By Andrew Porterfield
Biodiversity of insects has become an important issue in agriculture. Large-scale, intensive agricultural practices involve mechanically tilling the soil, managing pests with chemicals, and the use of plastic mulches and covers. While these practices control pests and increase crop yields, they can also reduce the populations of beneficial insects.
Organic and conventional farming practices can equally impact insect populations. In organic farming, tillage is often used instead of most pesticides to remove weeds. Tillage has come under criticism for encouraging soil erosion, disrupting insect populations, and even releasing carbon, thereby contributing to global climate change. Conventional farming can also use tillage, but tends to rely more on chemical soil treatments such as herbicides, fungicides, insecticides, and rodenticides, which can reduce the effects of pests on crops but also can significantly disrupt populations of beneficial animals.
For either organic or conventional farming, using plastic covers is common practice, including the use of black plastic mulch to retain water and boost soil temperatures. Another practice known as “strip tilling” plows a field so that the only soil that is disturbed are thin rows that contain crop seeds.
The tillage-beetle connection
A two-year study by Margaret Lewis, an entomologist at the University of Maryland, and Shelby Fleischer and Dana Roberts from Pennsylvania State University compared the effects of these farming practices on populations of beneficial ground beetles in squash and gourd fields. The researchers, whose results appear in the journal Environmental Entomology, discovered that strip tillage worked best at preserving the diversity of ground beetles. The study was the first to examine farming practices and insect diversity in annual horticultural production.
The researchers set up four different crop plots: two melon plots (one organic and one conventional) and two squash plots (one organic and the other conventional). They divided up strip tillage and row covers among the two types of fields and two types of crops. Commercial pesticides were applied to conventional fields, and pesticides approved for organic farming were applied to the organically farmed fields.
Then, from 2013 through 2014, the researchers measured the numbers of beneficial ground beetles, including Harpalus pensylvanicus, Cicindela punctulata, and a slug predator called Chlaenius tricolor. These carabid beetles are very sensitive to environmental disturbances and minor changes in climate. Individual species have developed specific preferences for habitat, which are influenced by temperature, soil moisture, prey, and competitors. Therefore, they are good bioindicators of the overall effects of farming practices on insect populations.
Comparing the effects on beetle populations from strip tillage, plasticulture, row covers, or no row covers, the team found that strip tillage was far less disruptive to beetles than plasticulture production. Over two years, the team captured 3,369 carabid specimens, which came from 38 different species. The first year, Harpalus pensylvanicus was the dominant species captured, followed by Cincindelia punctulata. The second year, H. pensylvanicus was dominant, followed by Bembidion quadrimaculatum oppositium, Bembidion rapidum, and Poecilus chalcites.
For all of these species, strip tillage had the least impact on beetle populations. Cicindela punctulata was found only in strip-tillage plots. Even relatively rare species, such as Chlaenius tricolor, Patrobus longicornus, or Colliuris pensylvanica, were sensitive to soil management techniques like strip tilling, indicating the importance of soil management to maintain diversity of the wide range of beetle populations. Row covers did not appear to have any effect on beetle diversity.
The study showed that even in annual horticultural production, “cultivation practices and field management strongly influence the carabid community.”
Multiple taxa benefits
Strip tillage and similar practices may also have an impact on other beneficial taxa. During a 2009 molecular analysis of the guts of 1,500 ground-dwelling specimens in field corn, 17 unique non-beetle taxa were seen feeding on corn rootworm eggs, and a number of taxa have been found to feed on diabroticite eggs in pumpkins. These taxa, which could be disturbed by conventional tilling, may also benefit from soil-conserving practices.
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Andrew Porterfield is a writer, editor and communications consultant for academic institutions, companies and non-profits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield on Twitter, or visit his Facebook page.