Less Is More When Plants Call for Aid

Oilseed rape (similar to canola) is a major crop prone to attack from herbivores. One pest is the Pollen Beetle, Brassicogethes aeneus, which is very much a problem in Estonia. A team of Estonian scientists have examined how farmers can help the plant call for a natural defence against these insects. Their research, published in Scientific Reports, shows that nutrition is the key to unlocking the plants’ alarm call.

Plants have a few ways to fight attackers. They can build structural defences, like thorns or spines, or create chemical defences to deter anything that bites them. But sometimes, they call for help. Plants can emit Volatile Organic Compounds (VOCs). VOCs released from a flower can attract pollinators, but they can use VOCs released from leaves to attract something else.

Oilseed rape, Brassica napus, attracts pollen beetles – who love pollen. They cause damage by attacking flower buds. They also like to plant their eggs on Oilseed rape, where the larvae feed. As they feed, the damage causes the plants to release VOCs, and other insects are paying attention. They use the VOCs as a signal that beetle larvae are around, and they lay their eggs into either beetle eggs or larvae. Here they grow as parasitoids, parasites that eventually kill their host as they eat them from the inside out.

If a plant wants to be able to release VOCs to call these parasitoids, then it needs the right nutrition. Valentina Zolotarjova and colleagues wanted to determine what fertilization works best for plants to produce these chemicals. To find out, they grew plants with varying amounts of nitrogen fertilizer and examined which plants did the best job of attracting parasitoids.

The team found that the beetles preferred to lay their eggs on the most fertilized plants. In contrast, the parasitoids were most attracted to the plants with a moderate amount of fertilization. Zolotarjova and colleagues suggest that the parasitoids navigate towards the plants with the most attractive smell, not the plants with the most beetle larvae. They also wonder if the more fertilized plants have better chemical defences, which makes the beetle larvae less palatable to the parasitoids.

Crucially, the scientists also found little difference in the yield between the moderate and highly fertilized plants. So while there is a benefit in promoting natural pesticides, there might not be a high cost in reduced yield for boosting this protection.

If this research can be developed, then there are plenty of potential benefits, such as reduced bills for fertilizer and pesticides for farmers. There are also benefits for the wider environment, as the parasitoids don’t attack other insects or pollinators indiscriminately the way some pesticides do. It will also reduce pests’ ability to adapt to pesticides, as pollen beetles currently do.

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Zolotarjova, V., Remmel, T., Kännaste, A., Kaasik, R., Niinemets, Ü. and Veromann, E. (2022) “Pollen beetle offspring is more parasitized under moderate nitrogen fertilization of oilseed rape due to more attractive volatile signal,” Scientific Reports, 12(1). https://doi.org/10.1038/s41598-022-18030-0