Aphids Don't Like Brussel Sprouts Either

Posted Feb 9, 2008 by Bob Ewing
Aphids that eat Brussels sprouts are smaller than normal and live in undersized populations, which has a negative knock-on effect up the food chain according to new research published today in "Science".
The press release says that, for the first time, it has been demonstrated that nutritional quality of plant food sources for herbivores has a far-reaching impact on an ecosystem as a whole. It can potentially impede important functions the ecosystem performs, such as the natural predation and control of agricultural pests.
Researchers compared aphids living on sprouts to aphids living on wild cabbage in a field experiment that took place on a farm in the Netherlands. They were able to note that sprouts were of a lower nutritional value for aphids than cabbage because the aphids feeding on them were smaller in size and there were fewer aphids living on them.
The team next followed the effects up through the food chain and discovered the implications of poor nutritional quality in plants spread throughout the extended network of feeding relationships in an ecosystem known as a food web.
What this means is that sprouts not affect the herbivore aphids that eat them, but also the primary parasitoid wasp predators that mummify and eat the aphids, and the secondary parasitoid wasps that in turn eat the primary parasitoid wasps.
The team analyzed the food webs associated with both types of plants. What they found was that food webs based on sprout-eating aphids are less complex and involve a less diverse network of predators than food webs based on higher-quality plants like wild cabbage.
Larger, cabbage-eating aphids produce larger primary parasitoid predators, which in turn attract more of the opportunistic generalist feeders among the secondary parasitoids; this leads to a greater diversity of species and complexity in the ecosystem.
This demonstrates that plant quality indirectly influences the foraging decisions taken by individuals higher up the food chain which ultimately determines the structure of the food web.
"The diversity and complexity of food webs have long been seen as good indicators of how well an ecosystem is functioning, and how stable it is, but until now we had very little idea of the processes that determine diversity and complexity. Our study has shown that changing just one element, in this case plant quality, leads to a cascade of effects that impact on predators across the food web."
"If we are to predict how environmental change is going to affect ecosystems and the functions they perform, an important part of the puzzle is to understand the mechanisms by which an effect on one species propagates through the complex network of interacting species that make up an ecosystem." Dr. Frank Van Veen, from Imperial College London's NERC Centre for Population Biology, said.
Dr Van Veen adds "Our aphid study certainly does not mean sprouts aren’t good for humans to eat – our nutritional requirements differ enormously from those of insects."
The research was jointly led by scientists at Wageningen University in the Netherlands and Imperial College London, and was funded by the Netherlands Organisation for Scientific Research (NWO) and the UK Natural Environment Research Council.