Burgling beetle targets plants with the heaviest security

Heavy locks, imposing gates and motion-sensing lights can help to fortify your home and safeguard your belongings against thieves. On the other hand, they can also advertise the fact that you have stuff worth stealing. Extra security can be a double-edged sword.

This is as true for plants defending their tissues as it is for humans defending their homes. Maize plants, like many others, protect themselves with poisons. They pump their roots with highly toxic insecticides called BXDs, which deters hungry mandibles. But these toxins don’t come free. The plant needs energy to act as its own pharmacist, so it distributes the poison to the areas that deserve the greatest fortification – its crown roots.

Maize seedlings grow roots either from the embryo itself (embryonic roots), or from the growing stem (crown roots). Christelle Robert found that the crown roots are especially important. They contain the most nutrients, and their loss matters more to the seedlings. As such, they receive the greatest investment of BXDs; they contain five times more of one particularly toxic compound called DIMBOA.

So, if plant-eating insects want to nibble on the most nutritious roots, they also swallow the highest amount of poison. Instead, they target the more lightly defended embryonic roots, which are less valuable to the plant. But the Western corn rootworm ignores these rules of engagement.

The larva of this beetle eats the roots of maize, corn and other cereals and it’s a significant pest that can ravage entire crops. Its success stems from its ability to turn maize’s defence against it. Robert found that the rootworm, unlike other insects, ignore the embryonic roots and head straight for the crown ones.

When Robert gave rootworms a mutant plant that couldn’t produce BXDs, it lost its interest in the crown roots. Rather than being deterred by the plant’s poisons, the rootworm actually uses them to track down the most nutritious meals. It’s a thief that uses the presence of tight security to find the buildings that house the richest loot.

The beetle is famed for its ability to rapidly evolve resistance to pesticides, and it certainly seems to be immune to the maize’s BXDs. In fact, it actually gains weight more quickly when it feeds on crown roots compared to other types. This ability could explain why the Western corn rootworm has spread throughout the maize-growing regions of the world, while other related beetles have been far more constrained.

Reference: Robert, Veyrat, Glauser, Marti, Gwladys, Doyen, Villard, Gaillard, Kollner, Giron, Body, Babst, Ferrieri, Turlings & Erb. 2011. A specialist root herbivore exploits defensive metabolites to locate nutritious tissues. Ecology Letters http://dx.doi.org/10.1111/j.1461-0248.2011.01708.x