Aphids (cotton)

Pests of Field Crops in Southern Africa

APHIDS

(Hemiptera/Homoptera: Aphididae)

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Text Box: The cotton aphid, Aphis gossypii, has a very wide distribution round the world and many host plants. In the sub-tropics, only females are found and these are blackish-green in the winged form and shades of yellow to green in the wingless form. Their colouring within a single colony varies tremendously with the juvenile stages tending to be yellow. The species is small to medium sized, (1-2 mm in length), with antennae about half as long as the body and cauda and cornicles dark. TOP

Text Box: This is a polyphagous species, with main hosts in the Malvaceae (including cotton and Hibiscus), and Cucurbitaceae, but in Sub-Saharan Africa, there are over 430 plant species from 93 families, as diverse as Euphorbiaceae to Zingiberaceae. Recorded hosts range from indigenous trees and herbs (Rhus, Bauhinia, Aloe and Colocasia) to cultivated species (such as Citrus, cassava, granadilla, guava) to weeds (such as Oxalis, Bidens and Amaranthus). TOP

Text Box: Cotton aphids are almost always present in crops but may reach severe numbers in seasons that are dry or during dry spells within the season. Since most cotton in the region is grown as a rain-fed crop, and mid-season droughts are common, they do quite frequently appear in heavy populations. There has been a good deal of research to establish the effect of aphids on cotton yields. In trial work carried out to test different aphicides at the Cotton Research Institute in Zimbabwe, yield increases in sprayed cotton when all other pests were controlled equally, ranged from less than 10% to nearly 40%. Sucking pests tap into the plant’s resources, causing indirect damage rather than direct damage to the fruiting part of the crop. The plant’s ability to compensate for this damage may change through the season. Late damage may be too late for the plant to compensate by putting up more bolls, or it may physically affect the quality of the cotton produced. In Zimbabwe, yield losses have been shown in some years and not in others, so that it seems that certain conditions may affect any proposed threshold level. The overall conclusion from years of trial work by the Cotton Research Institute was that the most striking benefit in controlling early sucking pests during the first eight weeks after emergence was in late-sown crops. Early-sown crops had time to compensate for the loss of yield from the base of the crop (the first bolls formed), but later-sown crops did not. An early infestation of aphids on young cotton can cause serious “cupping” of the leaves, and seems to debilitate the crop, but this usually only happens when rains are delayed and temperatures high. A late infestation of aphids on the foliage when bolls are well developed can have a similar effect to whitefly – the copious amounts of honeydew produced may cause “stickiness” of the resulting lint, and encourage the growth of sooty moulds, which stain the cotton. TOP

Text Box: Dark, winged females alight on cotton leaves and within a short while produce live young by parthenogenesis. They continue to produce offspring at a rate of at least 2-3 per day for their lifetime of up to three weeks. Wingless adults arising from these continue to feed and reproduce at the same rate, until heavy populations eventually give rise to further winged forms. The infestation is usually on the lower side of the leaf, and youngest leaves are favoured. TOP

Text Box: In many countries aphids are not regarded as a pest which requires spraying, but in parts of Africa where water stress is common and broad-spectrum insecticides are applied for repeated overlapping generations of bollworms, aphids may assume a greater pest status. Cotton is probably the most-sprayed crop grown in Zimbabwe. For many years, commercially grown cotton crops have been in the habit of receiving several sprays for bollworms, aphids and red spider mites. Both commercial and small-scale crops may receive three or four aphid sprays during the season, and this number only drops if there are good rains. As with all the major cotton pests, threshold values have been calculated for aphids after which sprays are deemed necessary. Aphids are counted on designated leaves of a specified number of plants per field by trained insect scouts. The number of accompanying natural enemies, which are also noted during scouting procedures, may affect the need for control. Handbooks and training courses are available from the Cotton Research Institute of Zimbabwe for growers. In terms of control, best results are achieved from systemic or at least translaminar insecticides, but if contact insecticides are used, then the application method must achieve good underleaf cover to make contact with the pest. Older insecticides used for cotton have been systemic organophosphates such as dimethoate and demeton-s-methyl. Newer soil applied carbamates such as aldicarb have given good control of early-season aphids, but this practice is not widely adopted because of the cost and toxicity of this chemical. Yet newer generation insecticides such as imidachloprid and acetamiprid have begun to be recommended, and carry the advantage of much lower toxicity, but are still very costly. Various forms of spraying equipment are in use, including the following: lever-operated knapsack sprayers with tailboom carrying up to three pairs of upwardly-directed nozzles to achieve good underleaf cover; hand-held ULV sprayers (battery operated with a spinning disc), modifying number of rows per swath and walking speed to achieve different spray volumes per hectare; tractor-mounted LV sprayers (either mistblowers or sleeve-boom sprayers); fixed-wing aircraft sprayers delivering 5-15 litres per hectare at swath widths of 20-30 metres. (With ULV application, molasses is added to water-based sprays to limit the evaporation.) Despite the fairly extensive spraying that is carried out for cotton aphids, a good deal of work has been carried out to investigate biological or integrated control. Cotton aphids have many predators, including lacewing larvae (Chrysoperla spp.), ladybird adults and larvae (Cheilomenes spp., Adonia sp., Platynaspis sp., Exochomus sp., Scymnus sp.), stinkbugs and various other small bugs, Syrphid fly larvae and spiders. In addition, a number of small parasitic wasps commonly mummify the aphids (Aphelinus and Aphidius spp.), although they may themselves be subject to hyperparasitism. These natural enemies, along with a fungus which attacks the aphids in wet seasons, may exert a considerable degree of control, but the abundance of natural enemies seems to vary from season to season, perhaps as a result of the very dry winters in Zimbabwe. Naturally they are adversely affected by most of the broad-spectrum insecticides which are applied for aphids and other cotton pests. Selective insecticides (such as pirimicarb) are available, and these have less effect on the natural enemies, but have generally been much more costly than the older broad-spectrum insecticides. TOP

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Aphid nymphs on the underside of a cotton leaf.

Leaf damage by aphids.

Honeydew on the cotton boll.