The Asian hornet, a formidable predator in its native range, has garnered significant attention due to its invasive spread and potential ecological impact. Understanding the dietary habits of these hornets is crucial for assessing their threat to native insect populations, particularly honey bees, and for developing effective management strategies. This article delves into the diet of Asian hornets, drawing upon various studies and observations to provide a comprehensive overview.
Asian Hornet Species and Distribution
The term "Asian hornet" can refer to several species, including the Asian giant hornet (Vespa mandarinia) and the yellow-legged Asian hornet (Vespa velutina). The Asian giant hornet, also known as the northern giant hornet or Japanese giant hornet, is the world's largest hornet, native to temperate and tropical East Asia, South Asia, mainland Southeast Asia, and parts of the Russian Far East. It prefers low mountains and forests, avoiding plains and high-altitude climates. The yellow-legged Asian hornet, on the other hand, has become an invasive species in Europe, causing concern due to its predatory behavior.
Predatory Behavior and Diet Composition
Asian hornets are intensely predatory, hunting medium- to large-sized insects. Their prey includes bees, other hornet and wasp species, beetles, hornworms, and mantises, with the latter being favored targets in late summer and fall. Large insects like mantises serve as key protein sources for queen and drone larvae. These hornets often attack colonies of other Vespa species, Vespula species, and honey bee hives to obtain adults, pupae, and larvae as food for their own larvae. Sometimes, they even cannibalize each other's colonies.
A single scout hornet, or sometimes a small group, cautiously approaches a hive, releasing pheromones to guide its nest-mates to the location. The hornets can devastate a honey bee colony, especially the introduced western honey bee, as their stings are ineffective against the heavily armored hornets. A small group of hornets can exterminate a colony of tens of thousands of bees in a few hours.
It's important to note that only hornet larvae can digest solid protein. Adult hornets can only consume the juices of their victims, chewing the prey into a paste to feed their larvae.
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Specific Prey Items
Based on an examination of larval waste products, the prey of V. mandarinia includes a wide range of insects, such as cluster flies, orange-legged drone flies, bristle flies, bronze birch borer beetles, western honey bees, western yellowjackets, German yellowjackets, aerial yellowjackets, bald-faced hornets, European paper wasps, golden paper wasps, paddle-tailed darner dragonflies, shadow darner dragonflies, large yellow underwing moths, blinded sphinx moths, and red admiral butterflies.
Dietary Studies and Methodologies
Several studies have investigated the diet of Asian hornets using various methodologies, including morphological analysis of prey pellets, DNA metabarcoding, and analysis of larval gut contents.
Morphological Analysis
Morphological analysis involves capturing foraging hornets returning to the nest with flesh pellets and identifying the prey remains under a microscope. This method has revealed that honey bees are a major dietary component, along with Diptera, carrion, and other arthropods. However, morphological identification can be challenging, especially when dealing with butchered remains.
DNA Metabarcoding
DNA metabarcoding is a molecular approach that involves analyzing all the DNA present in a sample to determine the proportion that comes from different species. This method has been used to study the diet of Asian hornets by analyzing worker jaw, stomach, or faecal pellet samples. While metabarcoding can provide valuable insights, it also has limitations, as it relies on assumptions about the amplification and detection methods used.
Larval Gut Content Analysis
Analyzing the gut contents of Asian hornet larvae offers several advantages. Larvae represent pooled samples of prey, as they are fed by successive workers that may have brought back different prey species. Additionally, larval gut contents are less degraded than faecal samples. DNA is extracted from the larval gut and sequenced to determine the species present.
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Diet of Vespa velutina
The yellow-legged Asian hornet (Vespa velutina) is a threat to biodiversity, pollinating insects, and beekeeping activities. The adult hornets predominately sustain themselves on carbohydrate sources from nectar, larval secretions or ripe fruits, they must continuously forage protein sources for the developing brood. This protein comes in the form of a ‘flesh pellet’: a small piece of proteinaceous material that the adult carves out from the protein rich thorax of a captured arthropod, or from carrion.
Regional Variations in Diet
The diet of Asian hornets can vary depending on the location and time of year. For example, a study in the Dordogne region of France found that predatory activity peaked in the first week of October, but honey bees made up a smaller proportion of the diet as the season progressed.
Impact on Honey Bees and Biodiversity
Asian hornets are known to prey on honey bees, and can have negative effect on honey bee survival, at least in some locations.
One notable thing missing from all these studies is the impact on native insect biodiversity and abundance.
Factors Influencing Diet
Several factors can influence the diet of Asian hornets, including the availability of prey, the size and location of the hornet nest, and the time of year. Potential prey items that are small, inconspicuous, rare, or difficult to catch are unlikely to feature in the hornet's diet. Conversely, prey that is large, obvious, numerous, and concentrated are likely to be targeted.
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Management Implications
Understanding the dietary habits of Asian hornets is crucial for developing effective management strategies. If hornet predation on honey bees is inversely related to the insect-richness of the environment then beekeepers should encourage landscape improvement that favor insect numbers and diversity. Similarly, notwithstanding the negligible impact on solitary and bumble bees, other pollinators may be depleted by Asian hornets; simply trying to fill the gap with imported honey bees may well not work.