Aphids (superfamily Aphidoidea) are a group of soft-bodied hemipteran insects equipped with piercing–sucking mouthparts (stylets) that tap directly into a plant’s phloem sap rather than chewing leaves. As a result, aphids are major agricultural pests, causing direct damage that often appears as leaf curling, puckering, twisted shoots, and deformed flowers and fruit.
Being sap-feeders, aphids excrete excess sugar as a sugary liquid called honeydew. Ants often feed on this honeydew and, in return, protect aphids from predators and other threats.
Another feature that makes many aphids successful pests is their rapid population growth, typically involving a long asexual phase in spring and summer and a brief sexual phase that produces eggs capable of surviving winter conditions.
Being an insect, an aphid’s body has three main parts: head, thorax, and abdomen.
The head bears a pair of sensory antennae, a pair of compound eyes, along with an ocular tubercle behind and above each eye. In most species, the ocular tubercle bears a triommatidium, a cluster of three ommatidia (visual units). Additionally, aphids have piercing-sucking mouthparts, comprising a rostrum (beak) containing stylets that interlock to form food and salivary canals. This beak helps probe plant tissues and tap phloem sap.
The thorax bears three pairs of thin walking legs. Although most aphids are wingless, some species may develop thoracic wings (alates or winged forms) for dispersal under stress conditions or in particular seasons.
The abdomen has 8 visible segments, with most aphids having a pair of tubes called cornicles on the dorsal surface of their 5th abdominal segment. These tubes exude a defensive fluid called cornicle wax, which quickly hardens in contact with air. In all aphids, the terminal segment of the abdomen bears a tail-like projection called the cauda, though its shape and size vary with species.
The name ‘aphid’ was coined by the Swedish botanist Carolus Linnaeus in 1758. It stems from the New Latin term aphis, which, the Oxford English Dictionary (OED) suggests, Linnaeus might have intended to draw from the Ancient Greek word korís, meaning ‘bug’. He might have misread korís as aphís, resulting in the current taxonomy.
All species of aphids are classified into a single extant and 8 extinct families (including a taxonomically uncertain group, incertae sedis).
These insects are found on all continents except Antarctica. Although they are found in tropical and subtropical regions, their diversity is the highest in temperate regions, especially across North America, Europe, and Asia.
Owing to their dietary preferences, aphids are found in habitats that support their host plants. They are abundant in agroecosystems (crop fields, orchards, vineyards, and plantations), but also occur widely in grasslands, shrublands, forests, and woodland edges.
Like many other hemipterans, such as scale insects and cicadas, aphids, too, primarily feed on phloem sap. However, around 60% of winged aphids ingest xylem sap, particularly after a period of dehydration, to replenish their water balance.[1]
Phloem sap is deficient in several essential amino acids (EAAs), and thus, aphids rely on endosymbiotic bacteria housed within specialized body cells called bacteriocytes to derive them.[2]
On average, adult aphids live for about 20 to 40 days, depending on environmental conditions such as temperature and food availability.[7] They remain in the nymph stage only for 5 to 10 days.
These insects undergo two reproductive patterns that are strongly influenced by their geographical distribution and climate.
In most species, the life cycle includes both asexual and sexual phases (holocyclic). During spring and summer, these species undergo rapid asexual reproduction by parthenogenesis, allowing populations to increase explosively. As autumn approaches, shortening daylength and cooler temperatures induce the production of sexual morphs (males and sexual females). After mating, sexual females lay overwintering eggs that survive harsh winter conditions. The overwintered eggs then hatch in the following spring into stem mothers or fundatrices. These females reproduce parthenogenetically and give birth to live nymphs, often exhibiting telescoping generations, where developing offspring already contain embryos of the next generation.
By contrast, some species reproduce only asexually by parthenogenesis throughout the year and do not produce a sexual generation or overwintering eggs. This pattern is called an anholocyclic life cycle and is more common in regions with mild winters, such as the tropics.
No known aphid species reproduces exclusively by sexual means.
Aphids are preyed upon by several birds and insects. Among insects, their most common predators are ladybugs (both adults and larvae), lacewing larvae, hoverfly larvae, predatory midge larvae, pirate bugs, big-eyed bugs, and some assassin bugs. Additionally, arachnids like crab spiders and predatory mites may feed on aphids opportunistically.
Birds, too, target aphids as their prey. For example, small insect-eating songbirds that pick insects from foliage, such as tits, chickadees, warblers, wrens, nuthatches, and even some sparrows and finches, feed on aphids, especially when they are feeding nestlings.
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