Positive Developments in Integrated Pest Control for Cotton in West Africa
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Positive Developments in Integrated Pest Control for Cotton in West Africa Dr. Ouola TRAORE, Agro-Soil Scientist, Senior Scientist Institute of the Environment and Agricultural Research (INERA), Cotton Program 01 BP 208 Bobo-Dioulasso 01, Burkina Faso E-mail: [email protected] Introduction Cotton is one of the leading cash crops in the West African subregion. It employs more than 10 million growers. In 2007, total output came to 1.2 million metric tons on approximately 1.5 million hectares, for an average yield of 800 kg/ha. Lint exports from these countries account for less than 15 percent of world exports but are second highest worldwide, trailing the USA. Yields recorded significant growth of 6 percent annually between 1960 and 1980, versus 2 percent worldwide. Beginning in the 1980s, though, yields virtually stagnated, or even declined. This situation is due to a number of factors: - dismantling of the extension system; - declining soil fertility; - declining world prices for cotton lint; - difficulties with pest control: Cotton is one of the most highly attacked plants in the world. Hargreaves has recorded more than 1,300 insect and acarid species on cotton plants, and this does not include nematodes and mammals. In the absence of pest and disease control, average harvest losses in the West African subregion range between 40 and 70 percent, depending on the agro- ecological zone and the year. This paper reviews integrated pest control efforts undertaken in the West African cotton sector. According to the International Organization for Biological Control (IOBC), integrated control is “a system for controlling noxious organisms that uses a set of methods meeting economic, ecological, and toxicological requirements, with priority given to the deliberate implementation of natural limiting factors and respect for tolerance thresholds.” For the cotton plant, integrated pest control requires full knowledge of the plant’s main pests and diseases and their natural enemies. I. Cotton pests in West Africa Cauquil (1986) classifies cotton pests according to the types of organs attacked on the plant. All parts of the plant are attacked (seeds, roots, vegetative and fruit-bearing organs). The principal pests and diseases of the vegetative and fruit-bearing organs of the cotton plant can be divided into three groups, according to the phenological stage of the plant: 1. Principal seed and root pests 1.1. Principal seed pests Insects of the Tenebrionidae family (Tribolium castaneum) attack the seed during storage. The seed also harbors Pectinophora gossypiella and Cryptophlebia leucotreta caterpillars in areas where these pests proliferate. At sowing, the seed and the young plantlet are attacked by diplopods (millipedes). The most harmful species belong to the genera Peridontopyge and Tibiomus. These species are dark in color, with alternating lighter-colored bands and blackish- brown bands. Their cross-section is round, and they range from 2 to 8 cm in length. Diplopods attack and hollow out the seed. They feed on it during germination. 1.2. Principal root pests These primarily consist of insects and nematodes. Insects Syagrus calcaratus: A small, shiny blue-black beetle with a tawny orange thorax and lower legs. The orange-headed adults feed on the leaves, in which they leave elongated perforations. The -1- larvae live in the soil, where they feed on the roots, which they decorticate by making rings around these organs, leading to a characteristic wilting of the plant. Nematodes Noxious nematodes belonging to the Nemathelminths have been identified in root ectoparasites and endoparasites. They fall mainly within the genera Pratylenchus, Rotylenchus, Meloidogyne, Scutellonema, and Helicotylenchus. 2. Principal leaf pests and their damage 2.1. Phyllophagous caterpillars Syllepte derogata This is the cigarette-shape “leaf-rolling” caterpillar. It is light green in color, often translucent with a black head. It attacks leaves at all stages of the plant’s development by spinning silky threads. Black excrement can be seen inside the leaf. Infestations of this pest are often localized within the field and may result in spectacular defoliation. It also attacks okra. Spodoptera littoralis The caterpillar may be brown, yellowish, or gray. It is characterized by two rows of black triangles on the back and a light line on each side. But these triangles may be present only at the front or rear of the body. The eggs are deposited in a pile on the underside of the leaves, where the young caterpillars are born and then begin to feed on the supporting leaves. Older caterpillars perforate the leaves and also attack the reproductive organs. Spodoptera also causes damage to cowpeas, groundnuts, etc. Anomis flava This is the “surveyor caterpillar,” so called because of its highly characteristic motion. The caterpillar is yellow, with a yellow or greenish-yellow head. It has five, very fine white lines on its back. It attacks leaves only. Damage from these caterpillars consists of circular perforations measuring 1 to 3 cm in diameter in the leaves. 2.2. Phyllophagous beetles Flea beetles These small, highly mobile insects are of several colors. They make lots of holes in the leaves of young glandless cotton plants. - Nisotra dilecta: blue - Nisotra uniformis: brown - Podagrica decolorata: yellow-orange They are also found on okra, the various Hibiscus (jutes, kenaf, etc.), and sometimes on glanded cotton plants. 2.3. Acarids There are two genera that are cotton pests in the subregion: Tetranychus or red spider mites and Tarsonemus mites, which are the most important. They are tiny, almost invisible to the naked eye, and live on the underside of the leaf. Tetranychus All the species encountered belong to the genus Tetranychus. They include T. urticae, T. neocaledonicus, and T. falcaratus. They are red in color, hence the name of red spiders. They are not very mobile. They feed on the underside of leaves with a necrosed appearance. They are usually unimportant. Tarsonemus The most important species is Polyphagotarsonemus latus, which is yellow-white in color and highly mobile. It appears on cotton plants in humid zones (rainfall above 1000 mm/year), especially Benin, Côte d’Ivoire, and Togo. It has a very short biological cycle, multiplies every five days, and causes substantial damage, particularly in damp, overcast conditions. Affected leaves show a number of successive symptoms, depending on the level of gravity: - the underside is dark green, glazed, oily, and shiny; - the sides of the limb roll downward; - the leaves look chapped and torn as if cut by a knife. -2- The plant takes on a spindly appearance with few or no fruit-bearing organs. Early or severe attacks result in heavy declines in output. 2.4. Piercing and sucking insects These include the Homoptera and the Heteroptera. The former are the most dangerous. Among the Homoptera, aphids, white flies, and jassids are of concern, while the Heteroptera include, in particular, mirids (Dysdercus sp. and Helopeltis schoutedeni). 2.4.1. Homoptera Aphids: Aphis gossypii These are yellow, yellow-green, or black-green in color, may be either winged or apterous, and have a very short biological cycle. They are polyphagous. They live in colonies on the underside of leaves that become arched and tense and shrivel downward. The waste matter, consisting of a sweet substance (honeydew), falls on the leaves and gives them a shiny appearance. Microorganisms (mushrooms) may develop on the honeydew and produce a black discoloration on the leaves and the seed cotton: this is called sooty mold. White flies: Bemisia tabaci The larvae are oval in shape, flattened, and green when they are young but yellowish when they are older. They are attached to the underside of the leaves. The adults are very small insects with two pairs of white wings. They are very mobile and flit around the plant. Large populations cause the leaves to yellow and disturb the plant’s development. As in the case of aphids, they also produce a honeydew that soils the cotton in open capsules. This pest is also highly polyphagous: it can be found on many other cultivated plants, particularly market gardening crops such as tomatoes. Jassids: Jacobiella fascialis These are tiny green insects with a characteristic, oblique type of motion. They pierce the leaves, which then take on a reddish appearance around the edges. 2.4.2. Heteroptera The genera and species that attack the cotton plant are very numerous in the subregion and particularly in the most humid zones. The main concern is: Helopeltis schoutedeni This is a mirid with an elongated form and orange-yellow or bright red coloring. It is found mainly in the most humid zones. It attacks the leaves, branches, stems, and capsules and produces brown or black cankers. In the case of early and severe attacks, the leaves become waffled and cracked, taking on the appearance of “claws.” The plant’s growth is slowed as a result. Other Heteroptera of lesser economic importance also bear mention: Anoplocnomus curvipes, Campyloma spp., etc. 3. Principal reproductive organ pests These include both insects and mammals. 3.1. Insects These are classified as carpophagous insects with exocarpal and endocarpal diets. 3.1.1. Carpophagous insects with an exocarpal diet Helicoverpa armigera The caterpillars are of variable color, with two light lateral lines and small hairs on the body. They attack and cause major damage to the flower buds, flowers, and capsules. The larvae are highly voracious. A single larva can destroy in one day between five and ten reproductive organs, specifically the squares, flower buds, and flowers. The excrement on flower buds and capsules that have been attacked is often abundant and expelled from the organ. In certain cases (second- generation infestation and shortage of the abovementioned organs), the caterpillar may attack young leaves and branches. H. armigera is highly polyphagous (cultivated plants: maize, sorghum, tomatoes, sunflowers, etc., as well as wild plants: Cleome viscosa, etc.).