🌿 Insects, Mites and the Diseases You Can Still Beat

Cassava is not an African plant. It is a New World crop, brought across the Atlantic centuries ago, and for a long time it grew in Africa with a great advantage: it had left its specialist pests behind. That advantage ended in the early 1970s, and the story of what happened next is the most important lesson in African crop protection.

The cassava mealybug, Phenacoccus manihoti, is native to South America. It was first observed in Africa in the early 1970s, in what was then Zaire, now the Democratic Republic of Congo, and in the Republic of Congo. Nobody planned it. It arrived, almost certainly on planting material, into a continent where nothing had evolved to eat it.

That last point is the whole mechanism, so make sure it lands. In its home range a pest is held down by a web of predators and parasitoids that evolved alongside it. Move the pest without that web and its population is released. It does not grow steadily; it explodes. And that is exactly what happened. By 1995 the cassava mealybug had reached essentially every cassava-growing area of the continent except Uganda.

What it does to the plant. Mealybugs are sap feeders that congregate at the growing tip. They look like small soft insects covered in a white, waxy, cottony coating, clustered in the folds of the youngest leaves and around the shoot apex. Their feeding, and the toxins in their saliva, cause the shoot tip to bunch up into a tight, distorted cluster of small deformed leaves, a symptom often described as bunchy top. The internodes shorten so the stem stops extending. Leaves yellow and drop. In heavy attack the plant is stunted, defoliated and produces little root.

And the roots are the point. A plant that loses its canopy stops filling roots. Reported yield losses reached up to 80 percent when the pest first arrived in a given area. That is the explosion phase. Over roughly five years the losses fell naturally, as farmers adapted and as indigenous generalist predators partially adjusted, to roughly 40 percent in highland and savanna zones and 20 percent in rainforest zones. Read those figures carefully: falling from 80 to 40 percent is not recovery. A chronic 40 percent loss, year after year, across a continent's main food-security crop, is a catastrophe that simply stopped making headlines.

Now look at the field diagnosis, because you need to tell mealybug from the virus problems in Module 7. Mosaic disease gives a mottled pattern of pale and normal green across the leaf blade, with distortion, and it is present on the plant from sprouting because it came in the cutting. Mealybug gives you visible insects, white cottony masses you can see and touch, damage concentrated at the growing tip, and a plant that was growing normally and then stopped. If you can see the insect, you are looking at a pest problem, and pest problems have a completely different set of tools available.

One more diagnostic habit worth building now. When you find mealybug masses, look closely. Among the white waxy insects you may see small, hard, swollen, brownish bodies. Those are mummies, mealybugs killed from inside by a parasitoid wasp. Finding them means the biological control described in the next lesson is working on your farm. It is the single most useful thing an unaided eye can tell you about mealybug management, and almost nobody looks.

What should you not do? The instinct is to spray. Understand what spraying costs you here. Broad-spectrum insecticide kills the parasitoid wasp and the generalist predators as readily as it kills the mealybug, and because the pest reproduces faster than its enemies recover, spraying can leave you worse off in the following weeks than if you had done nothing. That is not an argument that no insecticide ever has a role. It is an argument that in a system where a highly effective natural enemy is already established, an untargeted spray is often working against you. Any product, dose and safe-use decision is set by your national plant-protection or agrochemical regulator, and this course names no product.

What you should do is the cheap, unglamorous work: take cuttings only from clean, pest-free mother plants, because mealybug travels on planting material exactly as the viruses do; inspect your growing tips as a routine, not as a panic; and destroy heavily infested planting material rather than carrying it to a new field.

In the late 1970s Africa faced a continent-wide crop emergency with no chemical answer. Nobody could spray 9 million hectares of smallholder cassava. What happened next is the reason this lesson exists.

Entomologists went back to the mealybug's home range in South America and looked for what eats it there. They found a small parasitoid wasp, Anagyrus lopezi, formerly named Epidinocarsis lopezi. The work is strongly associated with IITA's biological control programme, led in the literature by Hans Herren and colleagues, though this course could not independently retrieve a dated primary programme history and does not present the leadership chronology as verified.

How the wasp works. A female wasp finds a mealybug and lays an egg inside it. The larva develops within the living mealybug and kills it, then emerges as a new adult wasp to find the next one. That is what a parasitoid is: not a predator that eats and moves on, but an insect that develops inside a single host and destroys it. The crucial property is that the wasp's population rises and falls with the pest's. When mealybugs are abundant the wasp multiplies; when mealybugs are scarce the wasp becomes scarce too, and neither is eliminated. The system settles at a low pest level and stays there without anyone paying for it again.

That is called classical biological control: introduce the natural enemy of an introduced pest, once, and let it establish permanently. It is the opposite of spraying, which must be repeated, paid for every season, and delivers nothing to the neighbour who cannot afford it.

The programme. Mass-rearing began and releases ran from 1981 onward, at roughly 150 release sites across some 20 countries, eventually reaching adoption in 29 African countries. A farmer in a village who never heard of the programme received the benefit anyway, because the wasp flies.

The results, and they are sourced. By the end of the release programme the parasitoid had brought the pest under control in an estimated 95 percent of fields. A technology catalogue summary puts the reduction in mealybug populations at roughly 90 percent. The effect is reported as permanent, present year-round, across all ecological zones tested except dry areas.

Now the economics, and this is where people stop believing the page. A cross-country economic assessment covering 27 African countries, representing 94 percent of continental cassava production in 1995 across roughly 9 million hectares, estimated total benefits of US$9.4 to 34.7 billion over 40 years, from 1974 to 2013, depending on the pricing scenario used. The total programme cost was estimated at US$46.9 million, discounted at 6 percent. Dividing the one by the other gives a benefit-cost ratio estimated at 199 to 738. Every US dollar spent is estimated to have returned somewhere between roughly 200 and 740 dollars.

Per hectare, the same analysis gives roughly US$26 of annual benefit against roughly US$5.20 of one-time per-hectare programme cost.

Why are those ratios so extreme? Because of the structure, not because of a mistake. A one-time cost bought a permanent, self-reproducing, continent-scale benefit that keeps delivering every year with no further payment. Nothing in ordinary farming looks like that. A fertiliser must be bought every year. A spray must be bought every season. A wasp that establishes itself is bought once and works forever.

But be careful with the figures anyway, and be careful in exactly the right way. These are ex-ante and long-horizon economic models, not audited cash accounting. They rest on assumptions about what cassava was worth, how much yield was actually saved, and what would have happened without the programme, and none of those can be observed directly. The right way to hold them is: the direction and the order of magnitude are strongly supported and widely reported, and the third decimal place means nothing.

What does this mean for you as a grower? Three practical things.

First, in most of Africa this control is already established and free. You do not need to buy anything. You need to not destroy it, which mainly means not blanket-spraying broad-spectrum insecticide across a field where the wasp is working.

Second, you can check whether it is working. Look for the parasitised mummies described in Lesson 1. If mealybug numbers are high and you find no mummies, that is worth reporting to your extension office, because it may mean the parasitoid is absent or has been knocked back locally.

Third, and most important for the rest of this course: this tool exists because the pest is an insect. You will see in Lesson 3 that the same approach worked against the green mite. You saw in Module 7 that nothing like it exists against a virus. When somebody offers you a biological solution to a problem, the first question is always what kind of organism the problem is.

The cassava green mite is the mealybug's story told a second time, with a different animal and the same shape. It matters both because the pest is real and because the repetition proves the first success was not luck.

The pest is Mononychellus tanajoa, with a related species M. progresivus also reported. Like the mealybug it is an accidental introduction from South America, and it began causing serious crop damage in Africa from the 1970s and 1980s.

What it does. Green mites are tiny, far smaller than mealybugs, and most people never see the animal itself without a hand lens. What you see is the damage, and it starts at the top of the plant on the youngest leaves. Look for fine yellow or whitish speckling, a stippled or mottled appearance on the young leaves, leaves that look dusty, dried and reduced in size, and in heavy attack a candlestick appearance where the growing tip loses its leaves and the bare stem stands up. Damage is worst in the dry season, which is exactly when it is easiest to mistake for drought stress.

That confusion costs farmers real money, so learn the distinction. Drought stress affects the whole plant fairly evenly, and older leaves wilt and drop first. Green mite damage starts at the youngest leaves at the top of the plant, gives a fine speckled stippling rather than a general wilting, and if you look at the underside of a young leaf with a hand lens you may see the mites themselves. If you have no lens, tap a young leaf over a sheet of dark paper and watch for tiny moving specks.

The losses, sourced. Reported yield losses in the absence of control range from 10 to 80 percent across agronomic trials in different African countries, with root yield losses reaching up to 50 percent in the complete absence of control measures in one synthesis. A multi-country survey across seven eastern and southern African countries found average losses of 10 to 30 percent, recorded 12 months after planting. Potential yield gain from control is cited at at least 30 percent under optimal planting conditions.

Notice the shape of those figures. The trial range, 10 to 80 percent, is much wider than the survey average, 10 to 30 percent. That is not a contradiction. Trials are often designed to expose the worst case with no control at all, while a survey measures what farmers actually experience in mixed real conditions. When you are handed a loss figure, ask whether it comes from a controlled trial or a field survey, because those two things answer different questions.

The solution followed the mealybug template. A South American natural enemy was identified and introduced, in this case a predatory mite, Typhlodromalus aripo, with a related species, T. manihoti, also used in some release programmes, including in Ghana from 2000 and 2007 onward. A predatory mite is not a parasitoid; it simply hunts and eats the pest mite. But the economic logic is identical, because it establishes, reproduces and persists.

The Ghana programme gives you concrete numbers. Approximately 2.3 million predator mites, 2,344,817 to be exact in the source, were released between 2007 and 2012, across seven regions and roughly 70 districts, averaging 23 communities reached per year.

The reported economics of that Ghana programme are extraordinary even by biological control standards: a benefit-cost ratio of 5,393.74 and an internal rate of return of 3,424 percent at a 20 percent discount rate over the period 2006 to 2046, falling to a benefit-cost ratio of 618.17 at a 50 percent discount rate.

Hold those exactly as you were taught to hold the mealybug figures. They are from an economic model with a forty-year horizon and a stated discount rate, not a cash ledger. The fact that the ratio drops from 5,393 to 618 simply by changing the discount rate assumption tells you everything you need to know about how sensitive such models are to their inputs. What the numbers genuinely establish is that a one-time introduction cost against a permanent continental benefit produces returns that no ordinary farm investment can match. What they do not establish is any precise figure.

What do you do as a grower? Much the same as for mealybug. Do not blanket-spray, because acaricides and broad-spectrum insecticides kill predatory mites efficiently and the pest mite rebounds faster than the predator. Keep the crop otherwise healthy, because a well-nourished, well-weeded plant tolerates mite damage better than a stressed one. Take cuttings from clean plants. And ask your extension office whether Typhlodromalus predators are established in your district, because if they are, your job is mainly to leave them alone.

Cassava bacterial blight, CBB, is caused by the bacterium Xanthomonas axonopodis pv. manihotis, reclassified in some literature as X. phaseoli pv. manihotis. It was originally discovered in Brazil in 1912 and has since travelled with cassava cultivation to Latin America, sub-Saharan Africa, Southeast Asia, India and the Pacific.

What it looks like. Bacterial blight usually shows first as angular, water-soaked spots on the leaves, often limited by the leaf veins so the spots have straight edges rather than round ones. The spots enlarge and merge, the leaf blights and dies, and the plant defoliates. On the stems and petioles look for dark streaks, wilting of the shoot, dieback of the growing tip, and a gum or exudate on the stem surface. Whole-plant wilting in a patch of the field, often spreading in the direction of rain or of your own walking path, is a strong sign.

How it spreads, and this is the practical heart of it. Rain splash. Wind-driven splash. Contaminated tools. Foot traffic. And, critically, contaminated seed and stem cuttings used as planting material. Four of those five are things happening on your farm, under your control, with your own hands and feet.

Now the figure that should change your behaviour. A single infected transplant is reported to cause a 30 percent yield loss in the first growing cycle, with cumulative damage reaching up to 80 percent loss by the third growing cycle where the disease is left unmanaged and infected material continues to be replanted.

Work through what that means, because it is the whole logic of the disease. One plant. First season, 30 percent of a field's yield gone. You then take cuttings from that field, as everyone does, and plant them next season. Now more plants start infected. Third cycle, up to 80 percent. The disease did not become more aggressive. You multiplied it, by doing the normal thing, taking cuttings from your own field.

History confirms the scale. Zaire lost a reported 75 percent of tuber yield in an early-1970s outbreak. Brazil lost a reported 50 percent in a 1974 outbreak.

The reported control measures are all things a smallholder can actually do.

1. Prune and destroy infected tissue. Cut it out, carry it off the field, burn or bury it. Do not leave it lying.

2. Use certified planting material, or at minimum visibly disease-free material from plants you have inspected yourself. This is the same instruction as Module 7 gave for the viruses, and the fact that it keeps recurring is the point: most of cassava's worst problems arrive in the cutting.

3. Crop rotation with roughly a six-month non-host interval. That means ground going out of cassava for about six months, planted to something the bacterium cannot live on.

4. Tool sanitation between plants and between fields. Your machete carries the bacterium from a diseased stem to a healthy cut surface as efficiently as any rain splash. Clean the blade. And do not work through a wet crop if you can avoid it, because wet foliage moves the bacterium onto your clothes and legs and then along the row.

5. Hot-water treatment of cuttings. This is worth its own paragraph. Hot-water treatment at 60 degrees Celsius is reported to show no sign of bacterial survival in one cited trial. That is a specific, low-cost, non-chemical treatment for planting material.

But read this next warning carefully, because it is exactly the kind of place where courses go wrong. This course also cites a different hot-water treatment, 49 degrees Celsius for 49 minutes, used against bacterial and vascular pathogens in the planting-material literature. Those are two different temperatures, from two different sources, aimed at different pathogen targets. Do not merge them into one universal cutting-treatment recipe. The available material gives a temperature of 60 degrees for the bacterial blight trial but this course does not have a verified immersion time to pair with it, and it will not invent one. If you intend to use hot-water treatment, get the temperature and the duration together, for your target pathogen, from your extension service or national research institute, because treating too hot or too long kills the cutting and treating too cool or too briefly leaves the pathogen alive.

One more reported result worth knowing, and worth being cautious about. A biological seed treatment using the bacteria Pseudomonas fluorescens and P. putida is reported to have produced a 2.7-times yield increase in one cited trial. That is a striking result from what reads as a single trial. Treat it as promising reported evidence, not established practice, and do not spend money on a product on the strength of it without asking your extension service what is registered and validated in your country.

Cassava anthracnose disease, CAD, is caused by the fungus Colletotrichum gloeosporioides f.sp. manihotis. It is a serious stem and defoliation disease across the humid, high-rainfall zones of West and Central Africa, and it has one characteristic that shapes everything about managing it.

The literature describes it as a weak pathogen that generally needs a wound to enter the plant. Mechanical damage or insect feeding opens the door. Understand what that means practically: anthracnose is largely a consequence of other damage. A field with heavy insect feeding, careless machete work, or animal and human traffic breaking stems is a field offering the fungus thousands of entry points. Reducing wounds reduces the disease.

The symptoms. Look for circular sunken leaf spots, reported at 10 to 30 mm in diameter. On stems look for pale brown cankers, dieback of the shoot tip, and stems that become brittle and deformed. In susceptible varieties the plant defoliates. Humid, wet conditions favour it, which is why it is a rainfall-zone problem and why planting timing matters.

The historical severity figures show it is not a minor complaint. In Zaire in 1975, 90 percent of local cultivars in one assessment were rated as severely affected. In the Congo in 1984, over 80 percent of plants were severely infected in certain regions. It has been recorded in Zaire, Nigeria, Mozambique, Angola, Tanzania, Uganda, the Central African Republic, Madagascar, Ghana and Cote d'Ivoire.

But the figures that matter most to your business are about planting material, and they close the loop this whole module has been drawing. Severely infected stems show germination rates of only 40 to 60 percent, and infected planting material is associated with a 50 to 75 percent reduction in cutting viability.

Think about what a 40 to 60 percent germination rate does to your field. You plant 10,000 stakes expecting a full stand at one metre by one metre spacing. Roughly four to six thousand come up. You now have a field with enormous gaps, which means light reaching the ground, which means weeds, which Module 7's companion agronomy makes clear are worth 30 to 50 percent of root yield on their own. You paid full price for planting material, full price for land preparation and full labour for planting, and you got a half-empty field. Then you spend the season fighting weeds in the gaps. The anthracnose did not just reduce your plant count. It multiplied your costs.

That is why a viability problem in planting material is a business problem, not a plant-health footnote, and why inspecting stems before you cut is worth more than almost any other half hour you spend.

The reported control methods are all cultural, and all available to a smallholder.

• Crop rotation, taking ground out of cassava.
• Removal of infected crop debris, so the fungus has nothing to overwinter in.
• Fallowing.
• Adjusting planting time to avoid peak humidity periods, which is a real and often overlooked lever. If you can shift planting so that the vulnerable early growth is not sitting in the wettest weeks, you reduce infection pressure without buying anything.
• Copper-based fungicides, with emerging interest in neem-based alternatives. Any specific fungicide product, its registration and its dose is set by your national regulator, and this course names no product and no rate.
• Resistant cultivar breeding. The source names breeding lines TME 30001 and 30211. Treat those as an example of how breeding lines are named, not as a current variety recommendation for you. As Module 7 established, current recommendations come only from your national root-crop institute.
• Quarantine and disease-free selection of planting material.

Notice that last one appearing yet again. Across this module and the last, four separate problems have pointed at the same intervention. Mosaic moves in cuttings. Brown streak moves in cuttings. Bacterial blight moves in cuttings. Anthracnose degrades cuttings so badly that half of them fail. Mealybug travels on cuttings too.

That convergence is the single most useful conclusion in the entire pest and disease section of this course. If you had to choose only one intervention and abandon the rest, choose clean, inspected planting material. It works against more of cassava's problems simultaneously than anything else you can do, and it costs attention rather than money.

One last practical instruction. When you cut stakes, look at every one. Reject anything with sunken cankers, brown lesions, brittleness, borer holes, white cottony masses, or discoloured tissue at the cut face. Rejecting a doubtful stake costs you one stake. Planting it can cost you a season.

You now know six problems: mosaic disease, brown streak disease, mealybug, green mite, bacterial blight and anthracnose. Trying to remember six separate management plans is hopeless. Instead, learn the structure, because these six sort into groups and the group decides the tools.

Start with the divide that matters most.

The two arthropod pests, mealybug and green mite, have documented, spectacularly cost-effective biological solutions already deployed continent-wide. Somebody found their natural enemy, released it, and it stayed. For most African growers that control is already present, already free, and requires nothing from you except that you do not destroy it with untargeted spraying.

The two viral diseases, mosaic and brown streak, and the bacterial and fungal diseases, blight and anthracnose, have no such answer. They currently rely on host-plant resistance breeding where varieties exist, clean planting material, sanitation, and rogueing to slow spread. Those tools are slower, less centrally coordinated and, for brown streak and bacterial blight in particular, incomplete in several countries.

Why the asymmetry? Because you cannot release a biological control agent against a virus, a bacterium or a fungus the way you can against an insect or a mite. Classical biological control needs a target organism with a specialist natural enemy that can find it, attack it and reproduce on it. A virus sitting inside plant tissue is not that kind of target. This is not a funding failure or a lack of effort. It is the biology.

So, the diagnostic question you ask first, every time, is: what kind of organism is this?

• If you can see the animal, insect or mite, you have a pest problem. Biological control may already be working. Look for evidence of it, protect it, avoid blanket spraying, and take clean cuttings.
• If the symptom pattern is systemic in the plant from sprouting, with mottled leaves or hidden root necrosis, you have a virus. There is no cure. Your tools are clean material, rogueing, and variety choice from your national list.
• If the symptoms are angular water-soaked leaf spots, gum on the stem and wilting that spreads with rain and traffic, suspect the bacterium. Your tools are sanitation, tool cleaning, rotation, clean cuttings and possibly hot-water treatment obtained correctly.
• If the symptoms are sunken circular spots and brown stem cankers following damage, in a humid zone, suspect anthracnose. Your tools are wound reduction, debris removal, rotation, planting timing and, again, clean cuttings.

Notice what appears under every single one of those four headings. Clean cuttings. At least four of the six problems move with planting material, or destroy it, and clean, certified or at minimum locally-inspected planting material is the single highest-leverage intervention available before any spray or biological agent is considered.

Now build the routine, because knowledge you do not act on regularly is not management. Here is a scouting round that catches all six in one walk of the field, and it takes twenty minutes on twenty plants.

1. Growing tip. White cottony masses and bunched, distorted shoot tips means mealybug. While you are there, look for the hard brown mummies that show the parasitoid is working.

2. Youngest leaves. Fine yellow speckling and a dusty, dried look means green mite. Tap one over dark paper if unsure.

3. Whole leaf blades. Mottled pale and green patterning with distortion means mosaic. Yellowish blotching along the veins may be brown streak.

4. Leaf spots and edges. Angular water-soaked spots with straight vein-limited edges means bacterial blight. Circular sunken spots of roughly 10 to 30 mm means anthracnose.

5. Stems. Gum exudate, dark streaks and wilting suggests blight. Pale brown cankers and brittleness suggests anthracnose. Brown streaking on green stems may be brown streak.

6. Roots, at every test-dig and at harvest. Cut across and look for brown corky necrosis. That is brown streak and nothing else in this course looks like it.

Write the counts down. Date them. Do it at least fortnightly through the first months and then monthly, and keep the sheets. A single walk gives you a snapshot; a season of walks gives you a trend, and a trend is what tells you whether your cutting source, your rotation and your sanitation are working.

Last, know the limits of what you can decide alone. Which strains are active in your district, which varieties are currently recommended for your zone, whether the predatory mite and the parasitoid wasp are established near you, which pesticide and fungicide products are registered and at what dose, and whether an inspected planting-material source exists: all of those are answered by your extension office, your national root-crop research institute and your national plant-protection regulator, not by any document. Making that phone call is part of the job.