Insect roles in nature


Text and photographs Marlies Craig or as credited

In the last article I started presenting the contents of a temporary exhibition that sadly fell victim to the pandemic. We covered the first four roles of insects in nature: pollination, seed dispersal, recycling and improving soil quality. In this article we continue the series, and look at roles 5: pest control, 6: weed control, and another role ‘X’ that goes beyond the content of exhibition – population control.

Role 5. Pest control – insects eat each other

What is a pest? A pest is something that directly harms our crops or livestock, or even us, something that is so numerous that we notice it and that does measurable damage. A lonesome caterpillar in the garden is not a pest.

Many insects that could become pests, never do, because enough of them get killed by their natural enemies. Natural enemies of insects include birds, lizards and other animals, but the most important natural enemies of potential pest insects are other insects. About a quarter of insect species eat other insects, thus keeping them in control.

Predators, such as the praying mantis, ladybird and assassin bug, catch their prey, then eat it up whole or suck it dry, depending on their mouth parts.

Flower mantis


Assassin bug

Aphids are plant pests that are found in many gardens. Numerous insects feed on aphids, such as this larva of a hoverfly, which looks like a caterpillar.

Parasitoids live inside their host’s body, like parasites, feeding on tissues and organs. They often start with non-vital organs, so that the host is able to continue to live, and even grow and develop. But ultimately the damage is so great that the host dies. The most important parasitoids are wasps and flies.

Parasitic wasp looking for a host.

This caterpillar has been eaten up from the inside by 100 parasitic wasp larvae. Now they are all coming out and pupating.

When an insect gets imported accidentally to another country it suddenly finds itself in a place where it has no natural enemies. With nothing to stop it from exploding in numbers and spreading everywhere, it can become an ‘invasive alien’.

Fall armyworm

Photo: G. Goergen

The fall armyworm (Spodoptera frugiperda) arrived in Africa in 2016. This voracious pest of maize and other crops is a threat to the food supply of millions of people. Two tiny parasitoid wasps, Trichogramma and Telenomus, lay their eggs on fall armyworm egg masses. The wasp larvae kill the eggs before they reach the larval stage, which is when they are most destructive to crops. Plans are being made to release these biological control agents.

Role 6. Weed control – insects eat plants

Plants are the producers, the foundation of the food chain. The environment depends on plants to prevent soil erosion, to assist in the nitrogen cycle, to be part of the water cycle, to produce oxygen, and most importantly, to be a source of food for others. Therefore, apart from their many other functions, plants’ main function is to get eaten.

Leaves that have been nibbled on are a sign that these plants fulfill their function in nature: they are feeding something, like this leaf beetle.

Most plants from foreign countries, when introduced, are not accompanied by their natural biological agents that keep them in check and in balance, as in their countries of origin. When they escape from where they were first introduced, they often outgrow and out-compete the indigenous plant life and become dominant and invasive.

This is on the South Coast not far from Durban. It may look like paradise but unfortunately it is completely overrun by invasive alien plants, such as Eucalyptus, Syringa, Ricinus, Canna, various vines, etc.

In this photo all the aliens are shown in red. If you are an insect, there is not much to eat here.

If there is no food for insects, there will be no insects and therefore no food for birds, bats, frogs, lizards, shrews, etc. Therefore there will be no snakes, owls, genets … or other insect-eating insects.

The wattles in the foreground, and all the other trees in this photo are invasive aliens.

Plants that don’t get eaten have an unfair advantage over those that do. If this carries on they can become invasive. Wattles for example, are perfect pioneer plants in Australia. They spring up, bind the soil, add nitrogen and support a whole range of local herbivores. Their seeds get consumed in large quantities by Australian insects, which then allows other plants to take their place. This does not happen in our country however. Here, they become invasive because none of our indigenous insects will eat them.

The seed feeding weevil from Australia, Melanterius maculatus, was introduced to control the wattle. As the wattle is a valuable commercial crop this biological control agent was specifically chosen as it only attacks the mature seeds and not the tree itself. Before its release, however, the weevil had to undergo stringent testing to ensure that it would only attack the targeted plant and not any of our indigenous plants.

At this point, after talking about weed control and pest control, there is another more sinister role that insects play in nature, one we prefer to ignore, and which did not feature in the museum exhibition: population control.

Role X. Population control – insects eat us

Insects do not just eat potential weeds and pests, they also eat us: us ‘higher’ vertebrates, us mammals, birds, reptiles and amphibians. They chew and nibble and suck on us and drink our body fluids. In the process – usually when they break through the germ barrier of our skin – they can transfer diseases. Though we do our best to work against this powerful evolutionary force, insects, and the organisms they transmit, are a formidable force that have until recently worked together to help keep the balance in nature.

Modern medicine is one of the key reasons why the human population has quadrupled in just 100 years, (since World War I), recently reaching 8 billion, after probably remaining below 200 million for millennia. We all know full well that humans are the root cause of so many planetary ills. But I do not believe in ‘letting nature run its course’ nor that people have to suffer and die in order to re-establish a healthy balance on earth. As an epidemiologist, my day job involves promoting human health and medicine. It is just that we have to find a way to survive and thrive, without thereby causing the demise of the other species on earth (and with it ourselves, for we owe our existence to them). This is not impossible, but we would need to stop feeding our addiction for ‘more more more’. It is our excessive consumption and greed that is killing earth, not the fact that we exist. But, this is a topic for another time.

For this issue, all I will do is present some of the culprits… or shall we call them ‘secret agents’? They are scattered across just a handful of insect orders.

Order Psocodea (lice)

Three louse species live on humans: the body louse, the closely related head louse, and the crab (or pubic) louse. 3000 other species of lice plague almost every kind of mammal and bird. Lice transmit epidemic typhus, also called camp fever, jail fever or war fever. It ravaged Europe repeatedly since the Middle Ages. At the end of World War I millions of people died of typhus in eastern Europe. Typhus is caused by Rickettsia bacteria, that live in the louse’s gut, and spread through louse faeces.

Human head louse

Human lice may thrive in dirty, overcrowded conditions typical of army camps and jails, but they also jump happily from the head of one squeaky clean and well-groomed Durban school child to the next. I got quite excited when my son came home with the lice from school one day. This gave me a chance to take lovely photos and videos of them, running along hairs, feeding, and doing their thing.

Persuading them that it wasn’t going to be a lasting arrangement, took some effort.

This is why a louse comb works.

Order Hemiptera, or ‘true’ bugs

The bite of a bed bug is painless, though infestations can cause itchy welts, allergic rashes and major discomfort, and they are considered an important public health concern.

Photo: CDC/PiotrNaskrecki

The global bed bug population is increasing by 100-500% per year. Even though over 45 different pathogens (disease-causing organisms) have been found inside bed bugs, there is no evidence that they actually transmit disease. This is very strange, however very fortunate for us! Do they contain some neutralising factor that disables the germs perhaps?

The kissing bug from South America on the other hand transmits Chagas disease, a type of sleeping sickness.

Photo: Erwin Huebner

Millions of people are suffering from Chagas disease, and about 20 000 die from it every year. Charles Darwin described how he was bitten by kissing bugs, and it is possible that Chagas disease contributed to his ill health and death.

Order Diptera

Flies are a hugely diverse group, with over 120 000 known species. Flies, as a group, have perhaps the most varied feeding habits of all insects. They fulfil all the important and valuable roles in nature covered in the insect exhibition. Only a tiny fraction of species harm humans in any way whatsoever. But these kind of make up for it, giving insects a bad name.

The worst offenders are the bloodsucking flies, especially the mosquitoes. Females mosquitoes need a blood meal for their eggs to develop.

Anopheles mosquitoes (the mottled ones) transmit malaria, filarial worms and some viruses. Malaria remains one of the deadliest infectious diseases in the world, with over half a million deaths per year.

Photo: CDC/James Gathany

Aedes mosquitoes (the black and white ones) also transmit filarial worms, plus haemorrhagic fevers such as dengue, yellow fever, West Nile virus, Rift Valley fever and others, such as the Zika virus. Dengue is spreading due to climate change.

Culex mosquitoes (the plain black ones) transmit Japanese encephalitis (in Culex) as well as other viruses.

Lymphatic filariasis, an awful condition otherwise known as elephantiasis, is caused by filarial worms (Wuchereria and Brugia), transmitted by a wide range of Anopheles, Aedes and Culex mosquitoes. The filarial worms block the lymphatic system, causing limbs and other body parts to swell up grotesquely, the skin to change colour and to crack.

Tsetse flies transmit African sleeping sickness, caused by a single-celled parasite (Trypanosoma) that invades the brain and nerves, causing sluggishness, coma and death.

Sand flies transmit Leishmania, the second deadliest parasite after malaria.

Photo: CDC/Frank Collins

Leishmania, a single-celled creature, breeds in the part of white blood cells that is needed to digest and kill invaders. This disease starts with skin sores (’white leprosy’) but then invades the rest of the body to become ‘kala azar’ (‘black fever’ in Urdu). This disease is already mentioned in cuneiform writings of ancient Nineveh.

Black flies (genus Simulium) transmit a filarial worm called Onchocerca, which causes ‘river blindness’. It starts with skin rashes, lumps and ‘lizard skin’, but the adult worms often settle in the eyeball.

There are also some really nasty parasitic flies that quite frankly eat us alive. The tumbu fly (Cordylobia anthropophaga meaning ‘man-eater’) is notorious for laying its eggs on wet washing hanging on a line. The hatched maggots burrow into human skin, where they cause awful boils. If they can, people in tumbu fly regions iron all their clothes, including socks and underwear, to kill the eggs.

Bot fly larvae parasitise humans, pets and livestock.

Photo: Geoff Gallice

The maggots burrow into the skin and breathe through an air hole, leaving a painful, oozing wound

Horse flies feed on farm animals, and bite people too. They transmit a few animal diseases.

Even the ubiquitous house fly is not innocent. It feeds and breeds in dung, carrion and garbage, and then comes indoors and walks, vomits and poops all over our food.

House flies (Musca domestica) can spread intestinal diseases like typhoid fever, dysentery and cholera, intestinal worms, salmonella and various eye infections – if it picks them up somewhere.

Order Siphonaptera (fleas)

Fleas puncture the skin with a blade-like tooth, inject saliva to prevent the blood clotting, then suck up the blood oozing out of the puncture wound with a siphon (a straw-like appendage).

Fleas transmit the plague (Yersinia pestis). The first widespread outbreak of plague of which we can be fairly certain occurred in the 500s AD. The worst pandemic, the ‘black death’, started around 1334 in eastern Asia. Within five years the plague spread across greater Europe, killing two out of three people. Plague epidemics continued to sweep across the world for the next 400 years. Millions died. Thankfully with the help of the microscope, scientists discovered germs and penicillin. So nowadays plague is just another bacterial disease that can be treated with antibiotics. Thank God for modern medicine!

Nowadays it seems we have less to fear from these insects, than from conspiracy theories about life-saving medical technologies. Public health has a hard time persuading some people to give up incorrect beliefs, for example about the danger of vaccines, the origin of viruses such as HIV, Ebola or Covid-19, or about big pharma abusing or withholding medicine. Conspiracy theories (attributing things to the actions of small, powerful groups with nefarious intentions) are not always false, because occasionally small powerful groups do indeed conspire to do evil things. But “for the most part,” says this article “conspiracy theories rely on sloppy thinking,” though they often have “some level of plausibility”. They spread by rumour. Apparently they are a universal phenomenon, because ‘gossip’ is hardwired into our brain – like homing in pigeons, or rolling balls in dung beetles. I won’t go on about it, but I will say this: anyone who has ever worked with infectious diseases, feels particularly despondent about this self-destructive quirk of the human mind.

Ultimately, what drew my attention as a university student, what attracted me to further study, was not my passionate love of insects (which has endured unscathed and intensified), but the disconcerting impacts of “insect role X”.

Author photo: Pat McKrill

About the author

Marlies Craig is an epidemiologist by training. She has worked for the Medical Research Council, then more recently for the United Nations’ Intergovernmental Panel on Climate Change, and is now employed at the WITS Reproductive Health Institute in the Climate Change and Health unit. Though she did originally study Biology and Entomology, her love affair with insects is very personal. In her book What Insect Are You? – Entomology for Everyone, she shares that passion with young and old, see She hopes to kindle in people of all ages enthusiasm and a deeper appreciation of nature and show them why and how they can make a difference. She also started a non-profit organisation called EASTER Action which hopes to promote awareness and action on biodiversity, climate change, and sustainable living, see


About Andrew Carter

Andrew Carter worked as Museum Officer at eThekwini Municipality for 34 years and was responsible for many of the displays at the Durban Natural Science Museum.