Every yam farmer knows the feeling of stacking a good harvest into the barn after months of hard work, only to return weeks later and find tubers that have gone soft, smell wrong, or are riddled with holes from insects that moved in quietly while no one was watching.

Yam storage loss is one of the most painful and preventable problems in Nigerian, Ghanaian, and South African farming. Research from the International Institute of Tropical Agriculture puts post-harvest yam losses across West Africa at between 30 and 50 percent annually. That means for every ten tubers a farmer stores, up to five may be lost before they reach the market or the kitchen table.

The insects responsible for most of that loss are not exotic or unknown. They are a small group of beetles and weevils that have been attacking stored yam across Africa for generations. What has changed is our understanding of how to stop them effectively using methods that are practical, affordable, and accessible to smallholder farmers.

Meet the Pests Destroying Your Stored Yam

Yam Beetle

Heteroligus meles, commonly called the yam beetle, is a large brown to black beetle that attacks yam both in the field during the growing season and in storage after harvest. In the field it bores into developing tubers from the soil, creating entry wounds that become infection points for fungal and bacterial rot. In storage the same beetles continue feeding on tubers, creating irregular tunnels through the flesh that cause rapid decay.

Adult yam beetles are robust, hard-shelled insects between 15 and 25 millimetres long. They are nocturnal and spend the day buried in soil or hidden in stored yam material. The damage they cause in storage is characterised by large irregular feeding cavities that quickly fill with rot.

Yam Weevil

Scyphophorus interstitialis, the yam weevil, is a smaller pest than the yam beetle but causes equally serious damage in storage. The adult weevil lays eggs inside small wounds or natural cracks in the tuber surface. Larvae hatch and bore through the tuber flesh creating a network of narrow tunnels packed with frass. The tunnelling weakens the internal structure of the tuber, accelerates moisture loss, and creates entry points for secondary fungal rot that spreads from the weevil tunnels outward through the flesh.

Weevil damage is often not visible on the tuber surface until the infestation is advanced. A tuber that feels slightly soft or sounds hollow when tapped is likely to show extensive internal tunnelling when cut.

Yam Moth and Scale Insects

The yam moth Dasyses rugosella lays eggs on the tuber surface and the larvae feed on the outer flesh, causing surface scarring that reduces market value and creates entry points for disease. Scale insects attach to the tuber surface and feed on sap, causing desiccation and shrivelling of affected areas during prolonged storage.

Why Yam Storage Losses Happen

Understanding what creates the conditions for storage pest outbreaks saves more yam than any single chemical treatment.

Harvesting yams before they are fully mature is one of the most common causes of heavy storage losses. Immature tubers have thinner skin, higher moisture content, and more natural wounds from harvesting that give pests easy entry. Allow yams to mature fully before harvest. In Nigeria and Ghana this means waiting until the foliage yellows and begins to die back naturally, typically between seven and nine months after planting depending on the variety.

Harvesting wounds that are not cured before storage are the primary entry points for both insects and disease. Every cut, scrape, or crack on a tuber surface from harvesting is an open invitation to yam beetles, weevils, and rot organisms.

Poor ventilation in the storage structure creates warm humid conditions that accelerate both pest reproduction and fungal disease development. Yam beetles and weevils reproduce fastest between 25 and 32 degrees Celsius with relative humidity above 70 percent, which describes the inside of a poorly ventilated barn in most Nigerian and Ghanaian farming communities during the dry season.

Storing damaged or diseased tubers alongside healthy ones guarantees that pests and disease from the bad tubers spread to the good ones within weeks. Sorting before storage is not optional. It is the foundation of every other storage practice.

Before You Store Anything: The Preparation Steps That Matter Most

Curing the Harvest

Curing is the process of allowing freshly harvested yam tubers to develop a thickened protective skin layer over all wounds and cut surfaces before placing them in permanent storage. It is the single most important step in preventing storage pest entry and reducing rot.

Lay freshly harvested tubers in a single layer in a shaded, well-ventilated area for seven to ten days after harvest. Do not pile them on top of each other during curing as this traps moisture and heat. The shade is critical because direct sun exposure during curing causes surface scorching that damages the skin rather than thickening it. During this period the cut surfaces at the seed yam end and any harvesting wounds will form a dry corky layer that significantly reduces pest and disease entry.

Sorting Before Storage

After curing, sort every tuber by hand before moving it into the barn or storage structure. Separate tubers into three categories. Sound tubers with no wounds, cuts, or soft spots go into primary storage. Tubers with minor surface wounds that cured properly go into a secondary storage section for early sale or consumption. Tubers that are soft, show signs of rot, have visible insect entry holes, or smell fermented must be removed from the storage area entirely and either consumed immediately or discarded away from the storage structure.

Never place a doubtful tuber into storage hoping it will be fine. One rotting tuber in a pile of fifty generates heat and releases moisture that accelerates deterioration in every tuber around it.

Treating Tubers Before Storage

Dust each tuber lightly with wood ash before placing it in storage. Wood ash creates an alkaline surface environment that is hostile to beetle and weevil eggs and larvae. Apply by rolling each tuber in a shallow layer of dry wood ash so the entire surface is lightly coated. This is a traditional practice used by Igbo, Yoruba, and Akan farmers for generations and it works because it is cheap, widely available, and genuinely effective as a pest deterrent at the tuber surface level.

For farmers with larger storage volumes, pirimiphos-methyl dust is a registered chemical option for treating stored yam in Nigeria and Ghana. Apply at 1 gram per kilogram of yam by dusting lightly over each layer of tubers as they are stacked. Pirimiphos-methyl provides residual protection against beetles and weevils for up to three months in storage. Observe the pre-harvest interval on the label before consuming treated tubers.

Neem leaf lining is another effective and low-cost treatment. Line the floor and walls of your storage structure with fresh or dried neem leaves before stacking yam. Place additional layers of neem leaves between each layer of tubers as you stack them. Neem contains azadirachtin and other compounds that repel yam beetles and weevils and disrupt larval development. Replace the neem leaves every four to six weeks as they dry out and lose potency.

You can also read: Neem oil pest control for African farmers

Building and Managing a Good Yam Storage Structure

The traditional yam barn remains one of the most practical storage structures for Nigerian and Ghanaian smallholder farmers when it is built and managed correctly.

A properly constructed yam barn has vertical wooden stakes planted firmly in the ground at intervals of approximately 60 centimetres. Horizontal cross-members are tied between the stakes at multiple heights, and individual yam tubers are tied at their seed end and hung vertically from the cross-members with adequate spacing between each tuber.

This hanging vertical storage method is significantly better than pit storage or floor piling for three specific reasons. Air circulates freely around every tuber surface, preventing the humidity buildup that accelerates pest reproduction and rot. Visual inspection of every tuber is possible without disturbing the entire stock. Any tuber that begins to deteriorate is visually obvious and can be removed before it contaminates neighbours.

Site your barn in a shaded location protected from direct afternoon sun but positioned to receive prevailing breezes through open sides. Avoid low-lying areas where cold air pools at night, as the temperature fluctuation between warm days and cold nights creates condensation inside the barn that raises humidity around the tubers.

For farmers using room or building storage, install ventilation openings covered with mesh at the top and bottom of the walls to create convection airflow through the storage space. Never store yam in a fully sealed room without ventilation.

Monthly Inspection Routine

Inspect stored yams every two weeks during the first two months after harvest, which is when the highest proportion of losses occur as any missed pest infestations or wounds that did not cure properly become apparent. After the first two months when the stock has stabilised, inspect monthly.

During each inspection walk the entire storage structure and check every accessible tuber for soft spots by gentle pressure, discolouration, unusual smell, and visible insect entry holes. Remove any suspect tuber immediately for assessment. If it shows active infestation or advanced rot, remove it from the storage area entirely.

Check the neem leaf lining condition during each inspection and replace any dried or decomposed sections. Check wood ash coating and reapply where it has been rubbed off during handling.

Prevention That Starts in the Field

Healthy yam in storage starts with healthy yam in the field. Tubers that escape beetle damage during the growing season arrive at storage in better condition and with fewer entry wounds.

Apply chlorpyrifos granules at 10 kilograms per hectare broadcast into the planting mounds at the time of planting to protect developing tubers from yam beetle damage in the soil. This reduces the number of beetle entry wounds that arrive with the harvest and become storage pest entry points.

Use clean certified seed yam from disease-free sources for planting. Seed yam carrying latent fungal infection or beetle damage transmits those problems to the new tubers developing around it underground.

Practice crop rotation by avoiding planting yam in the same plot more than two consecutive seasons. Yam beetle populations build up in soil where yam is grown repeatedly. Rotating with maize, cowpea, or cassava for one season reduces the soil beetle population significantly before returning to yam.

You can also read: How to prevent weevils in stored maize

The Mistakes That Cost Farmers the Most Yam

Skipping the curing stage because the market price is good and you want to sell or store quickly. Uncured tubers lose three to four times more yam to storage pests and rot than properly cured ones. The seven to ten days spent curing pays back many times over in reduced losses.

Stacking tubers directly on the soil floor of the barn without any barrier. Soil contact introduces moisture, soil insects, and fungal spores directly to the tuber surface. Always lay a thick barrier of dry neem leaves, dry grass, or wooden boards between tubers and the soil floor.

Treating storage pests with the same insecticides used for field crops without checking whether they are safe for use on food in storage. Some field insecticides leave residues on treated tubers that are not safe for consumption. Only use insecticides specifically registered and labelled for stored product use.

Waiting until you smell rot before inspecting the storage structure. By the time rot is detectable by smell it has already spread through multiple tubers. Inspection must be visual and physical, on a fixed fortnightly schedule, not triggered by odour.