Sandhill hornets (Dolichovespula arenaria), large black and yellow wasps related to yellow jackets (genus Vespula) and bald-faced hornets (Dolichovespula maculata), have similar behaviors and feeding habits. Mated females, queens, of both wasp species form colonies in nests made of paper. Sandhill and bald-faced hornets' nests are above ground, while the yellow jackets' nests are below ground. Like bumblebees, the queen lays several eggs in a small nest, foraging and feeding the young until they metamorphose into adult workers. Once the young workers leave the nest, they assume the tasks of foraging, feeding young, nest construction, and nest defense.
Over the summer, a nest may contain 4,000 rearing chambers and produce 6,000 or more workers – although normally less than 500 workers are alive at a given time. A large nest may be almost a foot in height and nine to ten inches in diameter.
Food of Sandhill Hornets
Hornets feed on insects, spiders, and nectar, with animal protein making up a larger portion of an adult hornet's diet that it does an adult yellow jacket's diet. Larvae are fed exclusively insects and spiders. Toward the end of the summer, the queen produces male eggs and a pheromone that causes some eggs to become fertile females, then stops laying eggs. As the last larvae approach adulthood, the workers shift from provisioning the young with protein to providing them sugar-rich foods such as nectar and tree sap.
Change in Diet is Essential
After reproductives mate they leave the nest and hibernate in wood piles, under the bark of trees, in rotting wood, under rocks, and in shallow burrows in the ground. They emerge in the spring to establish new colonies. Since these hornets are exposed to freezing temperatures over the winter, they store high concentrations of sugars (glucose and others) in their cells to prevent the cells from freezing.
Since animals can convert proteins into sugar but cannot convert sugars to proteins, the hornets obtain these sugars from the plants directly instead of using the energetically and metabolically wasteful processes of converting proteins and fats. Thus, the hornets conserve fats and muscle and enzymes until spring when they will be needed again for locomotion, energy production, and digestion.
Hornets Use Different Technique to Produce Paper and Sap
During the spring and summer, hornets tear off the outer bark of many plants such as lilacs (genus Syringa)and groundsel bushes (Baccharis halimifolia), chew the bark and mix it with saliva to produce a pulp that, upon drying, becomes paper to form the walls of the nest, the brood chambers, and the caps that seal the pupae inside the chambers to prevent the pupae from falling out.
In the fall, the hornets tear off the outer bark in much the same manner, but do not chew it up and mix it with saliva. Instead, they chew the phloem cells below the bark until the cells rupture releasing sap being transported to the roots. As the sap flows to the surface of the stem, the hornets lap it from the opening. After some time the sap begins to bubble, and the hornets move to new areas on the stem or to other bushes.
Other Animals Drink the Sap
As groundsel bushes flower in the fall, they attract a wide range of arthropods: spiders, assassin bugs, praying mantis (Mantis religiosa), butterflies and moths, bees, wasps, flies, and beetles. Of these, the spiders, assassin bugs, and mantis sit in wait for the other insects to approach the flowers. Most of the other insects feed on the flowers. Some insects, however, feed on the sap exuding from the stems the sandhills hornets have opened. Honeybees (Apus mellifera) and red admiral (Vanessa atalanta) and question mark (Polygonia interrogationis) butterflies are among these, and thermoregulate when on the flowers with their wings slowly opening and closing before flying back into the shade below the leaves to feed on the sap. Even birds, such as the yellow-rumped warbler (Dendroica coronata), forage on sap in these bushes.
This preference may be in part because, in addition to sugars, sap also contains proteins, vitamins, and amino acids in higher concentration than in flower nectar. Additionally, the insects may find respite from predatory insects when feeding in sites other than the flower heads of these bushes.
Since the injury produced by a hornet covers only a small area of the stem, the bushes are normally able to repair the damage and continue to provide for future generations of insects. The stem may become infected with bacteria or fungi and die, but this is a rare occurrence, usually affects only one stem on a plant, and has little effect on either the population of groundsel bushes or the suite of insects and vertebrates that use these bushes for food, hunting grounds, and shelter.