How cotton leaf worm responds to the defence mechanisms of plants

A new side to the triangular story of interactions among plant, pest and predatory insect emerges as a result of a study carried out by researchers from the National Centre for Biological Sciences, Bengaluru. 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. “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,” says Radhika Venkatesan, in whose lab the work was carried out.

Natural triad

Take the example of the trio: cotton plant, the worm and the predator wasp Bracon brevicornis. 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.

The experiment consisted of spraying the cotton leaf worm with plant volatiles and observing its change in immunity levels. “We exposed the [worm] to various plant volatiles for different time durations, then we collected the blood for immunological assays,” says Enakshi Ghosh who is a postdoctoral fellow at NCBS and first author of the paper published in Journal of Chemical Ecology. “As we observed that specific volatiles could modulate their immune status, we challenged the cotton leaf worm with its natural enemies — pathogen and parasitoid,” explains Dr Ghosh.

Induced immunity

Six plant volatiles such as beta-ocimene and linalool were used in the experiment and each had different effects on the immune system of the cotton leaf worm. “In the case of beta-ocimene treatment, immune functions were enhanced that helped the worm combat wasp better.” says Dr. Ghosh. That is, being immune, the worm is not immobilised by the wasp’s sting. This prevents the wasp from laying its eggs on the worm. “In the case of linalool exposure, the worms survived better against bacteria,” she adds.

“It is interesting that beta-ocimene mediated immuno-modulation helped the herbivore [worm] survive better against parasitoid-like stress only, while linalool exposure caused increased survival against pathogen,” says Dr. Venkatesan. “The elevated defense caused trade-offs like reduced pupal size and adult life-span in these primed larvae compared to controls,” she adds.

Some of the questions that are raised by this study include - what happens when the worms are exposed to a mixture of plant volatiles, and whether this immuno-modulation is specific to Spodoptera litura . “It would be interesting to see if, in the race of co-evolution, there is any mechanism the parasitoids are building to combat heightened herbivore immunity,” says Dr Venkatesan.