Researchers have discovered a new side to the triangular story of interactions among plant, pest and predatory insect in cotton plants.
This is the first study showing the impact of plant volatiles on cellular immunity of the worm [S. litura], causing elevated defense against natural enemies.
The team finds a link between the boosting of immunity levels in pest (cotton leaf worm, Spodoptera litura) and exposure to plant volatiles, which are aromatic vapours released by the plant when the worm chews the leaves.
When the worm feeds on the cotton plant’s leaves, the leaves release aromatic and volatile vapours into the air. These volatiles waft in the air and attract the wasps, which harm the cotton leaf worm.
Though the adult wasp is an independent entity, the wasp lays its eggs on the skin of the worm, and when the eggs hatch, the larvae feed on the worm itself, thereby ending up killing it.
In order to make this possible, the wasp first injects a toxic substance into the worm which immobilises it, so that the wasp can take time to lay its eggs on the skin of the worm.
Another example of such a triangle is the Cabbage worm Plutella xylostella, and the wasp Cotesia vestalis. In this case, the wasp injects the eggs into the body of the worm and the eggs hatch inside and feed on the worm as they grow.
During research, it was observed that the worm obtains immunity from plant volatiles. The worm is not immobilised by the wasp’s sting and thus prevents the wasp from laying its eggs on the worm.
The elevated defense caused trade-offs like reduced pupal size and adult life-span in these primed larvae compared to controls.