Insect-plant ecosystem of the Sinai Milkweed

Sinai Milkwee, Mt Sinai

Sinai Milkweed

When driving through South Sinai you may be excused for thinking it a dull and lifeless place, but you would be wrong, it just needs a subtle shift of focus. Go and inspect any desert plant and you will find an alternative universe, thronged with insects of extraordinary design exploiting valuable resources, fighting turf wars and repelling predators. (Please go to the end of the blog to see a gallery of insects.)

Without plants, the desert would be a much kinder place! By trapping the sun’s energy in sugars and stocking water in their cells, plants become targets for attack by every bug or beast desperate for sustenance in the inimical harshness of the desert. Plants need to defend themselves. Some deploy a protective shield of thorns while others synthesize a lethal arsenal of toxic chemicals. Their survival is at stake.

The Sinai Milkweed, Asclepias sinaica, has opted for chemical warfare. When nibbled or slashed, it exudes a milky latex containing bitter, toxic alkaloids. Large grazing animals find them repellent and avoid the plant which is therefore left to grow freely in the mountain areas around Mt Sinai. Insects, however, are not so coy.

Weevil Paramecops, St Katherine Protectorate, Sinai

Weevil Paramecops

The clever weevil, Paramecops sinaiticus, for example, circumvents the plant’s defences by cutting through the leaf midrib to stem the flow of toxic sap then nibbles the leaves from the tips in impunity. It also deposits its eggs under the skin of the seed pods so that when the larvae hatch they can burrow inside and feed off the seeds. And when the time comes for the larvae to pupate, they tear off the silky tufts that feather the seeds and weave themselves cocoons!

A warning display Usherhopper grasshopper, St Catherine National Park Sinai

A warning display Usherhopper grasshopper

The weevil is careful to avoid the plant toxins, but not the Usherhopper grasshopper, Poekilocerus bufonius. It feeds off the latex with relish, absorbs the toxins into its body fluids and concentrates them in a specialised poison gland on its abdomen. When attacked, it goes into spasm, squeezing the poison over its abdomen and blowing it into a foam with air forced out through its spiracles. Most predators find the spectacle disgusting and never approach the grasshopper again. As a reminder and warning, the grasshopper displays conspicuous yellow spots on its sleek black body. Should the predator persist and eat the grasshopper, it will suffer vomiting, paralysis and possibly death. The toxins are concentrated enough to kill a cat.

Tiger Butterfly, St Katherine Protectorate, Sinai

Another warning display Tiger Butterfly

In an extension of the grasshopper’s strategy, caterpillars of the Plain Tiger Butterfly, Danaus chrysippus, feed off the Milkweed, sequester the toxins into their body fluids and retain them after morphing into butterflies. In this way, they remain unpalatable to birds throughout their life cycle. Like grasshoppers, they also need to give a conspicuous warning signal which they do by flaunting their golden spotted wings. And in a further example of insect nerve, they have jettisoned the normal butterfly flitter in favour of a slow, straight flight, giving birds plenty of time to identify them.

Eggfly Butterfly, Mt Sinai

Subversive mimic Eggfly Butterfly

Another butterfly, however, threatens to undermine the strategy. The Danaid Eggfly butterfly, Hypolimnas misippus, is not at all poisonous but has developed almost identical markings to the Tiger Butterfly and the same languid flight so as to deceive birds into thinking it’s poisonous. The birds, however, need to learn by trial and error whether the butterflies are offensive or not, and if there are too many innocuous butterflies with the same display as the toxic ones, the birds will be emboldened to attack them all indiscriminately.

Predator Ladybird with Aphid prey, Mt Sinai

Predator Ladybird with Aphid prey

Aphids also feed off the plants, sucking quietly on the sap. They are yellow with black legs, suggesting that they too are advertising their toxicity. Unfortunately for them, their most common predator, the ladybird, is immune to the toxins and oblivious to their display. The aphids, however, have another strategy to drive off the ladybirds. They can call upon ants to protect them in a mutually beneficial arrangement. The sap that aphids feed off is very rich in sugars which can cause osmotic problems for them. They therefore secrete the excess sugars out of their anuses and dump it onto the surface of the plant where it is known as honeydew. The ants like to collect this and will protect the aphids in return by biting or stinging the ladybirds.

Such is the story of one plant with its colonies of clever, toxic, deceitful and militant insects, all competing for survival in a wilderness so vast in comparison that they pass unnoticed by all but the most inquisitive of visitors to the National Park.

It is also a story of evolution where the struggle for survival promotes innovative design and astute behavior.

This blog first appeared as an article for the Maadi Messenger, Cairo, to promote an awareness of ecosystems in the St Katherine Protectorate, Sinai

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