Robber flies, also known as assassin flies, are a captivating group of insects recognized for their predatory prowess. These bristly or hairy flies, ranging from medium to large in size, belong to the family Asilidae. In North America alone, over 1,000 species exist, with nearly 100 documented in the Upper Midwest. Their common name aptly describes their hunting technique: they are masters of aerial ambush, pouncing on prey with remarkable speed and ferocity.
A detailed view of a robber fly, showcasing its bristly body and predatory features.
Adult robber flies typically exhibit an elongated body shape, characterized by a slender, tapering abdomen and often a hump-backed appearance. Their coloration is generally muted, with most species displaying shades of gray, brown, or black. However, nature’s mimicry is evident in some species that cleverly resemble bumble bees, likely for protection or to deceive prey. These adult flies enjoy a lifespan of around three months and are commonly observed throughout the summer months. They thrive in open, sunny fields and are particularly abundant in semi-arid environments. Renowned for their speed and agility in flight, robber flies are capable of capturing insects significantly larger than themselves while airborne.
Robber fly in its natural habitat in Chile, demonstrating its camouflage within foliage.
Robber flies are highly responsive to movement, with flying insects being their primary targets. When disturbed or in pursuit of prey, their flight is usually short-distance, often accompanied by a distinctive loud buzzing sound. Male robber flies exhibit territorial behavior, which explains why large congregations of adults are rarely seen. Intruding males are quickly driven out or captured by resident males.
Female robber flies lay small, cream-colored eggs on various surfaces, including grass, plants, soil, bark, or wood, depending on the species. While some species lay eggs individually, most deposit them in large masses encased in a soft, chalky-white protective material. Certain species exhibit more elaborate egg-laying behaviors, such as creating shallow holes in the soil or placing eggs in crevices under tree bark.
The larvae of robber flies are small, cream-colored, and cylindrical, residing in soil or decaying wood. They are active hunters, moving around to find prey. Their feeding method involves piercing their prey’s body and extracting fluids. The larval diet generally consists of insect eggs and soft-bodied insects they encounter. Some species show a preference for grasshopper eggs or white grubs. Robber flies overwinter as larvae, pupating in the soil. Just before adulthood, the pupae rise to the soil surface, leaving the pupal skin partially exposed as the adult emerges. The complete life cycle for common species typically ranges from one to three years.
Adult robber flies are known as aggressive, generalist predators. Their diet is diverse, including butterflies, wasps, bees, dragonflies, grasshoppers, beetles, and other flies. Larger species are capable of inflicting a painful bite if handled carelessly, a testament to their potent mouthparts. Remarkably, some species frequently prey on formidable insects like hornets and paper wasps. At least six North American species primarily feed on grasshoppers, highlighting their potential role in controlling these agricultural pests. However, most species are indiscriminate feeders, consuming both destructive and beneficial insects, including honey bees and other pollinators.
A robber fly in New Zealand, captured with prey, illustrating its hunting success.
Saropogon dispar, for example, is a notable species in Texas known to frequent apiaries and cause harm to bee populations. In one instance, over 700 of these flies were eradicated from a single bee yard within three days. Another species, Proctacanthus milbertii, sometimes referred to as the “Missouri bee killer,” has a more varied diet. While honey bees constitute a small portion (around 4%) of its prey, grasshoppers and moths make up the majority. In Nebraska sandhills, P. milbertii is estimated to capture nearly 2% of the adult grasshopper population daily, with each fly consuming one to two prey items per day. Conversely, several robber fly species are often found on flowering shrubs like Prunus, mockorange, or flowering sumac, where they feed on flies and wasps attracted to the blossoms.
Promachus rufipes robber fly, showcasing its reddish legs, a key identification feature.
The larvae of certain robber fly species may play a role in regulating populations of carpenter bees, white grubs, and grasshoppers. They are known to consume small grubs, root maggots, wireworms, and other beetle larvae, as well as insect eggs in garden environments. Diogmites discolor has been reported to significantly impact white grub populations, destroying 12% of Phyllophaga (white grub) larvae at a Kentucky site.
Despite their predatory habits and potential for natural pest control, robber flies have not been widely adopted in biological control programs. Their precise role in naturally controlling pest species remains understudied, even though they are common in many regions. Adult robber flies require both sugar and protein for egg production. Encouraging nectar-producing flowers like goldenrod or Queen Anne’s lace in gardens might support robber fly populations, potentially enhancing their natural pest control services.
– Based on information from Susan Mahr, University of Wisconsin – Madison
