Aphids are tiny insects belonging to the Aphididae family. About 4700 species have been described worldwide, one third of those are present in Europe. They occur predominantly in the northern temperate regions of the world.
Aphids are arthropods, they have a segmented body and an exoskeleton made of a cuticle. This rigid exoskeleton prevents progressive growth. They are forced to go through successive stages of growth (larval stages), getting rid of the exoskeleton whenever it becomes too small (it is called the moulting). Aphids are insects, their body is composed of three parts, the head, the thorax and the abdomen. The head has a pair of antenna and compound eyes. The thorax has three pairs of legs and two wing pairs originally. However, in most of the species, there are two adult forms, the wingless (aptera) and the winged (alate). Aphids are Sternorrhyncha Hemiptera, their mouthparts are modified in stiletto shape for piercing the plants and collect the sap. Among the Hemiptera aphids have peculiar morphological characteristics. The antennae of these insects are usually 6-segmented with the last segment modified into a narrowed projection called the terminal process or unguis. Most aphids have two-segmented tarsi, the second segment bearing two claws. Aphids usually have two prominent structures on the posterior dorsum of the abdomen called cornicles or siphunculi. They excrete liquid drops containing alarm hormones. Despite these common features, there exist significant morphological differences between aphid species, color, shape of cornicles, forehead, tail, the length of the antenna, etc... can vary. Species identification can be done using these morphological differences.
Different shape of front of head: Myzus persicae with antennal tubercles developed (left) Melanaphis sacchari with a flat front (center) and Cerataphis sp. with two little horns on front of the head (right)
Aphids are plant feeders and extract phloem sap during their feeding process. Most of them are monophagous, they feed on a single plant species. Few are polyphagous and can attack many species of plants belonging to different families. Nearly all plants are potential hosts for one or several species of aphids. While most aphid species are not considered as plant pests, approximately 250 of them can be both direct pests of many of the world cultivated crops and excellent vectors of plant viruses.
Aphids extract phloem sap during their feeding process that can reduce the vigor of the plant. They also inject saliva that can distort the plant tissues, crumpling leaves, stunting and bending stems, or forming galls. Honeydew, a sweet liquid excreted by aphid is high in plant sugars and can serves as a medium for saprophytic fungi, the sooty mold. These fungi, often of black color, grow on the leaf surface and reduce the photosynthetic and respiratory properties of the plant. Expanding on fruits, they reduce their marketability or make them entirely unmarketable. Aphids can also serve as vectors of numerous plant viruses that can damage, stress, or kill a plant. Some species like Myzus persicae is known to transmit more than 120 different plant viruses.
Crumpling leaves of Prunus tianshanica caused by Brachycaudus pilosus (Mordvilko, 1929)
Gall of Pemphigus immunis Buckton, 1896 on Poplar
Obsessed as we are by our food resources, and the damage that causes aphids, we forget that these insects have also a role in the food web. They are an important source of prey for many insects, birds, bats and thus contribute to the balance of cultivated as well as natural areas.
From an anthropocentric point of view, the honeydew produced by some species is collected by bees to produce honey conifer! Think about it the next time you enjoy a spoonful of honey fir...
Ants collecting honeydew © Francesco Tomasinelli
The interest of biologists for aphids is mainly related to their life cycles, so special and unique in the insect world. The typical annual life cycle, cyclical parthenogenesis, consists of several parthenogenetic (asexual) generations of females, followed by the production of sexual morphs (males and egg laying females), that undergo one sexual reproduction event before winter. The parthenogenetic females are viviparous, they give birth to live young aphids. Only sexual female is oviparous and lays eggs. During the spring, eggs hatch on winter host, each giving rise to a morph called a fundatrix, which is apterous. These individuals produce several generations of apterous females until the population density becomes too high and winged dispersal forms are produced. During autumn, particular parthenogenetic females, the sexuparae, give birth to oviparous females and males. After copulation, eggs are layed on winter hosts.
Holocyclic heteroecious life cycle of Brachycaudus helichrysi (Kaltenbach, 1843)
Surrounding this general pattern we will find several types of cycle. When the whole cycle is completed we say that the species is HOLOCYCLIC. Some aphid species have lost the sexual part of the life cycle. They are described as ANHOLOCYCLIC. The great majority of aphids go through both the sexual and parthenogenetic phases of their life cycle on a single host plant, or on a small range of closely-related plants, without obligate changes between parthenogenetic and sexual phases. These species are described as MONOECIOUS. The others use two sets of host plant during their life cycle, one for the sexual phase, usually a tree, and another one for the parthenogenetic phase usually an herbaceous plant. These species are described as HETEROECIOUS.
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COPYRIGHT: Coeur d'acier et al. (2014). DNA barcoding and the Associated PhylAphidB@se Website for the identification of European Aphids (Insecta: Hemiptera: Aphididae). Plos One, 9 (6) to use the database in a publication. DOI:10.1371/journal.pone.0097620
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