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Melon aphid, Cotton aphidIdentification & Distribution Biology & Ecology Damage & Control
Identification & Distribution:Wingless females of Aphis gossypii vary in size and colour depending on conditions. In cool favourable conditions they are medium-sized and blackish green or green mottled with dark green. In hot conditions or when crowded they are smaller and are a very pale whitish yellow. Most commonly they are light green mottled with darker green. The dorsum has no dark sclerotized markings. The siphunculi are dark. The cauda is usually paler than the siphunculi and bears 4-8 hairs. The body length of apterae ranges from 0.9-1.8 mm. Aphis gossypii alates have 6-12 secondary rhinaria distributed on the third antennal segment and usually none on the fourth.
Marginal tubercles are only consistently present on abdominal tergites 1 and 7. The longest hairs on the third antennal segment are 0.3-0.5 times the basal diameter of that segment. The terminal process of the last antennal segment is 1.7-3.2 times the length of the base of that segment. The apical segment of the rostrum is 1.1 to 1.5 times the length of segment 2 of the hind tarsus. Micrographs of whole mounts in alcohol are shown below.
The clarified slide mounts below are of adult viviparous female Aphis gossypii : wingless, and winged.
Micrograph of clarified mounted aptera (first image) courtesy PaDIL. Copyright Rebecca Graham (Department of Agriculture, Western Australia) under Commons Attribution 3.0 Australian License. Alate micrograph (second image) courtesy Favret, C. & G.L. Miller, AphID. Identification Technology Program, CPHST, PPQ, APHIS, USDA; Fort Collins, CO.
The melon or cotton aphid is highly polyphagous and does not usually host alternate, reproducing all year round on its chosen host. In temperate climates it is most often seen in glasshouses on cucurbits (cucumbers and marrows) and begonias, and in gardens on ornamental Hypericum species. In the tropics Aphis gossypii is a major pest of cotton. It is distributed almost worldwide, and is particularly abundant in the tropics.
Biology & Ecology:
Aphis gossypii does not have a sexual phase in the tropics, nor in most of Europe. Some sexual reproduction may be taking place in southern France, although the primary host is unknown. Host alternation and a sexual phase occur more regularly in parts of east Asia and in North America. Several unrelated plants are utilised as primary hosts.
Colonies may or may not be ant attended. One colony was found around a leaf petiole at junction of leaf and petiole of Hypericum androsaemum - no ants were attending the colony, but a Myrmica ant was feeding at an extra-floral nectary on the plant as shown in the picture below:
On other occasions ants were definitely attending the aphids, as shown below with a Lasius niger ant.
Aphis gossypii is very variable in colour. The picture below shows a 'normal' coloured Aphis gossypii (mottled green) along with some darker aphids. It seems likely that the darker aphids were also Aphis gossypii, since Aphis fabae is not recorded from this Hypericum species.
Alternatively this may be a mixed species colony. Our photos have revealed many examples of mixed species colonies, but these are seldom referred to in the literature and there has been little if any work on the dynamics of such colonies. Hill (1987) points out that many ecological studies have failed because of the inability of the observer/recorder to recognise a mixed species population.
This particular colony had several predators actively reducing its numbers, including the syrphid larva shown above.
Damage and control
In temperate climates Aphis gossypii is considered an important pest of greenhouse crops such as cucurbits and ornamentals such as Begonia and calla lily (Zantedeschia aethiopica). Insecticide treatment may be recommended. Outdoors it is a pest of Hypericum androsaemum and Hypericum inodorum. Leaves may turn yellow and on ornamentals the large amounts of honeydew and exuvia may look unsightly. Soap solution may be used to reduce numbers. In the tropics it is a major pest of many crops including cotton, cucurbits, coffee, cocoa, peppers and okra. Aphis gossypii is known to transmit over 50 plant viruses.