Ants and scale insects have a well-known mutualistic relationship, in which ants protect the scale insects in exchange for honeydew, a sugary substance produced by the scales. This interaction was recently observed on a fallen mistletoe twig found on a sidewalk in Vacaville, California.
Mistletoe itself is more than just a symbol of holiday romance. According to the UC Statewide Integrated Pest Management Program (UC IPM), “Mistletoes are parasitic plants that absorb both water and nutrients from a host tree. Healthy trees can tolerate a few branches infected with mistletoe. However, if the infestation is severe, trees can weaken, have stunted growth or dead branches, or die completely. Symptoms are magnified on trees already stressed by drought or disease. Remove branches at least a foot below the mistletoe attachment before it produces seeds that will infest other limbs and trees.”
UC IPM also notes that heavy infestations of broadleaf mistletoe often occur when birds spread seeds throughout tree canopies while feeding on berries. Seeds may also fall from higher parts of an infested tree to lower branches, quickly spreading new infestations, especially among newly planted trees near older ones with significant mistletoe presence.
The recent observation included Argentine ants (Linepithema humile), identified by Professor Phil Ward of the UC Davis Department of Entomology and Nematology. These ants are considered invasive; they originate from South America and are known for forming large colonies that outcompete native ant species and disrupt local ecosystems.
This finding highlights not only the ecological impact of mistletoe as a plant parasite but also how its presence supports complex relationships among various insect species.
