Module 1 – Introductions
Lesson 1.1: Botany and History of Cotton
Cotton belongs to the genus Gossypium, family Malvaceae, which also includes Hibiscus, okra, baobab and cocoa. Relatives of cotton are therefore grown commercially or as ornamentals around the world. There are also many other species of this plant family that are neither commercial nor ornamental but which form part of the natural flora of large tracts of land. Insects living on one Malvaceae species are likely to adapt quite easily to another. For example,Pectinophora and Earias bollworms are found on Hibiscus, okra, and non-cultivated (wild) relatives of cotton as well as cultivated cotton.
Gossypium spp. are found in the tropical and warm temperate zones of all continents. Their natural habitat is the banks of the watercourses of seasonal rivers. Many are perennial.
There are 33 diploid species in the genus Gossypium. They have been divided into groups (designated A-G) that fit closely to the geographical distribution of the genus (see THE SPECIES OF GOSSYPIUM L. table). Members of a group can interbreed (hybridise), which allows the easy transfer of useful genes from wild to cultivatable species. Genetic crosses between groups requires special techniques.
Outgrowths of the seed coat occur across the genus (click for a photo of cottonseed with and without lint). Four species, G. arboreum and herbaceum (old World), and barbadense andhirsutum (new World) have dense, long (>15 mm), and straight outgrowths. These outgrowths form the basis of an industry that dates back more than 7000 years. The length of the ‘staple’ determines the value and end-use.
In South Asia, Thailand and China, arboreum and herbaceum provide short staple ‘desi’ cotton. ‘Indian’ cotton was for millennia the world standard. Agricultural records collected over the last 4000 years indicate that insect pest damage was not a major issue with desi cotton. Was this an attribute of the plants – tolerance or resistance to the pests? Or was this a reflection that the production environment was less intensive?
The long staple cottons of Egypt and the West Indies (sea island cotton) are produced frombarbadense. However, hirsutum (var. latifolium), known as (American) Upland cotton, now dominates world trade.
Applied entomologists always like to find out where a crop originated so that they can look for biological control agents in that area. In the case of cotton this appears to be southern Africa where the only ‘wild’ species with straight lint is found. This is Gossypium herbaceum, var.africanum. It is in fact the archetypal parent of all cotton. Somehow genes from the southern African cotton reached Latin America and the Indus valley (now Pakistan) because archaeological searches in both Peru and the Sind turned up cotton cloth fragments dating to 5000 BC.
31 countries produce more than 50 000 t of cotton in 2001. Of these, China (5.3 Mt), the USA (4.4 Mt), India (2.6 Mt) and Pakistan (1.7 Mt) contributed most.
Table of top 25 cotton producing countries
(from Cotton production by country, United States Department of Agriculture)
Commerce calls for the movement of unprocessed cotton around the world. This is because cotton is not always ginned and further processed in the country of origin.
Cotton and its relatives’ widespread distribution and trade has two important implications for cotton IPM. First, it does not matter where you grow cotton as there will already be pests there to attack the plants. Second, pests that have a stage and/or live in the lint or seeds are likely to be transferred around the world unless quarantine procedures are effectively and rigidly applied.
Diapause: In general terms, diapause is a resting stage when the development process halts at a certain stage to restart either at a certain fixed point in time or when the insect receives a stimulus.
We can talk about an obligate diapause when all the individuals in a generation always enter diapause or a facultative diapause when some members of a generation enter diapause or only those individuals that were stressed enter diapause. In addition we can distinguish between adiapause and aestivation, where the former term is reserved for an overwintering process, and the latter term denotes a means of surviving adverse conditions during summer or the ‘hot season’.
We can also distinguish between quiescence and diapause. Quiescence means a period of inactivity that restarts more or less spontaneously (for instance some insects become quiescent prior to moulting) while diapause is a distinct physiological event, involving extremely slow metabolism and definable stimuli to start and end it. This web site tells you more about diapause in the context of cultural control <http://ipmworld.umn.edu/chapters/ferro.htm>
Students interested in learning more about the cotton plant, cotton history, cotton technology and cotton anatomy should visit the sites listed below.
- Cotton’s Journey – http://www.cottonsjourney.com/Storyofcotton/page3.asp
Lesson 1.2: Growing Cotton
Unlike their wild relatives, modern cotton varieties and hybrids mature within one season. There is a continuing trend in cotton breeding for shortening the growing season and/or dwarfing the plant. This is seen as a means of avoiding bollworm attack and increasing the stand density to ensure good yields. Unfortunately, this process has often left us with varieties and hybrids that are susceptible to insect pests and diseases.
Hybridization has led to the transfer of disease resistance from agronomically unsuitable species into species with excellent quality lint and other desirable features. In seeking a crop-wide strategy we need to think in terms of growing a high yielding, disease resistant variety/hybrid and adopting effective (sustainable) pest management practices that allow the crop to achieve its potential.
Cotton is a heat-loving crop, growing actively in the range 14-38ºC. It requires approximately 3,000 degree days above a threshold of 10ºC to achieve physiological maturity. In tropical climates, this means a crop cycle is completed within 6-8 months.
Cotton is a commercial crop that has little value within the economy of the farm family other than as a generator of cash. The exception is that the stalks are retained for burning on cooking fires in some ‘traditional’ areas. Unfortunately, this practice ensures the carry over of the pest species that diapause in the stalks from one season to the next in the vicinity of the cotton fields.
The potential for a large profit makes the crop valuable and worthy of care and attention. However, the establishment costs are high and usually require credit to cover them. This may leave a farmer economically vulnerable if there is no other source of income and insect pest attacks call for the purchase of additional pesticides. This means that farmers take every step to maximise their yields (but not necessarily their profits).
Deep ploughing is needed to penetrate any cultivation – or salt – pan that has developed and to permit the even development of the extensive root system of cotton plants. The optimum soil pH is in the region of 6. Cotton will grow in most soil types but high yields are traditionally associated with deep black clay soils, such as Vertisols.
Cotton is normally direct seeded as a monocrop and thinned to the required density. The optimum crop management procedures call for the incorporation of organic fertilizer at a rate that at least compensates for the organic matter removed from the fields in the previous season. This ideal may only be achievable by small-scale farmers with access to livestock manure. Otherwise, fertilizer is applied as a basal dressing. Foliar fertilizers or side dressings can be applied if nutrient deficiencies appear later in the season, for instance, following heavy rainfall. Irrigation is applied according to local practices. Water logging and puddling are not conducive to productivity.
Lesson 1.3: Introduction to the Pest Groups of Cotton
In the previous lessons, we outlined some of the reasons why cotton has so many pest problems. These included the global distribution of Gossypium, the use of high-yielding but susceptible hybrids in a monocrop, and the ability of pests to disperse along with this globally-traded commodity. In this Lesson, we will introduce some of the major pest guilds which attack cotton and point out some of their major management issues.
Although numerous pest species attack cotton plants, most of the major pest species can be organized into 5 specific groups. These groups include
- Helicoverpa bollworms
- Other Bollworms
- Sucking Pests
Each group is briefly described on the following pages. More detailed information about each group and the species in those groups is available in the [References and Readings] section of this course.
Lesson 1.3.1: Introduction to Helicoverpa
Helicoverpa (Helicoverpa armigera) is possibly the world’s most serious insect pest. Consider the following –
- Helicoverpa’s range extends over four continents,
- it is polyphagous, consuming cotton, tomatoes and other vegetables, coarse grains (maize, sorghum and pearl millet), all grain legumes, and other crops,
- the crops it attacks are essential for food security or of high commercial value, and
- the larvae feed on and spoil or destroy the ripening fruit and seed pods of the crops it attacks, which are often the plant parts that farmers want to harvest.
Helicoverpa’s biology gives it many advantages, including –
- the ability to disperse widely and rapidly,
- a short generation time,
- over-wintering mechanisms, where needed,
- high fecundity,
- the larval ability to avoid parasites, predators, and insecticide sprays by burrowing into the fruit and buds of the host plant,
- the wide range of detoxification systems in its body that enables it to neutralize anti-insect resistance factors (secondary metabolites) in plants, and which provide insecticide resistance mechanisms, and
- ineffective natural control systems in agricultural environments.
In addition, human activity has extended its pest status by –
- creating farming systems that favour the rapid propagation of the insect, and
- applying insecticides inappropriately, resulting in further outbreaks of this and associated pests.
Cotton farmers know very well that this pest can appear in their crops suddenly and in large numbers. Fruiting bodies can disappear within a few days. Farmers and their advisors need a clear understanding of this ecology of this pest to operate effective counter-strategies.
Lesson 1.3.2: Introduction to the Other Bollworms
The term bollworm is not particularly useful. Whilst it clumps together a group of insects that are members of the order Lepidoptera, that is really where the similarity becomes thin – except, of course, that the caterpillars – the worms – do bore into the bolls of cotton plants. But they also strip the leaves, destroy buds and bore into the stems. Perhaps we should call them cotton caterpillars.
Among cotton pests they form a separate ‘block’ from the sucking pests. This is because they require a different management approach. Sometimes the management strategies for the two blocks are in opposition. This why the cotton IPM practitioner needs be aware of all the possible outcomes of the actions he or she might recommend.
Most species of bollworm (other than Helicoverpa) have spread all round the world (Table 1). This is because they are carried with the product they infest, both in the lint and the seed – and by the accidents that confound the most stringent of quarantine procedures. The implication is that even if a given species is not present now it could be one day.
Table 1: Non-heliothine bollworms and their distribution
|Common Name||Species & Family||Distribution|
|Pink bollworm||Pectinophora gossypiella Saunders Gelechiidae||Virtually worldwide|
|Pink spotted bollworm||Pectinophora scutigera Holdaway Gelechiidae||Australia and the Pacific islands|
|Spiny bollworm, Egyptian boll worm||Earias biplaga Wlk. Noctuidae||Widespread, important in India|
|Spiny bollworm||Earias insulana Boisd Noctuidae||Widespread|
|Spotted bollworm||Earias vittella F. Noctuidae||Widespread|
|Rough bollworm||Earias buegeli Rogenh. Noctuidae||Australia|
|Earias cupreoviridis Wlk. Noctuidae||China, Africa, SE Asia|
|Red bollworm, Sudan bollworm||Diaparopsis spp. Noctuidae||Africa|
Lesson 1.3.3: Introduction to the Sucking Pests of Cotton
Sucking pests of cotton form a ‘block’ of species that are, together, as important to cotton growers as the bollworms. They can infest a crop from the time of seedling emergence:
- They are able to reduce the yields of cotton as pests in their own right – but usually only if their population densities are enhanced by the misuse of insecticides. Sucking pests are often induced pests.
- Farmers are known to react to their presence and apply an insecticide in an attempt to kill them, irrespective of whether they have any potential to reduce yields. This brings forward the first insecticide application of the season and reduces the number of natural control agents in the fields that would or could have eaten or parasitised the bollworm eggs and larvae that arrive later in the season.
- This earlier-than-necessary insecticide application is the first step in the seasonal pesticide treadmill that results in the build up of insecticide resistance within and between crop cycles.
- Aphids, jassids and whitefly in particular are seen as ‘predator fodder’ and as such have an important role to play as attractants to the ladybirds, lacewings and other beneficial species that will contribute to the suppression of bollworms later in the season.
This group of insects has similar life systems and therefore competes for the same resources (space and food). This means that it is somewhat unusual for heavy outbreaks of more than one type of insect on a plant or even in a crop. Low densities of all types of sucking pests are likely to be found at any time. The dominant species will be the one best favoured by:
- a local source of infestation,
- the characteristics of a given crop stand and its management (variety, sowing date, pesticide and fertilizer regime, irrigation pattern, etc.),
- the spectrum of natural enemies, and
- the weather characteristics (rainfall, humidity, max and min temperatures, etc.) of a given season.
The most important sucking pests for Asia-Pacific cotton farmers are:
- aphid Aphis gossypii, (http://www.extento.hawaii.edu/kbase/crop/Type/aphis_g.htm)
- There is information on the cotton aphid, Aphis gossypii at http://www.ipm.ucdavis.edu/PMG/r114300111.html and http://www.daff.qld.gov.au/26_9615.htm
- Indian cotton jassid (jassids iare also known as leafhoppers) Amrasca devestans,
- Information on Tetranychus species (cotton mites) can be found at http://www.cottoninc.com/fiber/AgriculturalDisciplines/Entomology/Spider-Mites/SpiderMitesCottonMidsouth.pdf ; http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7405.html
- Thrips information can be found at http://ipm.ncsu.edu/cotton/insectcorner/text.html or http://msucares.com/pubs/publications/p2302.pdf,
- sweet potato whitefly information can be found at http://www.extento.hawaii.edu/kbase/Crop/Type/b_tabaci.htm or http://www.cicr.org.in/PDF/kycp_whitefly.pdf or http://www.cicr.org.in/PDF/kycp_whitefly.pdf
- Bemisia tabaci http://www.extento.hawaii.edu/kbase/Crop/Type/b_tabaci.htm
Lesson 1.3.4: Introduction to Cotton Weeds
What is a weed but a plant misplaced?
Poetic as that may be, weed management plays a significant and costly part in the annual program of cotton crop management – both while the crop is in the ground and during the annual fallow period. Cotton cultivation demands a nutrient rich and, if possible, moist soil. This environment favours weeds as well as the crop. Farmers in Asia are generally aware of the importance of weed control in cotton and give precedence over other farm tasks to this activity.
As in all aspects of pest management misguided actions carried out in the name of weed control can lead to undesirable, even disastrous, outcomes. The methods of weed control have been undergoing a process of change for a number of years all across Asia – albeit at different rates and in different directions. Hand weeding up to the time of canopy closure is the traditional method, especially when the seed is or was broadcast. Line sowing has given the opportunity for intercultivation with a bullock drawn ‘country plough’ or other traditional cultivation devices. Tractors have replaced bullocks on the farms of the larger or wealthier farmers. Hand weeding – traditionally a ‘woman’s job’, is also phasing out.
Urbanization provides highly paid jobs for labourers from rural areas, so that there are fewer hands available in the countryside. The demand for farm labour has not really changed, only the supply. Wage demands are consequently higher. Fewer farmers are willing to pay the prices for labour (and so more families are forced to move to the cities).
The net effect is that farmers, for the first time in this area of management, are virtually forced to turn to what amounts to ‘new’ technology in the form of herbicides. The IPM practitioner must therefore be prepared to guide his or her clients in deciding when and how to use herbicides.
It should also be remembered that weeds are reservoirs for the natural enemies of insect pests and that they are a source of fodder for livestock. Farmers should be made aware that there are penalties to pay for the ‘clean’ stands that are achievable with herbicides.
Weeds of Cotton in Asia and the Pacific
If we take the huge expanse of Asia and the Pacific Rim and the diversity of its environments and plants, it is easy to imagine that there is a similarly huge diversity of weed species. This is true, but what may be surprising is that there are a number of species that seem to be everywhere, wherever one turns. For instance:
- Cyperus rotundus L. nut grass or nut sedge, (http://edis.ifas.ufl.edu/FW015)
- Cynodon dactylon (L.) –http://www.hort.purdue.edu/newcrop/duke_energy/Cynodon_dactylon.html – and Sorghum halapense (L.) perennial grass weeds with a dense rhizome system,
- Echinochloa spp.
- Convolvulus arvense L. bind weed, a choking weed that seems impervious to herbicides and can regenerate from stem or root fragments
- Amaranthus spp. annuals that grow rapidly, carry virus diseases and produce copious amounts of seed.
For further information, please visit:
- Weeds photo gallery at http://www.ipm.ucdavis.edu/PMG/r114700999.html or http://ag.arizona.edu/crop/cotton/weeds/ctnweeds.html
Lesson 1.3.5: Introduction to Cotton Diseases
Cotton diseases can be devastating, sometimes causing total crop loss. Diseases can kill plants outright, or weaken them so that they are less resistant to drought and insect attack. All stages of the standing crop are susceptible, as is the stored product. In general, cotton crops are less affected by diseases in the Asia Pacific region than in the Americas, although disease resistance breeding is similarly less developed in Asia Pacific countries.
It is likely that outbreaks of some diseases are localized around specific hot-spots, where strong interactions between soil, climate, and variety facilitate infection. Disease propagules can lie dormant in soil, be carried on or in seeds, blown by wind, or transferred by insect or human vectors.
Preventative measures are very important when dealing with cotton diseases, as it is difficult to ‘cure’ sick plants. Knowing the disease cycle, correctly identifying vectors, recognizing symptoms, and understanding the influence of environmental factors on disease development are all important aspects of successful disease management.
Some of the more important diseases of cotton in the Asia Pacific region are listed below, along with a brief description of the disease. For further information, please visit the Diseases section in the [References and Readings].
Important Diseases of Cotton in the Asia Pacific region.
- Nematodes (Meloidogyne incognita) and Fusarium Wilt (Fusarium spp) The root knot nematode interferes with root function by forming galls on the roots. Fusarium wilt is nearly always associated with nematodes. Both are pandemic.
- Verticillium wilt (Verticillium spp). Defoliation and death of plants may occur. Disease severity depends on the number of root penetrations. Verticillium wilt is favoured by low air and soil temperatures.
- Bacterial wilt (Xanthomonas spp). This disease is widespread. Symptoms are watery lesions that dry out to irregular necrotic areas on the leaves and bolls. Leaves and bolls drop in severe cases.
- Leaf curl virus (CLCuV-PK) is found in Africa and Asia. It is transmitted by whiteflies. The edges of leaves curl upwards. Plant death follows an early infection. Older plants will be impaired in terms of reduced lint production although the quality is not affected.
- Cotton mosaic virus (tobacco streak virus) is found throughout Pakistan where it is a minor problem.
Interested persons may want to visit the following sites for more information on diseases and nematodes:
- Non-chemical Control of Cotton Seedling Damping-off in the Field –http://ag.arizona.edu/pubs/crops/az1006/az100610c.html
- Nematode Management Practice –http://www.cotton.org/tech/pest/nematode/practice.cfm
- Cotton Virus Diseases –http://ag.arizona.edu/pubs/crops/az1006/az100610f.html
Lesson 1.4: Introduction to Cotton IPM Principles and Practices
The first rule of IPM is ‘grow a healthy plant’.
The second rule is that ‘farmers should have ownership of the relevant knowledge so that they can understand the outcome of the pest management options available to them.’
The third rule is ‘there are no more rules.’
The explanation of these anarchical statements lies in the following definition:
IPM is a set of management activities that farmers implement to maintain the intensity of potential pests at levels below which they become pests, without endangering the productivity and profitability of the farming system as a whole, the health of the farm family and its livestock, and the quality of the adjacent and downstream environments.
Our favorite definition refers directly to the primary stakeholders – the farmers and their families. It focuses on the objectives of most of the world’s farmers – economic survival and sustainability.
Management processes are part of a continuum that is for convenience divided into activities that involve:
- Enhancing natural control processes,
- Challenging the pests’ life systems by carrying out specific cultural practices,
- Growing pest resistant varieties, and
- Applying pesticides (when all else fails or proves uneconomical)
The actual combinations of activities depend on the nature of the pest complex and the given (sociological, economic, environmental) context. However, the IPM specialist would usually think in terms of providing sets of alternative practices (the IPM toolbox) from which farmers can select according to seasonal conditions and his or her perceptions, values and needs.
If a given activity has an outcome that stops potential pests becoming pests and promotes the sustainability of the farming system, we can consider it a component of IPM. There is doubt, in some people’s minds, about including pesticide application as an IPM component. Practical experience shows that a situation specific decision needs to be made. Sometimes pesticide application is essential, sometimes it can be inappropriate. In all cases, if a decision is made to use pesticides, their application has to be carried out correctly and responsibly.
Interested persons may also want to visit some of the external sites listed below for additional information on IPM and pest management in cotton.