Ranger Report - March 2011

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Madikwe is a place of captivation, beauty and mind blowing tranquillity. To live in a place like this is not only an honour but it also installs conscientiousness. With summer still in full swing (although late summer) it is difficult to look in any direction without seeing a delicate little flower or some sort of perfectly created little insect crawling around on it. Then you hear the eerie rumbling and the bushes spread apart as a big grey behemoth passes by only to kick up a myriad of even more life that the frail human eye failed to perceive at first. All, big and small are designed and created by the creator to fulfil a specific purpose and to play a specific role in this circle of life and the attention to detail is inconceivable.

The other day I walked past a very tall Panicum maximum (Guinea grass). I noticed these little black things sitting on the inflorescence. On closer inspection I saw that the poor grass was absolutely covered in aphids or plant lice. This immediately booted me back 10 years and I could almost relive the lecture during my entomology studies. That is why I decided to talk a little bit more about them.

There are about 4 000 species of aphids in the world of which about 250 are serious pests to humans. Twenty-five percent of all plant species are infested with aphids, and though it is believed that the speciation of aphids has followed that of plants not all groups of plants are equally parasitised. Aphids are often parthenogenetic for part or all of their lives and both viviparous and oviparous at different times of year. Parthenogenetic (asexual) reproduction generally results in viviparity (live birth) and all aphids are both parthenogenetic and viviparous during spring and summer. Some aphids which are parthenogenetic during the summer produce sexual offspring (males and females) in autumn this is termed ‘cyclical parthenogenesis’ or a ‘holocyclical’ life cycle. The sexual males and females mate in the autumn. Sexual females, like asexual ones have 2 sex chromosomes i.e. XX. Males have only one sex chromosome i.e. OX. In theory this means males could produce sperm with either no sex chromosomes i.e. an O, or one sex chromosome i.e. an X. However, in reality sperm with an O sex chromosome degenerate very rapidly and never contribute to an embryo. This means that all offspring of a sexual mating must have XX as their sex chromosomes, because females always contribute an X chromosome, and therefore all aphids resulting from sexual matings are female. Ova within a viviparously reproducing female start to develop immediately after ovulation, this occurs long before birth (even human females are born with all the ova they will ever need throughout their life, though they remain undeveloped for many years.) This means that an embryo can exist inside another larger and more mature embryo. In fact a newly born summer aphid can contain within herself not only the developing embryos of her daughters but also those of her grand-daughters which are developing within her daughters. Parthenogenesis combined with this ‘telescoping of generations’ gives aphids an exceedingly rapid turn-over of generations meaning they can build up immense populations very quickly.

Aphids feed from the phloem of plants which they tap into with the stylets of their proboscis. They gain access to the phloem vessels from 3 main parts of the plant, stems, leaves, and roots. Their stylets, which are contained within the proboscis when the aphid is not feeding, are very thin and could suffer damage while being pushed into the plant or bend in an unwanted direction. Therefore aphids secrete a special liquid from the tips of their stylets which starts to harden as soon as it leaves the stylets forming a hard protective sheath around the stylets as they are slowly pushed into the plant in search of the phloem tubes. When the stylets reach a phloem tube the aphid injects saliva into the plant cell. It is suspected, but not known for sure, that this saliva helps prevent the plant cell from sealing the puncture (i.e. the aphid?s mouthparts) with special proteins. These special proteins are the plants normal defence mechanism. They are deposited on the wall of the cell around a puncture as a result of the drop in redox potential that occurs along the cell wall following puncture damage. Aphids insert their stylets slowly and it takes quite a bit of time to tap into a phloem tube, it can be anywhere from 25 minutes to 24 hours from starting to insert the stylets to actually getting something to eat. Once they penetrated the phloem of the plant the aphid starts to suck the sap. They now encounter another fundamental problem. All plants have water pressure known as ?turgor pressure? and in most species it is much more than the aphid?s body can stand. So they have developed a very cunning way of solving the problem. In a Lehman?s terms the hind gut and fore gut is connected with the true stomach and intestine making a loop in between this connection. This allows the aphid to let some of the plant sap through for digestion in the stomach and intestine while the excess is bypassed through this special connection. The excess is then secreted in droplet form that we all know as ?honey-dew?. This sticky, sugary droplet sometimes fall to the ground and that is what you sometimes find on your car after stopping under a tree.

Honey-dew is the reason that ants are associated with aphids. In fact many species of ants are so addicted to this sweet drink that they will protect the aphids from various predators and move them to new plants if the one they are on starts to wilt. Some ants even go as far to build small shelters over species that feed near the base of the plant and or to keeping root-aphids inside their own nests. Ants gain much nutrition from their relationships with aphids and the honey-dew excreted by aphids. Generally the amount of proteins and amino acids excreted in honey-dew varies between species and plant.
Most of us think as aphids as an unwanted pest in our garden, me too (which they are), but I hope I could open up a bit more on the world of these insects just to illustrate the complexity and utter attention to detail that we are surrounded by (nothing is left to chance).
Further to report I am happy to say that all is well here in Madikwe. We are now in late summer and autumn is around the corner. There is already a chill in the morning air and the jackets are out. It warms up quickly though and by noon the air conditioners are blasting away.

The wildlife is in great condition and with the high rainfall we had over the last couple of months the grass is standing six feet tall in some places. This resulted in happy and somewhat overweight members of the herbivore family. Most of the dams are full and it should take us through to the next rainy season.

A certain lion pride we call the Dipelo pride have taken great fancy to our new airstrip. We were there the other night with them when they decided to have a go at some wildebeest that also stay on the airstrip during the night. Silently and in darkness we followed them as they crept up to their unsuspecting quarry. When they were about 100m away they all fanned out into different directions. The youngsters stayed on the runway while others melted into the dark bush like phantoms. We slowly followed the adults leaving the youngsters on the runway in hungry anticipation. We only got fleeting glimpses of the adults as burning amber eyes caught our lights from time to time. When they got close we all switched engines of and all lights were turned off. The darkness was thick like soup and the sounds of a multitude of insects reverberated in our ear drums. Then all of a sudden the sounds of the insects where extinguished in a frenzy of pounding hooves that sounded more like a freight ship running aground. Slowly you could hear the stampeding hooves fade into the abyss and we waited anxiously for the cry of distress. Nothing happened though. We turned on our lights again and found the big female standing in the middle of the runway staring at a disappearing dinner. I am not sure what happened, but I think that excitement got the better of the lot and they launched their attack to early or they were spotted by a life saving eye. None the less it was still a memorable experience for all of us.

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Until next time

Regards
From the Madikwe Hills Team
Jaco Becker
Head Ranger

Ranger Report – March 2011