Plants trick moths by faking injury

Plants are the ultimate masters of defence. As they are unable to run away if being attacked, they have to find different ways to fight against their enemies. Many plants produce foul chemicals to ward off herbivores, while others produce structural defences such as thorns or dense hairs. Other plants recruit natural enemies of their herbivores through the use of a chemical signal called a synomone.

As we continue to learn more about the secret lives of plants, we discover more strange and wonderful strategies plants utilise to stay healthy. In the case of the rainforest plant species Caladium steudneriifolium a novel method of self defence has been discovered: pretending to be damaged.

You’re probably familiar with plants in the genus Caladium. Their tolerance to a bit of neglect, large leaves, and glossy sheen make them one of the world’s most popular house plants. Interestingly, another quality that makes these plants such a desirable addition to your home is essentially the same mechanism that allows the plants to deter predators: variegation.

Plants that are variegated have more than one type of genetic make-up within their tissues. This leads to differential leaf colour, causing white, or yellow  zones on some part of the plant. These unusual features can often be quite beautiful or interesting, and correspondingly plant breeders often propagate variegated varieties en mass. This gives us the interesting variegated varieties found throughout gardens worldwide.

In wild ecosystems, retention of genes causing variegation is a slightly different situation. Non-green zones on the leaf do not contain chlorophyll, and as  chlorophyll is required for plants to photosynthesise there is an associated fitness cost of having variegated leaves. However, in the wild – the species C. steudneriifolium exists in both variegated (1/3 of all plants) and uniformly green (2/3) types.

In 2009, a team lead by Sigrid Liede-Schumann noticed that variegation on the leaves strongly resembled damage caused by leaf mining insects. They noticed leaf miners were present in both morphs, and set out to test whether presence of a leaf miner was influenced by whether leaves were variegated or not. They expected that moths would preferentially select healthy leaves free of competition for their larvae to develop, selecting entirely green leaves over variegated leaves.

To test this, the team tagged 800 brand-new leaves near a car park in South Eastern Ecuador. These leaves were divided into four treatments shown in Figure 1 below. The team used either white correction fluid, or a transparent correction fluid thinner (shown in blue for clarity) to mark simulated variegations on the leaves. The team came back 3-months later and counted the number of leaves infested by leaf mining larvae within each treatment.

Post_2b
Figure 1: Experimental manipulation of C. steudneriifolium leaves

The team found infestation rates were 4-12 times higher in uniformly green leaves in comparison to variegated leaves. This was true regardless of presence of clear painted defoliation, suggesting mining moths use visual cues to select their host plant.

Alongside the direct cost of herbivory, when leaf mining insects emerge from the C. steudneriifolium leaf after completing development a wound in the plant is formed. Plant pathogens can opportunistically use this wound to infect  the host. By tricking moths into believing it is already in use, the moth passes up the ‘used’ leaf and seeks out a ‘fresh’ healthy host – which in reality, is more likely to be in use by the undetected larva of another moth than a leaf of the variegated plant. The variegated plant can then breathe a sigh of relief, enjoying it’s comparatively higher quality of life made possible by deception. A pretty clever strategy, even for a plant.

Reference:
Soltau, U., Dötterl, S., & Liede-Schumann, S. (2008). Leaf variegation in Caladium steudneriifolium (Araceae): a case of mimicry? Evolutionary Ecology, 23(4), 503–512. doi:10.1007/s10682-008-9248-2

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