The amount of honeydew produced by insects can be substantial. For example, first instars of the willow aphid see Apis Species Tuberolachnus salignus release more honeydew than their own body weight on an hourly basis. However, the rate of honeydew production by other aphids is generally considerably lower.
The fluid that comprises honeydew accumulates in the rectum and is then ejected as a single droplet via the anus at fairly regular intervals, usually once every 15 to 40 min. Both the volume of each droplet and the frequency of production are influenced by many factors, including aphid age, size, species, and host plant.
Honeydew can be hazardous for the insect producing it because droplets may smother the insect and promote microbial growth on or near the insect. Various mechanisms reduce these risks in insects both on exposed plant surfaces and in confined spaces, including galls. On exposed surfaces, aphids generally project the honeydew droplet from the anus to a distance of up to several centimeters by either a kicking action of one of the hind legs or by using the cauda (a small appendage dorsal to the anus) to catapult the droplet ventrally. The legs and cauda bear hydrophobic cuticular hairs that prevent the sticky honeydew from adhering to the insect surface. Honeydew production by some species has been reported to be interrupted in windy conditions, presumably to avoid being smothered by the honeydew.
Insects in galls or other confined spaces (e.g., subterranean forms) are not wetted by their honeydew because they are coated with hydrophobic wax secreted from cuticular glands. In addition, each honeydew droplet produced by these insects tends not to be projected away from the insect body but remains poised at the anus until it is coated by wax. In some insects, notably certain nymphal psyllids, the honeydew and waxes coalesce to form a gelatinous or crystalline substance (called lerp), which acts as a solid protective covering for the growing insects.
Honeydew production may serve purposes additional to the elimination of waste compounds, with implications for the pattern of honeydew production. This is illustrated by two phenomena: "honeydew-panting" and ant-tending.
The former behavior is displayed by certain aphids, notably T. salignus, at elevated temperatures. The aphids raise their abdomen almost at right angles to the plant surface with their mid- and hind legs extended, and small honeydew droplets are alternately protruded and retracted from the anus. This behavior may cool the aphids as a result of evaporational water loss from the droplets.
The ejection of honeydew from some insects is modified by the attendance of ants or other insects. When solicited by an ant, the insect releases the honeydew droplet slowly and holds it at the anus while the ant imbibes, and, if the droplet is not removed by an ant, the insect may repeatedly extrude and retract the droplet, as if advertising the availability of honeydew. However, certain obligately ant-tended species apparently excrete honeydew droplets only in response to solicitations by their attending ants. The cauda and rectal musculature of some ant-tended aphids is much reduced, presumably reflecting their dependence on ants to remove their honeydew.
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