Aphis nerii (Boyer de Fonscolombe)
Taxonomic placing: Insecta, Hemimetabola, Hemiptera, Sternorrhyncha, Aphidoidea, Aphididae.
Common name: Oleander aphid, milkweed aphid.
Geographical distribution: Common in tropical to warm temperate regions. This pest is thought to have originated in the Mediterranean region, where its major host plant, oleander (Nerium oleander Linnaeus) in endemic.
Host plants: Aphis nerii develops primarily on plants of the family Apocyanaceae, but may occasionally infest Compositae, Convolvulaceae, and Euphorbiaceae, only rarely on citrus.
Morphology: The body is bright yellow-pink with black appendages. The alate female head and protnotum are dark, with dark wing veins. The apterate female is yellow with black siphunculi, antennae, legs, and cauda. Nymphs are similar to the apterates, except that they are smaller. Sizes range from 1.5 to 2.6 mm in length.
Life Cycle: No sexual forms have been observed in the field, all adult morphs being parthenogenic and viviparous apterous or alate females. There are five nymphal instars. Under optimal conditions of population density and fresh plant material, apterate generations proliferate. Summer populations are low due to the high temperatures, but during mild winters A. nerii may be abundant. Alate forms occur under conditions of overcrowding and as plant quality declines, and the winged adults migrate to find new host plants.
Economic importance: The pest’s occasional infestation of minor host plants may have some significance in the transmission of several important non persistent virus diseases. Other than that, damage to ornamentals is primarily aesthetic. Aphis nerii is a phloem-feeder, excreting honeydew that subsequently serves as a substrate for sootymold.
Management
Chemical control: Such control is seldom called for, but if necessary, insecticidal soaps or oils can be used.
Biological control: many potential predators are deterred a priori by the presence of host-plant derived cardenolides, which are sequestered by the aphids and protect them. Aphis nerii is relatively insensitive to these toxins, and its aposematic bright yellow body color and associated black appendages serve as visual cues, signaling their unsuitability as prey. Cardenolides are secreted by the siphunculi. However, some parasitoids and generalist insect predators are not deterred by these toxins. When parasitized, the parasitoids (usually of the family Aphidiidae) “mummify” their hosts, i.e., the normally soft aphid cuticle hardens and the developmental stages of the parasitoid are protected. This however does not reduce aesthetic damage to the crop. .
References
Moran, N. A. 1992. The evolution of aphid life cycles. Annual Review of Entomology 37, 321-348.
Martel JW and Malcolm SB. 2004. Density-dependent reduction and induction of milkweed cardenolides by a sucking insect herbivore. Journal of Chemical Ecology 30, 545-561.
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