Cowpea aphid, groundnut aphid, bean aphid, black legume aphid.
Worldwide. Asia, Africa, North, South & Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, Fiji, Guam, Kiribati, Marshall Islands, New Zealand, Papua New Guinea, Samoa, Solomon Islands, Tonga, and Tuvalu.
Cowpea (Vigna unguiculata spp. unguiculata), French bean (Phaseolus vulgaris), lentil (Lens culinaris), lucerne (Medicargo sativa), mungbean (Vigna mungo), peanut (groundnut) (Arachis hypogaea), pigeon pea (Cajanus cajan), Vicia bean, yardlong bean (Vigna unguiculata spp. sesquipedalis).
Damage is done, i) directly by nymphs and adults sucking sap from young shoots, flowers and pods as well as injecting a toxin that causes stunting, ii) indirectly by spreading viruses, and iii) indirectly by producing honeydew which blankets leaves and is colonised by the black fungal growth of sooty moulds.
On peanuts, direct damage by aphids feeding on shoots causes stunted growth, curled and distorted leaves, and wilting. On cowpea, infestations cause similar damage: there is stunting, reduced growth and shrivelled pods.
Indirect damage occurs as this aphid spreads over 30 plant viruses, of which a number are important, for example: Alfalfa mosaic virus; Bean common mosaic virus; Bean yellow mosaic virus; Cucumber mosaic virus; Groundnut rosette virus; Papaya ringspot virus; Peanut mottle virus; Peanut mild mottle; Peanut stunt virus; Peanut stripe virus; Watermelon mosaic virus; Zucchini yellow mosaic virus.
Indirect damage also occurs as honeydew is excreted from the bodies of nymphs and adults as they suck sap, falling onto leaves below. Colonisation by sooty mould fungi reduces the ability of leaves to exchange gases and to photosynthesis.
In the tropics, females do not lay eggs, they give birth to living young, without fertilization; there are no males. There are four nymph stages. Nymphs are slate grey, but not shiny as the adults. Initially, the aphids are without wings, but as the colony develops and there is overcrowding (Photo 1), less food or, perhaps, less favourable climate, winged forms occur. Adults are glossy, dark brown or black, with two erect tubes at the rear (Photo 2). They live from 5 to 15 days.
Populations of the cowpea aphid are susceptible to periods of heavy rain, especially those on young plants. Spread of the aphid occurs on the wing, and in air currents that can transport them over long distances.
Aphis craccivora is one of the most common aphids in the tropics. It is important on food legumes, peanut (groundnut) and cowpea, in particular, but also on French beans (Phaseolus species) mungbean (and other Vigna species), pigeon pea, chickpea, alfalfa and several others. However, financial losses are not well recorded for any crop.
On peanuts, yield losses of up to 50% occur in parts of Africa if shoots become infested during early stages of growth. On cowpea, and again in Africa, direct damage results from early aphid infestations, causing reduced numbers of pods and seeds per pod. Elsewhere, heavy infestations have been reported (e.g., chickpea in Pakistan, lentil in India, and cowpea in Egypt), as well as 2 to 4-fold increases in yield where pesticides have been used to compare crops with and without aphid infestations (e.g., greengram in India).
Of the many viruses spread by Aphis craccivora, groundnut rosette virus, which is a complex of at least five viruses (plus so-called helper viruses and satellite RNA) is one of the most important. Plants develop mottles and mosaic leaf symptoms, on distorted and stunted shoots. If infections occur when plants are young, they may fail to develop pods. Because of its association with the disease, the aphid is considered the most important pest of peanuts in Africa.
Look for the adults clustered on the shoots, up to 2.5 mm long, with whitish legs, and wingless. Look for the nymphs that are roundish, dark or dusty brown. Note, distribution of the aphid may be patchy in a field, with some plants infested others not.
Ladybird beetles (e.g. Cheilomenes sexmaculata and Coccinella septempunctata), hoverfly larvae (syrphids) and larvae of lacewings, are important insect predators of aphids. Spiders, too, are important predators. Several parasitoid wasps are recorded on Aphis craccivoria of which Trioxys indicus (a braconid) is widely distributed, and there are also species of Lysiphlebus and Diaeretiella. Trioxys is known for tolerance to hot weather. There is an important hypeparasite of this wasp (Alloxysta pleuralis), so care is needed if Trioxys is to be introduced.
Note, ants tend aphids for their honeydew. By doing so, they protect the aphids from the activities of parasites and predators. To manage aphids, it is important to remove the ants, so that biological control can operate.
There are resistant varieties of chickpeas, peanuts and cowpeas bred for resistance to virus diseases.
If ants are present, find the nest, and if not too close to the plant with aphids, destroy the nest with boiling water. Without ants, predators and parasites may bring about natural control.
AUTHOR Grahame Jackson
Information from CABI Aphis craccivora (groundnut aphid) (2017) Crop Protection Compendium. (www.cabi.org/cpc); and Aphis craccivora Wikipedia. (https://en.wikipedia.org/wiki/Aphis_craccivora); and from Godfrey LD, et al. (2017) Alfalfa Cowpea Aphid UC/IPM. Agriculture & Natural Resources, University of California. (http://ipm.ucanr.edu/PMG/r1301511.html). Photo1 Mani Mua, SPC, Sigatoka Research Station, Fiji. Photo 2 Whitney Cranshaw, Colorado State University, Bugwood.org.
Produced with support from the Australian Centre for International Agricultural Research under project HORT/2016/185: Responding to emerging pest and disease threats to horticulture in the Pacific islands, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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