase the risk of some soil pests and diseases depending on soil type and crop rotation.
A significant component of natural enemy conservation is selective use of pesticides. Applying pesticides only when pests have reached threshold can help reduce harm to the natural enemies. Selecting reduced-risk insecticides belonging to chemical families that are less harmful to natural enemies is a positive step. Spot treating where the pest problem occurs can also reduce the risk to natural enemies. Frequent use of foliar fungicides has been shown to reduce the presence of entomopathogens (fungi) that control insects. Use foliar fungicides only when necessary. The use of systemic insecticide seed treatments can have a negative impact on natural enemies in two ways:
- indirectly, by suppressing the pest population year after year so that the fields are not able to sustain a natural enemy population
- directly, by the natural enemy feeding on prey that contains the insecticide, which then kills the natural enemy
Weather can have a big impact on natural enemies. Harsh winters tend to greatly impact some species and can delay their ability to respond to spring pest infestations. Cool wet conditions can also be more detrimental to natural enemies than the pest species, while warm moist conditions, particularly when the crop canopy is closing, can help promote entomopathogens. Hot dry conditions are harsh to many natural enemy species and increase the crop’s susceptibility to stress incurred by pests and diseases. Simply recognizing some of the key natural enemies of the pests of field crops can increase awareness of their importance and help determine if and when chemical control is necessary. The natural enemies of soybean aphids in particular have proven their value in keeping aphid populations below threshold.
