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The Adelgidae is one of three families of soft-bodied insects that have paired membranous wings. They feed on sap of a few genera of conifers (Pinaceae) - either stored nutrients in the cortical parenchyma cells, or on phloem sap. Together with the Aphididae and the Phylloxeridae they comprise the superfamily Aphidoidea. The Adelgidae has about 51 species, divided into 2 tribes (the Adelgini & Pineini) each of one genus. The Adelges genus has 23 species and genus Pineus has 28 species. Like their host plants, Adelgidae are endemic to Northern Hemisphere boreal and temperate environments - but some have been introduced to the southern hemisphere where they are invasive pests. Despite the broad geographic distribution of their host plants, the Adelgidae comprise just 1% of Aphidomorpha species.
Adelges nordmannianae gall on Picea orientalis, and alate gallicola.
Like their closest relatives, they exhibit cyclical parthenogenesis, but unlike the Aphididae they are entirely oviparous. The life cycles of species within the Adelgidae are variable and complex. The basic pattern is a holocycle (i.e. it includes a sexual generation) involving two different host plants: a primary host which is always a spruce (Picea sp.) and a secondary host which is always a conifer of another genus (e.g. Abies, Larix). Five generations make up the typical adelgid holocycle. Three generations are produced on the primary host, where sexual reproduction and gall formation occur; each generation has a distinctive morph (the sexuale, fundatrix, and alate gallicola). Two asexual generations are produced on the secondary host comprising the exule morph and the alate sexupara. Movements between the primary and secondary hosts takes place by winged forms. The complete life cycle of these species takes two years. Some adelgid species and populations have lost one of their hosts and experience only parthenogenetic reproduction on either a primary or secondary host - all non host-alternating lineages are entirely asexual. The anholocyclic life cycle on the primary host Picea consists of two parthenogenetic generations: fundatrices and gallicolae. The entire cycle is completed within one year. The anholocyclic life cycle on a secondary host (e.g. Pinus) consists of a simple series of parthenogenetic exulis generations.
Adelgids differ morphologically from 'true aphids' (Aphididae) in having fewer and shorter antennal segments, a reduced wing venation of the forewing, often a highly glandular body surface, in having no siphunculi, and in all the female forms being oviparous. All morphs produce wax - parthenogenetic apterous females are covered in flocculent wax. Although early workers used characters of the first-instar nymphs for species recognition, recent workers have looked in more detail at alatae.
Pineus pinifolii fore wing (notation amended). Image after Vickery, R.A. (1908).
Adelgid alatae resemble true aphids in having a distinct head, thorax and abdomen. Alatae have five antennal segments: segments III, IV and V each having a single (often large) sensorium. They have a pair of compound eyes plus three ocelli. There are marginal, pleural and mesial plates on the prothorax and abdominal segments. These plates are variously fused and faceted according to species. The wings are held roof-like (=tent-like) over the abdomen at rest. On the forewing, three 3 oblique veins arise from the subcostal vein: the cubitus veins (Cu1a & Cu1b in aphids) which are separate at their bases, and an unbranched media (M). There is no separate radial sector. On the hindwing there is only one oblique vein - the media. There are either four (genus Pineus) or five (genus Adelges) pairs of abdominal spiracles.
Newly hatched adelgids in all generations start out as 'crawlers' that have longer legs and antennae than other instars and they actively locate settling sites on their tree. With the exception of winged adults, the crawler is the only mobile stage in the adelgid life cycle. The first instars are very small, less than 0.5 mm long, and have the head, thorax and abdomen fused to form an oval shape. They have small three-segmented antennae, typically with a long segment III with a long apical seta. The eyes are reduced to three ocelli. The dorsal surfaces of first instar nymphs have a series of segmental sclerites bearing wax glands, the arrangement of which is of considerable taxonomic use.
Sexuales are apterous but have mouthparts. Female sexuales are smaller than parthenogenetic adults, with relatively short stylets and long, slender, four-segmented antennae. Unlike aphids, Adelgidae have the female copulatory organs formed into a reversed, tripartite ovipositor. Like the Phylloxeridae, and some Cerataphidini and Pemphiginae, sexual females lay a single large egg. Males are smaller, have longer legs, and are more active.
Genus Adelges [Adelgini]
Distinguished by having five pairs of abdominal spiracles, whereas members of the other genus of Adelgidae, Pineus, have only four distinct pairs. They are often identified from the shape of the galls on the primary host (wspruce) which are cone-like, often resembling miniature pineapples. Forms on the secondary host often produce abundant wax.
About 30 species worldwide. The primary host of species with a sexual stage in their life cycle is spruce (Picea), and the secondary hosts are fir (Abies), larch (Larix) and other conifers (but not pine). The complete life cycle of these species takes two years. Several species have lost sexual reproduction and host alternation, and instead live all year on spruce or the (original) secondary host.
Genus Pineus [Adelgini]
Distinguished by having only four distinct pairs of abdominal spiracles. Wingless adults on secondary hosts are pear-shaped or globular and have a fused and pigmented head and prothoracic shield. They generally secrete white wax-wool.
About 23 species through Europe, Asia and North America. The primary host of species with a sexual stage in their life cycle species is spruce (Picea), and the secondary host is pine ( Pinus). The galls on spruce are usually on the shoot tips and are less compact than those of Adelges. Several species have lost sexual reproduction and host alternation, and instead live all year on spruce or pine.