We all know that birds eat crop-destroying bugs, so we might think that farmers would welcome insectivorous birds to their fields with radiant rakes or happy hoes. But not so fast! Research by Ingo Grass and his colleagues alerts us to the reality that not all insects are created equal. Some insects eat crops, but some insects eat insects that eat crops.
Aphids are one of the worst scourges of the agricultural world. They suck the phloem sap from many plant species; this action can kill the plant directly, and also cause infections by plant pathogens and viruses. Fortunately for farmers, many animals enjoy eating aphids, including birds such as the Eurasian Tree Sparrow, Passer montanus, and insects such as ladybird beetles and hoverfly larvae.
Grass and his colleagues knew that sparrows eat both aphids and hoverflies, but they did not know how the effects of bird predation on these insects cascaded down to the oats and wheat crops grown near Gottingen, Germany. Their research tested the hypothesis that sparrows eat so many hoverflies that aphid abundance actually increases (despite also being eaten by sparrows), and oat and wheat abundance decreases (top food web in the diagram below). If so, they reasoned that removing the birds would increase hoverfly abundance, thereby decreasing aphids and increasing grain abundance (bottom food web).
The researchers set up an experiment with 11 nest boxes strategically placed between an oat field and a wheat field. Each box was equipped with a camera, so the researchers could see what the parents fed to their nestlings. In addition, Grass and his colleagues set up eight 4 X 5 meter plastic mesh exclosures which excluded birds, but allowed insects free access. They periodically surveyed 50 plants in each exclosure and in equal-sized control plots for hoverflies and aphids over the course of the sparrow breeding season. Because these birds can have three broods, this project kept them (the sparrows and the researchers) busy from early May to late July.
The birds fed very little on the two grain fields during the first brood, but towards the end of their second brood, they turned their attention to feeding on insects from the two grain fields, and later to eating the ripening grain. One important finding is that bird predation severely reduced hoverfly abundance. By early July hoverfly abundance was about 1 per 50 shoots when birds were present, and more than 3 per 50 shoots when birds were excluded (top graph below).
How did hoverfly consumption translate to aphid abundance? As you can see from the bottom graph, by early July, aphid abundance without birds was considerably lower than aphid abundance in the presence of birds. Taken together, these findings indicate that European Tree Sparrows consume hoverflies, which ultimately leads to an increase in aphid abundance.
Grass and his colleagues conclude that insectivorous birds can interfere with natural pest control of cereal production in central Europe. When birds were experimentally excluded, aphid densities declined 24% in wheat and 26% in oat crops. European Tree Sparrows were doubly bad for the crops, as they also harvested substantial quantities of grain from these fields to feed their third brood. The researchers argue that management of biological control systems for agriculture requires a broad food-web perspective that accounts for trophic cascades, such as the interactions that occur among sparrows, hoverflies, aphids and various types of economically important grain crops.
note: the paper that describes this research is from the journal Ecology. The reference is Grass, Ingo, Katrin Lehmann, Carsten Thies, and Teja Tscharntke. 2017. Insectivorous birds disrupt biological control of cereal aphids. Ecology 98 (6): 1583-1590. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.