The soybean aphid Aphis glycines, an invasive insect pest, is the greatest threat to soybean production in the United States. First detected in the United States in 2000, this East Asian pest has spread rapidly through the Midwest and costs growers about $30 million to $50 million per year in reduced yield. By 2005, farmers had responded with a 20-fold increase in insecticide use, spraying millions of acres of soybeans that formerly went untreated — at increased costs of $10 per acre.
Long-term study of ladybird beetles at KBS had documented their importance as predators of other aphids in wheat, poplar, and other crops. In 2002, KBS researchers detected the arrival of Harmonia axyridis, an Asian species of ladybird beetle new to the United States. Using experiments in which fine-mesh cages were used to keep predators away from soybean plants, they showed that ladybird beetles could keep aphids below economic thresholds: inside the cages aphid populations quickly built to levels that would otherwise justify spraying. Outside the cages ladybird beetles, and especially the new arrival, kept aphid populations low late into the growing season — but only where there was sufficient natural habitat in the landscape to provide food and shelter for the predators throughout the year.
Economic analysis showed that in 2008 ladybird predation provided biocontrol equivalent to $13-$79 per acre in reduced pesticide use and averted yield loss. The value of this natural biological control of the aphid in four states (Iowa, Michigan, Minnesota, and Wisconsin) was estimated to be at least $239 million per year.
KBS researchers also found that biological control is greater where the surrounding landscape is diverse with many different types of crop and non-crop habitats, animals, and insects. For example, they found that the increased presence of corn within one mile of a soybean field tended to reduce biological control because corn is a less favorable habitat for ladybird beetles and other insect predators. Thus, due to a 19% increase in corn acreage from 2006 to 2007 in response to grain ethanol demand, which reduced landscape diversity, the supply of beneficial predators to control pests in soybeans was reduced. For 2008 this loss was valued at about $58 million for the four states studied. Overall, landscape simplification in the Midwest is estimated to cause between $34-103 million in direct costs to growers each year
We scaled up our investigation to see if landscape simplification had similar effects across cropping systems and continents. Using studies conducted in Europe and North America, we conducted a quantitative meta-analysis analyzing how natural control of pest aphids was related to simplification of the surrounding landscape in multiple crops. We found that landscape simplification was consistently associated with as much as a 46% reduction in aphid control.
Shifting patterns in insecticide use can have continued to impact the soybean system. KBS research has found that widespread adoption of seed treatments and foliar applications to control soybean aphid at the regional scale have led to a regime shift in Harmonia population dynamics reducing its abundance at KBS. Researchers continue to study this dynamic system to learn how manipulation of landscape structure could be employed to stabilize natural enemy communities and reduce reliance on insecticidal controls.