Landscape Diversity Enhances Biological Control of an Introduced Crop Pest in the North-Central USA
This article looks at soybeans, examining whether or not landscape diversity enhances the amount of biological control agents, such as Coccinellid beetles. They chose to look at Aphis glycines, commonly known as the soybean aphid.
Insects are critical for their roles in predation of crop pests, as well as decomposition and pollination. The landscape surrounding agricultural systems influence the diversity and abundance of natural predators. Habitat complexity, quality, and patchiness all play a role in the supply of biological control. Agricultural systems are often somewhat unstable, in that tillage, harvest, and pesticide application means that arthropod populations have to frequently resettle plants, something that is assisted by natural reserve areas.
Invasives can be quite problematic as well, which is the case of Aphis glycines, introduced to the US in the summer of 2000. They feed on the leaves, stems, and pods of soybean plants, leading to reduced growth and yield. Many native and exotic predators feed on them, however generalists are the most sought after. Some common generalist predators include:
- Coccinellidae
- Harmonia axyridis
- Coccinella septempuncata
- Hippodamia variegata
- Hippodamia convergens
- Coleomegilla maculata
- Hemiptera: Anthocoridae
- Orius insidiosus
- Neuroptera: Chrysopidae
- Chrysoperla carnea
- Diptera
- Aphidoletes aphidomyza
- Leucopis spp.
- Syrphidae
These predators have been shown to reduce populations of soybean aphids, although the level of suppression varies between studies. This is likely the result of different landscape compositions. This study aims to determine if landscape diversity and composition may explain biological control variance of A. glycines. The hypothesis was that “landscapes composed of a high proportion of non-crop habitat such as grasslands and forests would provide overwintering habitat and alternative prey resources to natural enemies.”
Methods: 26 sites across Iowa, Michigin, Minnesota, and Wisconsin were observed, with exclusion cages on some plants, and none on others, so that predators cannot affect the aphids in the exclusion cages. They were measured after 7 and 14 days, and checked for different generalist predators, which were recorded. Additionally, the landscape complexity and composition was analyzed and correlated with the results.
Discussion of Results
Aphid populations in exclusion cages grew much more quickly compared to those in open fields, and crossed economic thresholds much more frequently. Many species of predators were collected; nine species of Coccinnelidae, with the greatest percentage being held by Harmonia axyridis (70% in 2005, 72% in 2006). Syrphidae, Chrysopidae, and Anthocoridae were also abundant.
Natural biological enemies of pests are very important. This study demonstrated that soybean aphid suppression was affected by the diversity of the agricultural landscape, and that coccinelid beetles responded to the presence of natural habitat. It found that low numbers of natural predators cause a significant reduction in pest species if there is predation earlier in the year, before pest populations are better established. Low diversity landscapes like those used in monocropping corn and soybean systems have reduced biological control, and in 45% of their studies, increased landscape complexity reduced pest pressure. In 15% of the studies, pest pressure increased, while the remaining 40% didn’t seem affected. This variation is likely tied to “differences in the impact of landscape complexity of specific natural enemy populations.” An occurrence of natural habitats within ~1.5 km explains the highest percentage variance in abundance of predators.
This is very encouraging conversationally – it means that having preserved natural ecosystems can be quite beneficial to farming systems. This is an incredible symbiotic relationship, and can encourage farmers, especially those in organic systems, to have conservation lands bordering them, which would mean not only more preserved ecosystem, but better biological control! TESC’s organic farm is a good example of one with this benefit – the forest provides a valuable buffer zone for insects.