Willow trees (Salix) are common, fast-growing, adaptable plants found in diverse Northern Hemisphere habitats. They play a significant role in many ecosystems, serving as a foundational food source for a wide array of animal life due to their widespread distribution and ability to thrive in moist environments.
Mammals That Consume Willows
Numerous mammalian herbivores rely on willow trees as a food source, consuming different parts of the plant depending on the season and their size. Large browsers like moose, deer, and elk feed on willows. Moose diets can be 50% to over 80% willows year-round, stripping green leaves in summer and consuming twigs in winter. Deer eat foliage and tender twigs, while elk forage on them, especially in late summer and fall.
Smaller mammals also use willows. Beavers fell willow trees primarily for the bark and cambium layer, especially during autumn and winter when other vegetation is scarce, also using branches for dams and lodges. Rabbits and hares consume the bark, smaller twigs, foliage, and seeds. Porcupines favor the tender, nutritious bark of young willow switches, especially in spring. Voles feed on willows.
Seasonal variations influence which parts of the willow are consumed. During spring and summer, when new growth is abundant, mammals like moose focus on the green leaves and tender shoots. As winter approaches and herbaceous forage becomes less available, many herbivores shift to eating woody twigs, bark, and buds. The nutritional content of willow twigs can vary, influencing foraging behavior, with younger, smaller diameter twigs often favored for their protein content.
Insects That Feed on Willows
Willow trees host a diverse array of insects that feed on various parts of the plant. Aphids, such as the large willow bark aphid (Tuberolachnus salignus), form dense colonies on willow bark and stems, sucking sap. These sap-sucking insects excrete a sugary liquid called honeydew, which can attract other insects like wasps and flies, and may lead to the growth of black sooty mold. While heavy aphid infestations can cause leaf distortion and stunted growth, they typically do not severely harm healthy, established trees.
Various types of caterpillars also feed on willow leaves. These larvae, the immature stage of moths and butterflies, can cause significant defoliation, including those of the mourning cloak butterfly. Willow leaf beetles (Plagiodera versicolora) and their larvae chew on leaves, causing discoloration, holes, or a skeletonized appearance. Sawflies, which are caterpillar-like insects, feed on young, tender leaves, sometimes leaving only the midrib.
Gall-forming insects and mites induce unusual growths on willows. These galls can appear on leaves, shoots, or roots, with their shape and location depending on the specific inducer. Gall midges, sawflies, and eriophyid mites are common gall-formers on willows, causing various deformities like leaf folds, swellings, or “pinecone” shaped galls. Wood-boring beetles, like the cottonwood borer, tunnel into the woody tissue, primarily the sapwood, which can weaken the tree structurally over time.
Why Willows Are a Vital Food Source
Willow trees are widely consumed by various animals due to their widespread availability, rapid growth, and nutritional composition. They are common in many moist habitats, such as along riverbanks and wetlands, making them accessible to numerous species. Their ability to grow quickly, with some species adding up to 10 feet in a single year, allows them to recover from browsing pressure and provide a continuous food supply, which is particularly beneficial for herbivores.
The nutritional content of willows makes them a palatable and valuable food source. Young shoots and leaves contain relatively high protein levels, which are especially important for growing animals. Willow bark also contains salicin, a compound similar to the active ingredient in aspirin, which may offer analgesic or anti-inflammatory effects that some animals instinctively seek. While some willows produce tannins that can deter grazing, many species remain highly palatable.
Willows support diverse food webs within their ecosystems. They provide a consistent food source throughout the year, especially during winter when other forage is scarce. The consumption of willows by primary consumers, such as mammals and insects, in turn supports predators that feed on these herbivores. This interconnectedness helps maintain biodiversity and healthy animal populations.