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The Unwelcome Guest: Introducing the Yellow-legged hornet to America

The Yellow-legged hornet (Vespa velutina, also called “Asian hornet” in Europe), often mistaken for its larger cousins (Vespa crabro and Vespa mandarinia), has been making a global buzz. Originally from China, it began its Western march by invading France in 2004 and has since spread throughout Europe. Now, it’s taking flight in the US, posing new challenges. However, it’s not an unbeaten path. French beekeepers have faced this threat for many years, gaining invaluable insights into the Yellow-legged hornet’s behavior and ecology. With the support of Véto-pharma, they’ve gathered the knowledge and tools to tackle this intruder. Together, we’ve accumulated a broad understanding of the Yellow-legged hornet, which we’re eager to share with the American beekeeping community.

Physical Characteristics of Vespa velutina

Slightly smaller than its European cousin and much tinier than the Asian Giant Hornet, Vespa mandarinia, the Yellow-legged hornet stands modest at about 2-3cm [0,78-1,18 inches]. Sporting a mostly black abdomen with a standout yellow fourth segment, it’s rightly dubbed “the yellow-legged hornet”. Its vibrant yellow-orange face further sets it apart. Spotting this winged invader is step one to safeguarding our ecosystems.

avispa asiática Stephane de Greef 2020

Nest Structures and Where to Spot Them

Imagine a delicate paper structure crafted from chewed wood pulp and saliva. That’s an Yellow-legged hornet’s nest for you!

avispa asiática
nido de avispa asiática

In the early stages, the primary nests whipped up by the queen (termed “foundress”) from March to June are small, about the size of a mandarin. Built with a trio of protective layers, these nests have their entrances at the bottom and snuggle up in sheltered nooks such as shrubs or trees.

nido de avispa asiática
nido de avispa asiática

But here’s where things get grand. As summer hits its stride, Asian Hornets either upscale their primary real estate or go house-hunting for a spacious new home. Mature nests, often nestled high in tall trees, are marvels of natural architecture. Towering up to a meter [40 inches] in height, they prefer the serene ambiance of watersides, like riverbanks. Fun fact: these nests are exclusive nursery chambers for their brood – no food storage here.

Avispero asiático en rayos X
Asian hornet nest in X rays © Eric Darrouzet

By year’s end, nests boast a whopping 12,000 cells but dwindle to a modest 2,000 members come October. To thwart their expansion, savvy French beekeepers deploy “spring trapping” techniques. But hang tight, we’ll delve deeper into this in a forthcoming article.

Lifecycle and Colony Patterns

The life of a Yellow-legged hornet revolves around four main stages: the egg, larva, pupa, and the adult. Here’s a look into their fascinating life cycle (depending on the state and climate you live in, months may vary):

  • Spring Emergence: After a long winter hibernation, queens emerge around February to March. They’re often found in close proximity to their previous nests or sometimes even within abandoned ones. Initially, they’re on a hunt for sugar to replenish their energy. Once revitalized, they embark on constructing the primary nest and lay their first batch of eggs by April or May. An interesting fact is the larval growth. Unlike bees, hornet larvae undergo “sloughing” three times in three weeks, allowing them to grow before they cocoon themselves in a silky case. It takes a full 45 days from egg to adult transformation.
  • Summer Predation: As the warmth of summer sets in between June and September, the colonies are in full swing. This is what beekeepers term the “predation phase“. Worker hornets are on a dual mission during this time – fetch sugars for energy and capture proteins, like bees, for the growing larvae in their nests. And their mode of hunting bees is somewhat grisly; hornets decapitate bees and retain only the protein-rich thorax, which is then mashed into “meatballs” for the larvae. The hunting radius? Approximately 800 meters from their nest, though they sometimes venture further.
  • Autumn, the reproduction phase: As the leaves turn golden, the Asian hornet colony transitions into the reproduction phase around the end of September and October. New queens and males are birthed. To nourish them, workers seek out sugars, causing a noticeable uptick in their outside-the-nest activities. Post mating, males meet their end, while the queens store sperm for future egg-laying. These new queens then scout for overwintering spots, ensuring the continuity of their species. By the arrival of winter, the old queens and existing colony fade away, leaving only the new queens to champion the next generation. A word of caution for the curious: if you ever come across an abandoned nest, handle with care. Some overwintering queens might just be nestled inside, and a warm touch could awaken them.

Impact of hornet predation on bees

The siege of the Yellow-legged hornet, in summer-fall, is a challenging period for bee colonies. Worker hornets hover at hive entrances, effectively blocking bees from venturing out. As a result, bee activity reduces drastically, affecting their ability to gather nectar and pollen. This not only depletes their food reserves, jeopardizing their winter survival, but also induces stress. Studies have indicated oxidative stress in bees during hornet predation, leading to premature aging. It’s noteworthy that hornets don’t discriminate; they target all hives in an apiary without preference.

Feeling enlightened? Stay tuned as we discuss the French beekeepers’ decade-long battle against the Yellow-legged hornet and guide you on spotting and decimating nests.

In the meantime, why not test your new-found hornet wisdom? Dive into our Yellow-legged hornet quiz and see if you’re geared up for the hornet showdown!