Spring has arrived in the Washington area and with it luxuriant growth on many trees and shrubs. Tiny liquid jewels of honeydew adorn the leaves of aphid-infested plants. While exploring one of my favorite landscape beds, I discovered every sprig of the resident roses and spireas festooned with aphids and dripping with droplets of sweet sticky liquid called honeydew. Why are aphids so abundant at this time of year and where does this honeydew come from? As plants break bud in spring to begin a new season of growth, production of new leaves and blossoms results in a steady flow of water and minerals from the roots to the shoots and the transport of nutritious sap within plants through a specialized vascular tissue called phloem.
Phloem is a favorite feeding station for many types of sap-sucking insects, including aphids. Aphids of all description are having a heyday during these glorious days. During this season, many species of aphids are parthenogenetic, meaning that they reproduce asexually – all ladies, no guys - and the female aphids give live birth. No wonder populations seem to explode! A newly born nymph inserts its stout beak into the plant’s phloem to feed. The beak encloses a tube connected to a miniature pump in the aphid’s head. The pump passes food along to the digestive system of the aphid, where nutrients are extracted and used for growth and development. Phloem sap is relatively poor in nitrogen but high in carbohydrates. For aphids to obtain enough nitrogen for growth and development, large volumes of plant sap must be imbibed. Excess residue of this extraction process is excreted by the aphid as the sweet sticky liquid called honeydew. Both nymphs and adult aphids produce copious amounts of honeydew.
Aphids are not alone in their craving for sweets. Insects of all description need a source of energy to power their good and dastardly deeds and most of this energy comes from carbohydrates. For plant feeding insects, most carbohydrates come from leaves they eat or from rich stores of nectar found in flowers or specialized liquid-producing glands called extra-floral nectaries. However, my sojourn to the aphid patch revealed several visitors tanking up on aphid honeydew. Some of the most notorious guests of aphids are many species of ants that serve as their body guards. To learn more about the intimate relationships between aphids and ants, please visit the previous episode entitled entitled "Aphids and their bodyguards."
Legions of aphids line branches and leaves in spring.
A gangly crane fly sips droplets of honeydew on the surface of a leaf.
The wonderful books “The Ants” by Bert Hölldobler and Edward Wilson, and “Aphid Ecology” by A.F.G. Dixon, and the article “Honeydew as a food source for natural enemies: Making the best of a bad meal?” by Felix L. Wäckers, Paul C.J. van Rijn and George E. Heimpel, were used as references for this Bug of the Week.