Many insects in several orders, including Diptera, Hymen-optera, Lepidoptera, Coleoptera, and Neuroptera, feed on honeydew that has fallen onto plant or other surfaces. Among these insects are herbivores (e.g., tephritid flies, butterflies, and moths) and many entomophagous taxa, such as chrysopids, coccinellids, syrphids, tachinid flies, and hymenopteran parasitoids. A number of nectivorous birds in Mexico and Australia forage on honeydew and lerp; lerp is also consumed by flying foxes in Australia. Other small mammals and reptiles also feed on honeydew, and dipterous vectors of human diseases (e.g., mosquitoes and phlebotomine sand flies) may rely on honeydew for an energy source.
Because it is usually freely accessible on leaf surfaces, it can readily be imbibed by insects that lack the specialized mouthparts needed to exploit floral nectar. However, several features of honeydew reduce its availability and suitability as food. First, there is the tendency of honeydew sugars to crystallize. Second, the performance of various predators and parasitoids is generally lower on honeydew than on nectar. Third, plant-derived secondary compounds in certain honey-dews are toxic to other insects. It has been suggested that insects have selection pressure to produce honeydew of little nutritional value; hence, potential competitors and natural enemies may be one factor shaping honeydew composition.
When animals consume honeydew as it is voided they are described as "tending." By far the most widespread group of tenders are the ants, including most species of the subfamilies Myrmicinae, Dolichoderinae, and Formicinae. Other insects reported to tend homopterans include polybiine wasps (e.g., Brachygastra and Parachartergus spp. associated with membracids and planthoppers, respectively) and silvanid beetles (e.g., Coccidotrophus spp. with the mealybug Pseudococcus breviceps). It is believed widely that only insects tend honeydew-producing homopterans, but it has been demonstrated recently that several Madagascan gekkoes stimulate planthoppers of the family Flatidae to release honeydew droplets on which they feed.
In the interactions involving ants, both the ants and their tended homopterans generally benefit from the association, which is therefore described as mutualistic. Access to honey-dew has been shown to enhance the rate of increase of ant colonies, but the magnitude of the nutritional benefit varies widely with ant species and environmental circumstance. Predominantly predaceous species feed on honeydew only very occasionally; some ants switch between tending and preying on homopterans, depending on the nutritional quality of the honeydew (as shaped by plant physiology) and the nutritional needs of the ant colony, and honeydew accounts for more than half of the diet of many temperate wood ants of the genus Formica and is the dominant, even sole, food of certain subterranean ants, such as Acropyga spp., and of Solenopsis (fire ants).
An indication that honeydew is an important food source for many tending ants is that the ants protect the tended homopterans from predators such as lacewings, syrphids, and coccinellids. In addition, certain ant species enhance their supply and quality of honeydew by transporting their tended homopterans to suitable parts of the host plant where the phloem nitrogen content is high and concentration of toxic plant chemicals is low (e.g., Lasius and aphids of genus Stomaphis), and some members of the genus Acropyga that tend coccids bear live coccids in their mandibles during the nuptial flight. The homopteran partner benefits from the protection from natural enemies and ant-mediated removal of honeydew, as frequently indicated by elevated rates of population increase in field conditions.
Honeydew has been used as a source of food by people. Encrustations of honeydew produced by scale insects have been eaten since biblical times in the Middle East. Certain groups of Australian Aborigines and American Indians also used lerp from psyllids and honeydew from scale insects as a source of sugar. In Central Europe, large amounts of honeydew are consumed indirectly, because the honeydew of aphids on conifers is the principal, and sometimes sole, source of food for some honey bees. The honey produced from this source, often referred to as Wald Honig (forest honey), is considered of inferior in quality to floral honey but is, nevertheless, consumed extensively.
Honeydew may also be an important source of carbon and nitrogen for microorganisms. For 2 decades, this topic has been influenced by an as yet experimentally unsupported hypothesis that the use of nutrients in insect honeydew by soil microorganisms would mobilize soil nutrients and enhance nitrogen fixation and thus promote plant nutrition.
In agricultural contexts, the growth of molds on deposited honeydew can depress plant photosynthesis and crop yield and contaminate fruits, vegetables, and flowers, making them unmarketable. For example, sooty molds arising from untreated infestations of greenhouse whitefly can halve the yield of glasshouse tomato crops, and cotton growers in the United States have suffered financially as a result of the "gray cotton" caused by sooty mold growing on cotton lint contaminated with whitefly and aphid honeydew. Other detrimental effects of honeydew are sticky sidewalks, glazed windshields, and gummed up harvesting machines.
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