Plant fertilisers can affect pest populations

11 October 2010 by Adele Walker

Conventional fertilisers may reduce competition between different pests on the same plant, allowing them to become more abundant, a recent study has shown.

Cabbage damaged by the diamondback moth

Cabbage damaged by the diamondback moth

Researchers from the Centre for Ecology & Hydrology, Imperial College London and the University of Southampton, wanted to test the idea that organic plants are better defended against pests than those grown using conventional methods.

Their approach was based on the fact that different pest species compete with each other on the same plant. It's not a straightforward competition though, as the different species often don't encounter each other directly - either because they feed at different times of day or eat different bits of the plant.

Instead the feeding action of one can cause the host plant to behave in a way that's detrimental to the other, perhaps by activating the plant's defences. For example, the action of lots of insects feeding on a plant's roots might cause it to change its root chemistry, which will affect the quality or amount of growth above ground and therefore the quality of food available to insects that feed on its leaves.

The consensus of earlier studies is that there is a relationship between the quality of food available to a plant and the pests that feed on them. The researchers' own recent field trial showed that different fertilisers played a role in this. But until now, entomologists have studied competition between insects, and little attention has been given to the role of plants.

"People haven't really looked at the effects of different types of fertilisers on the relationship between plant chemistry and insect competition," says Dr Joanna Staley, lead author of the team's report which is published in the Proceedings of the Royal Society B.

Diamondback moth larvae

Diamondback moth larvae

Dr Staley and her colleagues took a closer look at the effects of fertilisers on pests of the cabbage, Brassica oleracea, grown in the lab. The pests in question were the cabbage aphid Brevicoryne brassicae, which feeds on the fluids in the plant, and the diamondback or cabbage moth, Plutella xylostella, which eats the leaves. Because they feed on different parts of the plant, the two species are unlikely to be in direct competition for food.

The scientists grew the plants with either: no fertiliser; organic chicken manure; an animal- and mineral-derived John Innes fertiliser; and ammonium nitrate, which is widely used in conventional agriculture. They then looked at the effects of the fertilisers on a number of factors, including the plants' growth and the abundance of the two pests.

The key result was that, with no fertiliser or under the organic fertiliser treatment, there was clear competition between the two species of pests. But on those brassicas treated with ammonium nitrate, competition ceased.

"These results may indicate that the use of ammonium nitrate increases the quality of the plants for the pests," says Staley. "For example, by increasing the concentration of nitrogen in the leaves, which is a key nutrient for aphid population growth."

This increased nitrogen content may allow the aphids to overcome any negative changes in the plant quality caused by their competitors feeding on the same plant.

In general, when pests are in competition with each other they may limit each others' numbers, though in practice this competition is often 'asymmetric' with one species gaining dominance over another.

So the relationship is not straightforward and Staley is keen to point out that there are numerous other factors involved. For example, interactions with other insect species, and the use of conventional pesticides.

Nevertheless, the work is an important step towards a fuller understanding of these relationships and the possibility of more effective and less invasive approaches towards pest control.

'Plant nutrient supply determines competition between phytophagous insects' - Staley, et al 2010. Proceedings of the Royal Society B. doi:10.1098/rspb.2010.1593