Roughly 4000 species of aphids have been described world-wide and the vast majority of these are specialized, having one or at most a few closely related hosts. Only four of the 20 described subfamilies of aphids have host plant alternation. Primary hosts are generally woody plants with conifer, oak, birch, elm, walnut, willow and maple families being particularly favored. In these families with no secondary host, the entire life cycle is spent on a particular host. Overwintering eggs are deposited by the sexual generation in the fall. These hatch the following year and pass through several generations of parthenogenetic females. Females are viviparous--extruding young aphids that have undergone rapid internal embryonic development. Both winged and wingless asexuals are generally produced. Possibly more winged forms as the host becomes crowded or otherwise less suitable? Winged forms disperse the population to other host plants. In those four subfamilies having a secondary host (Eriosomatinae, Hormaphidinae, Anoeciinae, Aphidinae), the overwintering eggs, upon hatching, seek out a generally herbaceous host and pass through several asexual generations before producing a sexual generation that returns to the primary woody host??
Aphids tap the phloem of plant foliage, twigs, or roots, and plant turgor actually forces the fluid into the aphids (separate the aphid from its beak and sap will continue to flow). This food source is rich in sugars and other carbohydrates and the excess 'honeydew' is extruded through the anus. This is a rich food source for ants, and consequently one of the easiest ways to find aphids is to look for ants feeding on the honeydew produced by the aphid population. Ants will defend this resource against a variety of predators and parasites, but they are not always successful. Aphid predators and parasites are numerous and include: Coccinellidae (ladybird beetles), various Neuroptera (lacewings), various flies (some Syrphid larvae, Chamaemyiid flies, some Chloropids), various parasitic Hymenoptera (Braconidae--Aphidiidae; Alloxystidae). Some of the latter are even hyperparasites--preying not on the aphids but on the primary parasite that is feeding internally on the aphid.
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