Cassava Farms in Pest Control - IITA...

International Institute of Tropical Agriculture

About this booklet

This booklet is one in a set of field guides prepared by the International Insti-tute of Tropical Agriculture (IITA) to increase the technical knowledge of exten-sion agents and enhance the integration of plant protection and plant produc-tion practices in farmers’ efforts to grow a healthy crop of cassava. The bookletis based largely on the extension and farmer training experience of the regionalproject “Ecologically Sustainable Cassava Plant Protection” (ESCaPP), 1993–1997. ESCaPP was executed by IITA’s Plant Health Management Division(PHMD), in collaboration with national agricultural research and extension sys-tems in Bénin, Cameroon, Ghana, and Nigeria, and funded by the Division ofGlobal and Interregional Programmes of the United Nations DevelopmentProgramme (UNDP).IITA is one of 16 nonprofit international agricultural research and training cen-ters supported by the Consultative Group on International Agricultural Re-search (CGIAR). Their shared mission is the alleviation of hunger and poverty intropical developing countries by generating appropriate plant production andprotection technologies which benefit the poor and enhance agricultural pro-duction while preserving the natural resource base. At IITA, PHMD is dedicatedto sustainable plant protection of primary food crops in Africa. The division’sresearch philosophy is to identify and correct the ecological imbalances in agri-cultural systems causing pest problems and to provide environmentally andeconomically appropriate options for integrated pest management (IPM).

For more information contact:The Director

IITA Plant Health Management DivisionBiological Control Center for Africa

08 B.P. 0932Cotonou, Republic of Bénin

Fax: (229) 35 05 56Tel: (229) 35 01 88

E-mail: [email protected] visit IITA’s website at: http://www.cgiar.org/iita

Braima James, John Yaninek, Peter Neuenschwander,Anthony Cudjoe, Wester Modder, Nnamdi Echendu, Muaka Toko

Pest Controlin

Cassava Farms

Pest Controlin

Cassava Farms

Pest Controlin Cassava Farms

IPM Field Guide for Extension Agents

Braima JamesInternational Institute of Tropical Agriculture, Plant Health Management Division,Cotonou, Bénin

John YaninekInternational Institute of Tropical Agriculture, Plant Health Management Division,Cotonou, Bénin

Peter NeuenschwanderInternational Institute of Tropical Agriculture, Plant Health Management Division,Cotonou, Bénin

Anthony CudjoeDepartment of Plant Protection and Regulatory Services, Ministry of Food andAgriculture, Pokoase, Ghana

Wester ModderInternational Institute of Tropical Agriculture, Plant Health Management Division,Cotonou, Bénin

Nnamdi EchenduNational Root Crops Research Institute, Umudike, Umuahia, Abia State, Nigeria

Muaka TokoInternational Institute of Tropical Agriculture, Plant Health Management Division,Cotonou, Bénin

© IITA 2000ISBN 978-131-174-6Printed in Nigeria by Wordsmithes Printers, Lagos

Contents

What are the objectives of this guide? ------------------------------------------- 4

Introduction -------------------------------------------------------------------------- 4

What are the common pests in cassava farms? -------------------------------- 6

Why are cassava pests important? ----------------------------------------------20

When are cassava pests likely to cause severe losses? ----------------------- 22

How can I best control cassava pests? ------------------------------------------ 24

Summary ----------------------------------------------------------------------------- 35

5

IPM Field GuidePest Control in Cassava Farms

4

What are the objectives ofthis guide?

This field guide has been prepared to help youto:

• identify pests in cassava farms,

• specify how the pests can damagecassava,

• specify how the pests multiply and spreadin cassava farms,

• identify and recognize the role of thenatural enemies of cassava pests, and

• combine the most appropriate practicesto control pests and grow a healthy cropof cassava.

Introduction

Insects, mites, spiders, and other creatures oc-cur in cassava farms. Some of these creaturesare harmful while others are beneficial. Theharmful creatures are called pests becausethey feed on and damage cassava leaves andstems (Figures 1 and 2) and roots, causinglosses to the farmer. Some of these pests areeasily seen. However, there are others such astiny mites which may not be easily noticed es-pecially if you are not trained to look forthem. Even though the damage caused bypests may be obvious, this does not necessar-ily mean that the pest is causing yield loss. Pestcontrol measures should be undertaken onlywhen the pests are becoming very abundantand pose a high risk of yield loss, and/or thecrop looks unhealthy.

The beneficial creatures do not feed on cas-sava at all. Some feed on weeds, flowers, anddead plants. Others pollinate flowers or feedon pests. Those that feed on pests are called“natural enemies” (Figure 3). Natural enemiesare your friends because they help to controlpests on the farm.

Figure 3:Ladybird beetlefeeding oncassava mealybug(as seen enlargedunder themicroscope)

Figure 2:Cassava plants

debarked byvariegated

grasshopper

Figure 1:Cassava leavesdamaged by cassavamealybug

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6

Figure 4: Cassava mealybug on theunder surface of a cassava leaf

What are the common pestsin cassava farms?

The pests of cassava are insects, mites, andvertebrates. The pests attack and feed on dif-ferent parts of cassava plants. Some feed onthe leaves and stems while others feed on thestems and roots.

Leaf and stem feeders

The common leaf and stem pests of cassavaare cassava mealybug, cassava green mite, var-iegated grasshopper, and whiteflies.

Cassava mealybug

Appearance: The cassava mealybug, Phena-coccus manihoti is commonly found at cassavashoot tips, on the under surfaces of leaves(Figure 4), and on stems. The insects are cov-ered with large amounts of white waxy mate-rials. They are wingless, pink in color, oval inshape, and have very short body filaments(Figure 5).

Two other kinds of mealybugs occur oncassava. These are the green mealybug,Phenacoccus madeirensis, and the striped mea-lybug, Ferrisia virgata. You should not confusethese with the cassava mealybug. The greenmealybug is greenish white and not pink. Thestriped mealybug occurs mostly on surfacesof cassava stems (Figure 6). It has two long tailfilaments, two dark stripes running along itsupper body surface, and produces longerthreads of white materials than the cassavamealybug. The green mealybug is more com-mon on cassava than the striped mealybug.

Crop damage symptoms: The cassavamealybug sucks sap from cassava leaves and

shoot tips. The pest reduces the lengths ofthe internodes and causes the leaves to clumptogether into “bunchy tops” (Figure 7). Thepest also distorts the stems (Figure 8), driesup the leaves and eventually, if the attack isparticularly severe, it defoliates the plants (Fig-ure 1). The damage is more severe in the drythan in the wet season.

Reproduction: Populations of the cassavamealybug are all females. The insect lays eggswithout mating. A single insect can thereforestart a severe infestation. You may noticemasses of golden yellow eggs within coloniesof the pest. The pest is more abundant in thedry than in the wet season.

Method of spread: Newly hatched cassavamealybugs are tiny, light, and easily blown bywind from plant to plant. Also, the pest sur-vives on stem surfaces and is spread by beingcarried by farmers on cassava stem plantingmaterials.

Other crops attacked: The cassava mealy-bug feeds on cassava and no other food crops.

Figure 8: Cassava stem distorted bycassava mealybug

Figure 6: Striped mealybug oncassava stem

Figure 7: Cassava shoot tip with“bunchy top” caused by cassavamealybug

Figure 5: Body form of the cassavamealybug (as seen enlarged under themicroscope)

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Cassava green miteAppearance: Cassava green mite, Monony-chellus tanajoa, lives on the under surface ofyoung cassava leaves (Figure 9). Mites arewingless, very tiny, and appear as specks to thenaked eye. In the farm, you can see them moreclearly if you look at them under a hand lens.The nymphs (immature mites) are green incolor and turn yellowish as they get older. Redmites also occur on cassava, mostly on theolder leaves, but they are not common and donot cause serious damage.

Crop damage symptoms: Cassava greenmite sucks sap from cassava leaves and shoottips. The pest causes tiny yellow chloroticspots the size of pin pricks, on the upper leafsurfaces (Figure 10). You should not confusechlorotic spots caused by the pest with thechlorotic patches of cassava mosaic disease(Figure 11). Young leaves attacked by cassavagreen mite become small and narrow (Figure12). The pest kills the terminal leaves and asthese drop the shoot tip looks like a “candle-stick” (Figure 13). Cassava crop damage by thepest is more severe in the dry than in the wetseason.

Reproduction: Populations of cassava greenmite consist of eggs, nymphs, and adult malesand females. The pest mates before laying eggs.It is more abundant in the dry than in the wetseason.

Method of spread: Cassava green mite istiny, light, and easily blown by wind from plantto plant. Also, it survives on stem surfaces andis spread by being carried by farmers on cas-sava stem planting materials.

Figure 9: Cassava green mite (as seenenlarged under the microscope)

Other crops attacked: The cassava greenmite feeds only on cassava and not on otherfood crops.

Figure 12: Cassava shoot tip withsmall and narrow leaves caused bycassava green mite

Figure 10: Cassava leaf with chlorotic (pale)spots caused by cassava green mite

Figure 11: Cassava leaves with chlorotic(pale) patches of cassava mosaic disease

Figure 13: Cassava shoot tip in“candlestick” condition caused bycassava green mite

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Variegated grasshopper

Appearance: Adults of the variegated grass-hopper, Zonocerus variegatus, are green andhave bold yellow, black, white and orangemarkings on their bodies (Figure 14). Thenymphs are black with yellow markings on thebody, legs, antennae and wing pads (Figure 15).The young nymphs gather in large numberson weeds (Figure 16) and low-growing crops.

Crop damage symptoms: The variegatedgrasshopper chews cassava leaves, petioles,and green stems. It defoliates the plants anddebarks the stems (Figure 17). The pest dam-age is more common on older than onyounger cassava plants, and is more severe inthe dry than in the wet season.

Reproduction: After mating, female varie-gated grasshoppers lay many egg pods justbelow the surface of the soil. The egg podslook like tiny groundnut pods. The egg layingsites always have vegetation which casts shadeon the ground and keeps it moist, soft, andsuitable for egg laying. These sites are usuallyclose to cassava fields and small in surfacearea. In most of West and Central Africa, adultgrasshoppers can be seen in large numbers atsuch sites, usually between March and May.The eggs start to hatch at the beginning of themain dry season, usually in October and No-vember.

Method of spread: The variegated grasshop-per spreads by flying from farm to farm. How-ever, the insect does not fly over long distances.It spreads faster in areas where the forest hasbeen cleared than in thick vegetation.

Other crops attacked: In addition to cas-sava, the variegated grasshopper also feeds oncitrus, cashew, cowpea, plantain, vegetables,and many other crops.

Figure 17: Cassava plants defoliatedand debarked by the variegatedgrasshopper

Figure 14: Adult of the variegatedgrasshopper on cassava

Figure 16: Nymphs of the variegatedgrasshopper on Siam weed,Chromolaena odorata

Figure 15: Nymph of the variegatedgrasshopper

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Spiraling whitefly

Appearance: Adults of the spiraling whitefly,Aleurodicus dispersus, are bright white in color.Adults and nymphs of the insect occur inlarge numbers on the undersurfaces of cas-sava leaves covered with large amounts ofwhite waxy materials (Figure 18).

Crop damage symptoms: The spiralingwhitefly sucks sap from cassava leaves. As itfeeds, it secretes large amounts of honeydewwhich supports the growth of black mold onthe plant (Figure 19). The blackened leavesdry up and drop.

Reproduction: After mating, females of thespiraling whitefly lay eggs on the undersurfaceof leaves. The eggs occur in spiral patterns(like fingerprints) of white material secretedby the insect on the leaves (Figure 20). Theinsects are numerous mainly in the dry sea-son.

Method of spread: The spiraling whiteflyspreads by active flight and by being trans-ported on stem planting materials.

Other crops attacked: In addition to cas-sava, the spiraling whitefly feeds on manytypes of fruit trees (for example, citrus, ba-nana, plantains), vegetables, and ornamentalplants.

Figure 20:Spiral pattern ofwhite secretionswithin which eggsof the spiralingwhitefly arelocated

Figure 19:Cassava plant

blackened underattack by the spiraling

whitefly

Figure 18:Adults of thespiraling whitefly

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14

Bemisia whitefly

Appearance: Adults of the whitefly, Bemisiatabaci, have bright white wings, as in the spiral-ing whitefly. The insects are, however, smallerthan the spiraling whitefly and are notcovered with white material (Figure 21). Theadults and nymphs occur on the under-surfaces of young leaves. The nymphs appearas pale yellow oval specks to the naked eye.

Crop damage symptoms: Bemisia white-flies suck sap from the leaves, but this doesnot cause physical damage to the plant. Asthey feed, the insects inject the plant with vi-ruses which cause cassava mosaic disease(Figure 11). This is the main reason why theinsect is an important cassava pest.

Figure 21: Adults of the Bemisia whitefly (as seen enlargedunder the microscope)

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Stem and root feeders

The common stem and root pests of cassavaare termites, cassava root scale, cassava whitescale, and vertebrates.

Termites

Appearance: Many different kinds of ter-mites damage cassava stems and storageroots. Termites live in soil or in nests abovethe ground. They can also be found in tunnelson the surface of cassava stems. Termite nestscontain worker, soldier, queen, and king ter-mites. Worker and soldier termites are theones you normally see when you break openthe nests. The workers and soldiers are smallinsects with white or brown bodies andbrown heads. They may or may not have wings.

Worker termites cause all the damage tocrops and feed all the other members of thenest. The soldier termites fight off other crea-tures which may enter or destroy the nest.

Crop damage symptoms: In newly plantedcassava farms termites chew and eat stemcuttings (Figure 22). These grow poorly, dieand rot. In older cassava farms, termites chewand enter the stems (Figure 23). This weakensthe stems and causes them to break easily.Termite damage occurs mostly in the dry sea-son.

Reproduction: King and queen termites pro-duce all the other members of the termite nest.They are always hidden in special chambers inthe nests, and you are unlikely to see them.

Other crops attacked: In addition to cas-sava, termites attacks many other crops in-cluding maize, yam, and groundnut

Cassava root scale

The cassava root scale, Stictococcus vayssierrei,seems to be restricted to parts of CentralAfrica.

Appearance: The cassava root scale livesunderground on the storage roots, feederroots and submerged stems of cassava. Theinsects are reddish-purple or brown in color,oval in shape, and look like ticks on cassava(Figure 24). They lack wings and are attachedfirmly to the plant.

Crop damage symptoms: Cassava rootscale attack causes the storage roots to besmaller than normal and deformed.

Method of spread: It is not yet known howthe cassava root scale spreads.

Other crops attacked: In addition to cas-sava, the cassava root scale attacks yam,cocoyam, and groundnut.

Figure 24: Cassava root scale onunderground cassava stem

Figure 22: Cassava stem cuttingdestroyed by termites

Figure 23: Stem of a mature cassavaplant chewed off by termites (termitenest in background, center)

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Cassava white scale

Appearance: The cassava white scale, Aonido-mytilus albus, is found mainly on cassava stemsurfaces (Figure 25). The females are wingless,firmly attached to the stems, and coveredwith white material. The males have wings.

Crop damage symptoms: The insect suckssap from cassava stems. This causes the stemsto lose a lot of water and die.

Method of spread: Males of the cassavawhite scale can fly. However, the pest spreadsmainly by wind and the transport and plantingof infested stem cuttings.

Vertebrate pests

The common vertebrate pests of cassava arebirds, rodents, monkeys, pigs, and domesticanimals. The bird pests are usually bush fowlor francolins (Francolinus sp.) and wild guineafowl. These birds feed on storage roots thathave been exposed. They also scratch the soilsurface to expose the storage roots (Figure26). The remaining portions of the attackedroots later rot. Birds are particularly a prob-lem where cassava is planted in soils that areloose and easy to scratch away.

The major rodent pests of cassava arethe grasscutter or cane rat (Thryonomysswinderianus), the giant rat (Cricetomysgambianus), other rats, mice, and squirrels.Among these, the grasscutter (Figure 27)causes the greatest damage to cassava. It cutsdown and chews the stems, and also feedson the storage roots. Pigs dig, uproot, andfeed on cassava storage roots; monkeysdamage cassava in a similar manner. Cattle,goats, and sheep defoliate cassava by eatingthe leaves and green stems. Figure 26:

Cassava storageroot pecked away

by birds

Figure 27:Grasscutters

Figure 25:Cassava whitescale on cassavastem

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Why are cassava pestsimportant?

Cassava pests are important because they re-duce the yield from the crop. They cause foodand income losses from cassava in the follow-ing ways.

Loss of roots: Damage caused by pests tocassava leaves and green stems interferes withthe way the plant makes food for storage inthe roots. This will reduce the growth of theplants, the number of storage roots they canform, and the ability of the storage roots toswell with food and mature for harvest (Fig-ures 28 and 29). However, most cassava vari-eties can lose a lot of leaves before the rootyield is reduced. Farmers should be dis-couraged from rushing to tackle control mea-sures at the first signs of damage.

Loss of planting material: Some of thepests reduce the ability of cassava stem cut-tings to sprout. For example, the variegatedgrasshopper kills the axillary buds (“eyes” ofstem cuttings) by debarking the stem (Figure17); the white scale (Figure 25) kills the axillarybuds by covering and dehydrating the stems;cassava mealybug distorts and destroys cassavastems (Figure 8); and termites weaken thestems by chewing and burrowing into them(Figures 22 and 23). Other pests contaminatecassava stems and make them unhealthy forplanting. Examples of such pests are the cassavamealybug (Figure 5), cassava green mite (Figure9), and spiraling whitefly (Figure 18).

Loss of leaves: In areas where cassava leavesare used as food, leaf-feeding pests “rob”farmers and other consumers directly of leafyvegetables (Figure 30). For example, the cas-

sava mealybug and variegated grasshopper de-foliate cassava plants (Figures 1 and 2). Cas-sava mealybug and cassava green mite distortcassava leaf shape and size (Figures 7 and 12).Cassava mealybug and spiraling whitefly con-taminate cassava leaves with whitish waxymaterials and sooty mold (Figures 4 and 19).Cassava green mite and spiraling whitefly dis-color the leaves. These kinds of damage to theleaves will also reduce the ability of cassavaplants to make sufficient food for storage inthe roots.

Carrier of cassava diseases: The whitefly,Bemisia tabaci (Figure 21), sucks sap from theleaves but causes little physical damage to cas-sava by doing so. However, during feeding theinsect picks up viruses which cause cassavamosaic disease (Figure 11). It can later spreadthe viruses to other healthy cassava plants asit feeds.

Increase in weed growth and soil ero-sion: Pests that defoliate cassava plants en-courage weed growth in farms because thecassava plants are no longer able to block sun-light from reaching the weeds growing under-neath. In loose soils, defoliation of cassavaplants will expose the soil to erosion.

Damage to other crops: In addition to cas-sava, most of the pests also feed on and dam-age other crops. Examples of cassava peststhat feed on a wide range of crops are thevariegated grasshopper, whiteflies, termites,and cassava root scale.

Figure 29:Good cassavastorage root

yield

Figure 30:Good cassavaleaf harvest

Figure 28:Poor cassavastorage root yield

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When are cassava pests likelyto cause severe losses?

The presence of pests in cassava farms doesnot always mean that they will cause severelosses in food and income. The appearances ofpests and pest damage can be misleading. Insome cases, cassava plants recover from thedamage and provide good leaf, stem, and rootyield. It is therefore very important to knowthe conditions under which pests can be seri-ous problems. The following pointers will helpyou to know when pests are likely to causesevere losses in cassava farms.

The origin of pests: Some cassava pestshave always been in Africa. These are knownas “native pests”. Examples of native pests arethe variegated grasshopper (Figures 14 and15), termites, and cassava root scale (Figure24). Some other pests are new to Africa, andare only recently being seen on cassava plants.These kinds of pests were introduced by acci-dent to Africa from other continents (Figure31). They are called “introduced pests”. Ex-amples of introduced pests are the cassavamealybug (Figure 5), cassava green mite (Fig-ure 9), and spiraling whitefly (Figure 18). Thesepests are frequently introduced without thenatural enemies which kill them in the areasfrom where they come. Hence they usuallymultiply and spread very rapidly causing se-vere crop damage.

Cassava varieties: Losses caused by pestsare less severe on some cassava varieties thanon others. Generally, not much is knownabout cassava varieties which can toleratedamage by the pests. However, the IITA vari-ety TMS 30572 and the national varieties 8017

and 8034 in Cameroon, and MS6 and NR8082 in Nigeria are good varieties against thecassava green mite (Table 1).

The stage of plant growth at attack: Gen-erally, young cassava plants suffer more frompest attack than older plants. At 3–4 monthsafter planting, the roots of most cassava variet-ies start to swell with food. At about 7 monthsafter planting, the plants have formed the num-ber of storage roots they will carry duringtheir growing period. This number will not in-crease much after this time, but the storageroots will continue to swell with food untilthey are harvested. Therefore, if pests attackcassava farms aged 7 months or less, the losseswill be greater than if older plants are attacked.

The plant parts attacked: Pests whichdamage the plant parts that you harvest “rob”you directly of food and income. For example,when cassava storage roots are attacked theplants do not replace them with more rootsnor do the roots become bigger to compen-sate for the damage. Pests which cause thiskind of damage are the cassava root scale andvertebrates which feed on the roots. How-ever, when pests attack cassava leaves theplants may produce new leaves and later pro-duce a good root yield.

The season of attack: Many cassava pestsare dry season pests. They will cause greateryield loss in cassava planted at the end of thewet season (late planting) than at the begin-ning of the season (early planting).

The frequency of attack: Cassava plantsusually recover from initial pest attack by pro-ducing new leaves. However, the plants may notrecover from continued attack by the pest.

Figure 31: Map showing the origin and introduction of cassava mealybug and cassava greenmite from South America to Africa

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How can I best controlcassava pests?

The best way to control pests is to grow ahealthy crop of cassava rather than simply aimat killing pest organisms. In order to grow ahealthy crop, you will need to combine plantproduction and plant protection practices.

IPM practices at planting

Many integrated pest management (IPM) prac-tices in cassava are appropriate at planting.These include site selection, soil improvementpractices, selection of appropriate varieties andplanting materials. These practices are coveredin the companion field guide “Starting a CassavaFarm”. Table 1 lists some cassava varieties thatcan withstand pest attack better than others.For example, the IITA variety TMS 30572 andthe national varieties 8017 and 8034 inCameroon, and MS6 and NR 8082 in Nigeriaare good against the cassava green mite. In se-lecting a variety to grow against pests youshould find out if the selected variety also hasother features you may want.

Many cassava pests are spread by carryingand planting infested stem cuttings. The mainstem-borne pests are cassava mealybug (Fig-ure 5), cassava green mite (Figure 9), spiralingwhitefly (Figure 18) and white scale (Figure25). These pests survive on cassava stemsand leaves and are easily carried to new fieldsin this way. The companion field guide “Start-ing a Cassava Farm” presents the generalguidelines on how to select healthy stem cut-tings to grow a healthy cassava crop. In se-lecting healthy planting material you shouldlook for cassava plants with robust stemsand stem branches, lush foliage, and minimal

stem and leaf damage. You should avoid se-lecting planting material from cassava plantswith stem-borne pests or their damagesymptoms.

Cassava white scale normally occurs on a fewcassava plants in cassava farms. During thegrowing period of the plants and after cassavaroot harvest, try and destroy the stems in-fested with the pest. Do not store cassavastems with any of the stem-borne pests. Re-move such infested stems from cassava stembundles in storage.

If it is difficult to get sufficient quantities ofhealthy stems you should treat the stem cut-tings against certain pests. For example, youcan plant affected stem cuttings in a horizontalmanner by laying them flat and burying themcompletely in the soil to kill pests on the stemsurface. You can also dip cassava cuttings in di-luted solution of a suitable pesticide (for ex-ample, 1% Rogor) to kill the pests. If pesticidesare to be used, you should consult the label forguidelines on their application methods andhow to avoid personal and environmental haz-ards associated with their use. In areas wheretermites are particularly a problem, you cansmear the cut ends of cassava stem cuttingswith a watery paste of soil mixed with kero-sene. This can limit termite damage.

When cutting cassava stems into stem cut-tings for planting, you should select the middlebrown-skinned portions of the stems. Theseparts sprout and ensure plant vigour betterthan the top green stem portions. The topgreen stems dehydrate quickly and are easilydamaged by pests. Avoid unhealthy stem cut-tings as planting material.

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Figure 32: “Cassava trees” for rearing natural enemies of pests

Figure 33: Wooden sleeve cages forrearing natural enemies of cassavapests

The numbers of most cassava pests are higherand their damage is more severe in the dryseason than in the wet season. It is thereforeadvisable to plant cassava early, at the begin-ning of the rains. This allows the crop to growmore vigorously and better withstand pestdamage than in late planting.

IPM practices after planting

The common IPM practices after planting arebiological control, microbial control, and cul-tural control.

Biological control

Many of the insects that you find in your cas-sava fields are “natural enemies”. Natural en-emies feed on other insects, including impor-tant cassava pests such as mites, mealybugs,scale insects, and whiteflies. The natural en-emies commonly found in cassava fields in-clude several kinds of beetles, predatorymites, and tiny wasps. The tiny wasps are called“parasitoids”. There are also some microbesthat cause diseases in pests, but you cannotusually see these. Natural enemies are yourfriends in the fight against cassava pests. Using(or allowing) natural enemies to control yourpests is called “biological control”.

Cassava originally came from South America andmany of its important pests, including the cas-sava mealybug and cassava green mite, were alsobrought to Africa from elsewhere. Biologicalcontrol is especially effective against these “in-troduced” pests. In this kind of biological con-trol, scientists identify where the pest camefrom and then go to that “home region” and findthe most effective natural enemies that controlthe pest and prevent it from becoming a pest

there. The most promising natural enemies arethen carefully tested to ensure that they willcause no harm in the new locality. When scien-tists are completely sure that a natural enemy issafe, they will bring it to the place where it is tobe used as a “biological control agent”. Scientistsinitially rear large numbers of natural enemies(Figures 32 and 33) and release them in cassavafarms. Usually natural enemies only have to bereleased once in a particular area and then theyreproduce, multiply, and spread on their own,providing permanent control without farmersor plant protection services having to take anyfurther action.

Biological control does not eradicate pests. Itreduces their numbers to low levels that donot cause severe damage to the crop. As thenumbers of a pest drops, the number of natu-ral enemies will also drop. Similarly, if the pestnumbers increase, the natural enemies willalso increase. In this way both pest and naturalenemies remain in balance in the locality. Be-cause the natural enemies may take a shortwhile to build up, you may sometimes seesome plants with pest damage symptoms,even in farms where biological control is al-ready in action. This should not cause anyalarm. As long as the pest’s natural enemiescontinue to survive in the general area, theywill soon arrive on the infested plants, multiply,and prevent the pest from causing severedamage.

The common natural enemies used in biologi-cal control are predators and parasitoids.Sometimes, farmers can actively help them towork better. For the most part, the farmerneeds only to avoid doing anything to disruptthe good job that natural enemies are doing.

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Figure 36:Shoot of Mallotusoppositifolius

Figure 35:Wild poinsettia,

Euphorbiaheterophylla

Figure 34:Predatory mite(yellowish) feedingon cassava greenmite (as seenenlarged under themicroscope)

Above all, farmers should avoid the use of pes-ticides on cassava because natural enemiesare easily killed by pesticides. Wherever pos-sible, use pest control measures that do notharm natural enemies.

Predators control pests by feeding on andkilling them. Predatory ladybird beetles (Figure3) can help to control cassava mealybug orcassava white scale. Predatory beetles are alsosometimes seen feeding on cassava greenmite, but the most important biological con-trol agents of mite pests are predatory mites,called “phytoseiids” (Figure 34). Phytoseiids oncassava resemble the cassava green mite, buttheir body surface is shinier, and they runfaster than the pests. In the farm you will seephytoseiids and the cassava green mite clearlyonly if you looked at them using a magnifyingglass. Among predatory mites, Typhlodromalusaripo is the most effective against the cassavagreen mite. The predator occurs mainly onyoung leaves at cassava shoot tips. It spreadsby wind and by being carried on stem cuttings.

Farmers can increase the spread of predatorymites by plucking and carrying cassava shoottips with the predator from one field to an-other. Farmers can also increase the survivaland spread of these predators by growing cas-sava varieties whose new leaves clump to-gether at the shoot tip. These will attract thepredators better than varieties whose youngleaves are widely spread. Even though farmersmay not grow such varieties for food or sale,they can grow a few plants in farms to attractthe predators.

Farmers can also leave certain weeds such asEuphorbia heterophylla (Figure 35) and Mallotusoppositifolius (Figure 36) on cassava farms to

attract phytoseiids. The predators can live onthese weeds when their food is scarce on cas-sava, ensuring that they are there to providebiological control when the cassava greenmite comes up again. During weeding, farmerscan leave the weeds to grow along the mar-gins or in other parts of cassava farms — but,of course, not so many that they competewith the crop. This cultural practice will be es-pecially useful at sites where cassava is growncontinuously, with little or no fallow.

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Parasitoids are natural enemies that kill in-sect pests by living and growing inside them.The parasitoids reared and released againstcassava pests are mainly tiny wasps which laytheir eggs inside the pests. The eggs hatch intolarvae which eat the internal tissues of thepest, growing and killing it. The body of thedead pest does not rot but it becomes hard.This hardened body is called a “mummy”. Thelarvae grow into adult wasps inside the mum-mies. Later, tiny wasps emerge from thesemummies and kill more pests by laying eggs inthem.

The wasp Apoanygyrus (= Epidinocarsis) lopezi(Figure 37) is the most effective natural en-emy against the cassava mealybug, and it hascontrolled the pest in most of Africa. Anothertiny wasp, Encarsia haitiensis (Figure 39), is acommon natural enemy of the spiraling white-fly. Mummies of the cassava mealybug arebrown (Figure 38) while those of the spiralingwhitefly are black (Figure 40).

The wasp used in the biological control of thecassava mealybug prefers mealybugs that arelarge in size. Large mealybugs are found onvigorously growing cassava plants. Soil im-provement practices which promote vigorouscassava plant growth will therefore improvebiological control of the mealybug by thewasp.

Biological control is safe because natural en-emies attack only the pests against which theyhave been reared and released; they do notattack other insects or plants. Biological con-trol is effective because natural enemies stayon the farm permanently and reproducequickly to respond to any pest outbreak. Bio-

Figure 38: Mummy of the cassavamealybug (as seen enlarged under themicroscope)

Figure 37: A parasitoid wasp of cassavamealybug (as seen enlarged under themicroscope)

Figure 40: Mummies of the spiralingwhitefly (as seen enlarged under themicroscope)

Figure 39: A parasitoid of spiraling whitefly(as seen enlarged under the microscope)

logical control saves farmers the cost of pesti-cides and avoids the dangers of pesticideswhich can poison people, livestock, and theenvironment.

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Microbial control

Microbial control is a special form of biologi-cal control in which the natural enemies are“microbes” (fungi, bacteria, or viruses) that killthe pests by causing diseases in them. These“microbial control agents” may occur natu-rally on cassava farms and, like other naturalenemies, they do their job without harmingthe crop or affecting people.

Fungi have been found that kill the variegatedgrasshopper. The fungi are spread as “spores”which are like tiny seeds. The spores land on apest, germinate, and the fungus then pen-etrates the body of the pest, growing and kill-ing it within a few days. When a diseased grass-hopper dies, its dead body may remain firmlygripped to the plant (Figure 41) or drop to theground. “Biopesticides” consisting of fungusspores mixed in oil are being prepared by sci-entists as commercial products against thegrasshopper. The product can be sprayed onweeds such as the Siam weed, Chromolaenaodorata (Figure 42), to kill newly hatchednymphs which gather in large numbers on theweed (Figure 16). The product can also besprayed directly on cassava to kill nymphs andadults of the grasshopper on the plant.

Biopesticides can be sprayed using the sameequipment as ordinary pesticides. However,biopesticides are much safer than chemicalpesticides because they are not poisonous topeople and domestic animals. Moreover,biopesticides do not kill natural enemies, sothey can be used to kill one kind of pest with-out disrupting other kinds of biological con-trol in the cassava farm.

Figure 41: Nymph of the variegated grasshopper killed byfungal disease

Figure 42: Shoots of Siam weed, Chromolaena odorata

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Cultural control

The variegated grasshopper can also becontrolled by cultural practices. In any year,the abundance of the variegated grasshopperdepends largely on the number of egg podsthat survive in the soil during the wet season.The destruction of egg pods will thereforereduce the numbers of the pest. Farmers canlocate and mark egg-laying sites early in thewet season. At a later stage they can then digup the soil at the sites to expose and destroythe egg pods. The digging up of eggs should bedone before the eggs start to hatch early inthe dry season, for example, in October inmost of West Africa.

The variegated grasshopper does not lay eggpods deep in the soil. Therefore, it is easy todig out the egg pods. However, egg pod de-struction needs to be carried out over a widearea in the wet season in order to control thepest effectively. This will require the participa-tion of many farmers on many neighboringcassava farms. If only one farmer destroys theeggs in and around his/her farm, the pest willlater invade the farm from the neighboringfarms and bushes. Extension agents can orga-nize the community of villages to dig up anddestroy the egg pods on as many farms aspossible.

Certain weeds, for example, the Siam weedChromolaena odorata (Figure 42), harbor im-mature stages of the variegated grasshopper(Figure 16). From the weeds the pest willmove onto cassava plants. You can thereforediscourage the pests from gathering in yourfarm by removing these weeds in your farm.

Vertebrate pests of cassava are usually diffi-cult to control. A number of cultural practiceswill however help to reduce the damagecaused by these pests:

• make good seedbeds for planting cassavaso that the storage roots will not be easilyexposed later on; if the storage roots areexposed, cover them with soil to preventthem from being attacked and eaten bybirds and rodents;

• fence farms to prevent entry by grasscut-ters, cattle, sheep, and goats; set traps inthe fence against grasscutters and otherrodents;

• weed cassava farms on time and slashweeds and vegetation around the farm todiscourage grasscutters and other ro-dents;

• organize the village community to hunt forgrasscutters in your area;

• grow “bitter” cassava varieties where pigsand monkeys are a severe problem; pigsand monkeys prefer “sweet” cassava vari-eties;

• harvest cassava storage roots as soon asthey are mature; this will reduce the lengthof time they can be exposed and damagedby the pests.

Summary

To control pests and grow a healthy crop of cassava:

• Identify the common pests, their damage symptoms, and natural enemiescorrectly; know the conditions under which the pests will cause severe losses.

• Select sites with dense vegetation, deep loamy soils, and flat or gently slopingland to plant cassava.

• Improve the soils by manuring, mulching, and intercropping.

• Grow cassava varieties that tolerate the common pests in your area.

• Plant healthy stem cuttings or treat the stem cuttings against pest damage;avoid transporting and planting cassava stems infested with stem-borne pests; afterharvesting, destroy cassava stems infested with stem-borne pests.

• Plant cassava mainly at the beginning of the wet season; avoid late planting

• Use natural enemies against cassava pests.

• Do not spray pesticides on cassava as these will kill the natural enemies ofcassava pests.

• Dig egg-laying sites of the variegated grasshopper in the wet season to expose anddestroy egg pods of the pest.

• In the control of bird, rodent, and other vertebrate pests of cassava, fence farms andset traps in the fence; cover exposed storage roots with soil; organize villagesto hunt for grasscutters; weed your cassava farm on time to discourage rodentpests; and harvest cassava storage roots as soon as they are mature.

Pest Control in Cassava Farms

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Acknowledgements

Special thanks to the United Nations Development Programme and the Austrian governmentwhich provided funds, and to the following institutions which provided materials, information, andservices for the production of the set of cassava IPM field guides:

• Agricultural Development Programmes (ADPs) in Abia, Akwa Ibom, Anambra, Benue,Cross Rivers, Rivers, Delta, Edo, Enugu, Imo, Kogi, Kwara, Ogun, Ondo, Osun, Oyo, andPlateau State Governments, Nigeria

• Centre d’Action Régionale pour le Développement Rural (CARDER), Bénin

• Centro International de Agricultura Tropical (CIAT), Cali, Colombia

• Crop Services Department (CSD), Department of Agricultural Extension Services (DAES),and Plant Protection and Regulatory Services Department (PPRSD), Ministry of Food andAgriculture, Ghana

• Federal University of Technology, Owerri, Nigeria

• Fourah Bay College, University of Sierra Leone, Freetown, Sierra Leone

• IITA Eastern and Southern African Regional Centre (ESARC), Uganda

• Institut de Recherche Agronomique et du Développement (IRAD), Cameroon

• Institut National de Recherche Agronomique du Bénin (INRAB), Bénin

• National Root Crops Research Institute (NRCRI), Umudike, Nigeria,

• Rural Training Centre (RTC, Presbyterian Church) in Fonta and Kumba, Cameroon

• Sasakawa Global 2000, Bénin

• Service de Protection des Végétaux et du Contrôle Phytosanitaire (SPVC), Bénin

• Southern African Root Crops Research network (SARRNET), Malawi

• University of Agriculture, Abeokuta, Nigeria

• University of Buea, Buea, Cameroon

• University of Cape Coast, Cape Coast, Ghana

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