Chemical Warfare Plants vs butterflies and animals


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Do the plants win? Do the butterflies win? Or is it a win-win situation? Do plants ‘call' butterflies? And what do elephants and butterflies have in common?


Plants vs butterflies and animals

Certain plants have the desired effect of keeping insects away, as gardeners in the know use a mixture of chillies and garlic as a spray-on insect repellent, telling these ‘pests' or ‘goggos' to bug- off. Some plants even use chemicals to ‘communicate' with each other.

During the mid ‘80s a study was done on kudu by Professor Wouter van Hoven. In the study area there were more kudu than the veld could support and several kudu were later found dead. A subsequent post mortem revealed saturated stomachs, yet the animals had died of starvation.

How was this possible? When a certain plant is browsed on, it emits chemicals that permeate the air, warning surrounding plants that a browsing kudu is present. The ‘warned' plants immediately raise their tannin levels. The hook thorn, Acacia caffra, raises its tannin levels in the leaves by up to 94 percent in only 15 minutes! This tannin level increases to a remarkable 282 percent after an hour.

Tannin makes the leaves bitter and also unpalatable. The tannin complicates the digestion of the leaf material to such an extent that it has a negative influence on the fermentation in the rumen and the animals starve, even though they have ingested sufficient foliage. Now the kudu browse very little on a single plant and tend to move on before the tannin level increases too much, unless the kudu are confined.

Is this chemical warfare? Perhaps only a polite warning

Strangely, browsing by elephant has the exact opposite effect; plants' tannin levels drop, suggesting that the plants can distinguish between different animals feeding on them. The reason plants may ‘prefer' elephant browsing on them, may be due to the fact that elephants only digest about 60 percent of their food. Elephant are often the catalyst for many plants' seeds to begin their sprouting process. Once the seeds have passed though the intestines of an elephant they are stimulated to germinate.

Some butterflies of the family Nymphalidae are also attracted to animal urine and faeces.

The current understanding is that these butterflies are supplementing necessary minerals and salts by sipping at the faeces. It has been found that a pheromone found in female Indian elephant urine is very similar to a butterfly pheromone. This pheromone is the fifth most used pheromone in about a hundred different butterflies and moths.

In fact the pheromone of a moth, Trichoplusia ni, with a similar pheromone structure to an Indian elephant approaching oestrus may be used to synthesise the elephant pheromone. Could it be that this similar and familiar smell is attracting the butterfly to the urine, reasoning it may be a female butterfly of the same species? Perhaps, once here, they forget their original intent and sip at the urine. Maybe a placebo effect.

The bitterness caused by a concentration of tannins may be unpalatable for some, but very attractive to others. Vernonia plants are known for their bitterness which is infused and used to treat stomach ailments and colic, yet their flowers prove to be a big hit as a nectar source when they are in season.

Is the flower bitter too? Perhaps not, because the syringa, Melia azedarach, has toxic berries, but the flowers are well visited during spring and seem to be non-toxic – albeit only to butterflies. Certain plants like the paintbrush, Castelleja indivisa, can compartmentalise or partition their toxins to certain parts of the plant – eg the leaves and fruit, where it does not want any parasites, and have no or very little toxins in the petals or nectar. The toxicity of syringa berries also seems to vary from tree to tree and area to area.

The butterfly larvae of some Lepidoschrysops species or ant-blues use pheromones or air permeating chemicals for their own means. The egg is laid on a flower and the emerging larva burrows or eats its way into the flower where it then eats the ovaries or young seeds. After moulting twice, the larva is in its third instar.

At this stage the larva emits a pheromone resembling the smell of the ant brood. Camponotus ants attending the flowers then find these ‘lost souls' and safely ‘return' them to the nests. Once in the ant nest the butterfly larvae devour the other ant larvae. It also pupates in the nest and after eclosion - emerging from the pupa - the wet-winged adult butterfly races to the surface.

The reason for racing is that it has now lost it's ‘ant smell' and is seen as a foreigner and perpetrator in the nest and the soldiers give chase. Nature has some amazing secrets that still astound man, and if we all learn something new, we can all win.



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